What causes the smell after rain?

Most people notice a distinctive smell in the air after it rains. It's frequently linked with spring, as the smell of fresh cut grass is associated with summer. You'll find it in a lot of poetry and also on many inspirational lists of things to be happy about. But what causes it?

As it turns out, the smells people associate with rainstorms can be caused by a number of things. One of the more pleasant rain smells, the one we often notice in the woods, is actually caused by bacteria! Actinomycetes, a type of filamentous bacteria, grow in soil when conditions are damp and warm. When the soil dries out, the bacteria produces spores in the soil. The wetness and force of rainfall kick these tiny spores up into the air where the moisture after a rain acts as an aerosol (just like an aerosol air freshener). The moist air easily carries the spores to us so we breathe them in. These spores have a distinctive, earthy smell we often associate with rainfall. The bacteria is extremely common and can be found in areas all over the world, which accounts for the universality of this sweet "after-the-rain" smell. Since the bacteria thrives in moist soil but releases the spores once the soil dries out, the smell is most acute after a rain that follows a dry spell, although you'll notice it to some degree after most rainstorms.

Another sort of smell is caused by the acidity of rain. Because of chemicals in the atmosphere, rainwater tends to be somewhat acidic, especially in urban environments. When it comes in contact with organic debris or chemicals on the ground, it can cause some particularly aromatic reactions. It breaks apart soil and releases minerals trapped inside, and it reacts with chemicals, such as gasoline, giving them a stronger smell. These reactions generally produce more unpleasant smells than bacteria spores, which is why the after-the-rain smell isn't always a good one. Like the smell caused by the bacteria spores, the smell of chemical reactions is most noticeable when it rains following a dry spell. This is because once the chemicals on the ground have been diluted by one downpour, they don't have the same reaction with the rainwater.

Another after-the-rain smell comes from volatile oils that plants and trees release. The oil then collects on surfaces such as rocks. The rain reacts with the oil on the rocks and carries it as a gas through the air. This scent is like the bacteria spores in that most people consider it a pleasant, fresh smell. It has even been bottled and sold for its aromatic qualities!

These are a few common rain smells, but there are also all sorts of other scents after it rains. There is lots of aromatic material that the moisture and impact of rain can stir up, and the moist atmosphere following a downpour is particularly good at carrying these particles through the air. So, when you talk about the after-the-rain smell with a friend, you may mean one thing while your friend is thinking of something else. You'll both agree, however, that the air has a much stronger aroma to it after a good rain.

What is the UV index and how is it calculated?

If you have read How Sunburns and Sun Tans Work, you know that your skin is sensitive to ultraviolet (UV) radiation. The UV index is a value calculated each day that helps you to know how much UV radiation will reach you if you go outside. By knowing the index value for the day, you can take appropriate precautions. These precautions help you avoid sunburn in the short term and skin cancer in the long term.

The UV index is calculated based on four factors:

• The thickness of the ozone layer over your city (detected using satellites)
• The cloud cover over your city (clouds block UV radiation to varying degrees)
• The time of year (in winter, UV radiation is lower than in the summer because of the sun's angle)
• The elevation of your city (higher elevations get more UV radiation)

The UV index value ranges between 0 and 10, with zero being minimal UV exposure risk and 10 being maximal UV exposure risk.

Why does the sky get dark at night?

The answer to this seemingly simple question may boggle your brain. It's actually a famous cosmological problem, formally known as Olbers' Paradox. (Heinrich Olbers was a German astronomer who popularized discussion of this subject in 1826.) You might think that the question can be explained away by the effect of distance -- not so. To fully understand the perplexity, picture stars of equal brightness distributed evenly in concentric layers around Earth, like shells around a nut. The same amount of light should reach Earth from each layer, because although the amount of light to reach us from each star decreases with distance (by 1/d^2), the number of stars in each layer increases, effectively balancing out the distance effect.

If Star Layer A is twice as far from Earth as Star Layer B, then the amount of light that reaches us from each star in A is only one-fourth the amount of light that reaches us from each star in B; but there are four times as many stars in A as there are in B.

If the distance between A and B is 2 units, then each square in A is one-fourth as bright as each square in B; but there are four times as many squares in A as there are in B.

So light lost to distance does not account for the darkness of night. Obscuration by dust is not the answer, either, as any dust in the path of light would heat up and eventually reradiate. Most modern cosmologists have settled on two theories to account for the darkness. The first one states that red shift (see Echo and Doppler Shift), which indicates that space itself is expanding, decreases the amount of light reaching us. The other explanation -- generally considered the main one -- is that the universe is not infinitely old. If it were, the sky would in fact be infinitely bright, because light from every point in the universe would have had time (eternity) to travel to every other point. As far as we know, there is no edge of the universe, only an edge of time. The finite age of the universe limits how much light we see.

What is the heat index that the weatherperson talks about during the summer?

During an average day, your body burns about 2,000 calories (when you are exercising heavily, it burns a lot more). That means that during waking hours, you are burning about 2 calories a minute. These 2 calories have the ability to raise the temperature of 1 kilogram of water 2 degrees C. If you weigh 50 kilograms (110 pounds), your body temperature rises one-twenty-fifth of a degree C (one-twelfth of a degree F) every minute. (For more information on calories and your body, see How Calories Work.)

Your body needs a way to dump that excess heat. If it doesn't, then body temperature rises into the danger zone in a matter of 30 minutes. Up to about 80 degrees F (24 degrees C), it's easy to dump excess heat simply through radiation (this is why air temperature "feels" comfortable at up to about 80 degrees F). Above 80 degrees F, your body does not have enough surface area to get rid of the heat fast enough, so your body turns on your sweat glands to make evaporative cooling possible.

Evaporative cooling works great if the air is dry. In high humidity, however, it doesn't work very well -- the sweat cannot evaporate because the air is already saturated with humidity. In high temperature/high humidity environments, your body can get into a dangerous situation where it cannot radiate or evaporate the heat away. The heat index that you see on the evening news is designed to make you aware of these dangerous situations.

The heat index takes the day's temperature and humidity into account and calculates what the temperature would be if the air were at 25-percent humidity or so (very dry). On this scale, high humidity can make you excruciatingly hot because your body has no way to eliminate excess heat. For example, 100 degrees F with 100-percent humidity is the equivalent of 195 degrees F at 25-percent humidity -- nearly the boiling point of water!

What if I were struck by lightning?

Initially this seems like a fairly straightforward question. As it turns out, there are several ways a person can be struck by lightning, and the type of strike dictates the impact it can have on your body.

• 
  
  Direct strike - a cloud-to-ground lightning strike hits you or something you're holding, like a golf club, dead-on instead of reaching the ground.
• Side flash - lightning strikes something close to where you are standing and then jumps from that to you.
• Contact potential - while you're touching something, like a fence-post or a tree, lighting strikes that object and the current travels from the object through the point of contact into your body.
• Step voltage - you're sitting with your feet together in front of you, knees up and rump settled on the ground near a spot where a cloud-to-ground lightning strike hits. As the lighting current disperses, it travels through your body by entering one point, say your joined feet, and exiting another, your rear end.
• Surge voltage - while you're using some type of electrical appliance or a telephone, lighting strikes the source of power or network connected to the device and you receive a shock.

The worst kind of lightning experience is a direct strike - statistically, it's the most fatal. Being hit by a side flash or through contact potential are the next in the level of severity, with step voltage third and surge voltage last. Basically, the amount of current and voltage going through your body lessens with each of these types of strikes. If you're a victim of a direct strike, the full impact of the lightning courses through your body. In the other scenarios, the intensity is lessened because some of the energy is dispersed elsewhere.

The circulatory, respiratory, and nervous systems are most commonly affected when a person is struck by lightning:

Circulatory: Reportedly, the majority of fatalities resulting from direct strikes are due to cardiac arrest. Ironically, were someone nearby with an automatic external defibrillator, to administer another electric shock to the heart, the victim might survive.

Respiratory: The greatest threat to the respiratory system is paralysis. Artificial respiration is required so the victim won't die from lack of oxygen.

Nervous: When the central nervous system is affected, a number of side effects can occur such as dementia, amnesia, temporary paralysis, impaired reflexes, memory gaps and anxiety or depression.

Video Gallery: Why Lightning Strikes

Lightning will strike anything that stands high above the ground. Trees are a favorite target. No one knows how lightning determines its jagged path. Brought to you by the Discovery Channel.

Extreme weather has been causing death and destruction around the world. Near the Xinjiang province of China, high winds brought disaster, blowing 10 carriages off the tracks, leaving three passengers dead and 34 injured. In Sydney, fierce rain storms caused flash flooding where one man was struck by lightning and 11 school children barely escaped injury when their classroom was hit.

How do I avoid getting struck?

More than 1,000 people get struck by lightning every year in the U.S., and more than 100 of them die as a result of the strike. Lightning is not something to toy with. There are several precautions you can take to guarantee your safety in a storm.

If you're outside:

• Always look for appropriate shelter in a building or a car. Most people think it is the rubber tires that keep you safe in a car because they do not conduct electricity. Actually, in strong electric fields, rubber tires become more conductive than insulating. The reason you are safe in a car is because the lightning will travel around the surface of the vehicle and then go to ground. This occurs because the vehicle acts like a Faraday Cage. Michael Faraday, a British physicist, discovered that a metal cage would shield objects within the cage when a high potential discharge hit the cage. The metal, being a good conductor, would direct the current around the objects and discharge it safely to the ground. This process of shielding is widely used today to protect the electrostatic sensitive integrated circuits in the electronics world.
• Avoid taking shelter under trees. Trees attract lightning. Put your feet as close together as possible and crouch down with your head as low as possible without touching the ground - remember step voltage - you only want one contact point with the ground. Never lay down on the ground for the same reason; you never want the current to have the ability to pass through your body.

If you're inside:

• Stay off the phone. If you must call someone, use a cordless phone or cell phone. If lightning strikes the phone line, the strike will travel to every phone on the line and potentially to you if you are holding the phone.
• Stay away from plumbing pipes (bathtub, shower). Lightning has the ability to strike a house or near a house and impart an electrical charge to the metal pipes used for plumbing. This threat is not as great as it used to be, because PVC (polyvinyl chloride) is often used for indoor plumbing these days. If you are not sure what your pipes are made of, wait it out.
  

What is St. Elmo's Fire? What is called St. Elmo's Fire?

If you were to look outside your home during a thu­nderstorm and see a tall streetlamp glowing with­ blue flames, you might be tempted to call the fire department. Then you might notice that the streetlamp is on fire but isn't actually burning -- and the water from the fire hose isn't putting out the flames. At this point, you might be about ready to call a priest, but that, like the call to the fire department, would be unnecessary. The phenomenon you're witnessing is actually St. Elmo's Fire. (Which has nothing to do with a 1980s coming-of-age film starring a young Emilio Estevez.) ­

St. Elmo's Fire is a weather phenomenon involving a gap in electrical charge. It's like lightning, but not quite. And while it has been mistaken for ball lightning, it's not that, either -- and it's definitely not fire. Early observers of the phenomenon, mostly sailors on high seas during thunderstorms, seem to have understood they weren't looking at actual fire, because instead of abandoning ship, they took comfort in the sudden glow atop the masts. Such famous figures as Magellan, Caesar and Columbus experienced St. Elmo's Fire on their journeys. And Pliny the Elder, who seems to have documented absolutely every natural phenomenon back in the 1st century A.D., beat everyone else to the punch when he described blue flames appearing out of nowhere during thunderstorms.

Sailors tended to attribute the glow to "St. Elmo," a mispronunciation of St. Ermo or St. Erasmus, the patron saint of Mediterranean sailors. They believed the fire was a sign of salvation from the saint, since the phenomenon occurs most often toward the end of a storm. Benjamin Franklin and Charles Darwin viewed the weather event through a decidedly more scientific perspective. But regardless of interpretation, it's clear they were all observing the same phenomenon. And contrary to popular belief, St. Elmo's Fire doesn't only occur at sea.

­As with all electrical phenomena, St. Elmo's Fire is about electrons. So, what is St. Elmo's Fire if it's not a form of lightning? Find out in the next section.

Causes of St. Elmo's Fire: The Fire That's Not a Fire

Like lightning, St. Elmo's Fire is plasma, or ionized air that emits a glow. But while lightening is the movement of electricity from a charged cloud to the ground, St. Elmo's Fire is simply sparking, something like a shot of electrons into the air. It's a corona discharge, and it occurs when there is a significant imbalance in electrical charge, causing molecules to tear apart, sometimes resulting in a slight hissing sound.

Al Barry/Three Lions/Getty Images
Like St. Elmo's Fire, neon tubes glow with the light of a sustained spark.

The first step in generating St. Elmo's Fire is a thunderstorm. As you can learn in How Lightning Works, a thunderstorm creates an electrically charged atmosphere. There is a charge difference between the storm clouds and the ground, and this difference creates voltage, or electrical pressure. In between the clouds and the ground, the atoms in the air undergo changes; most important to our discussion, electrons move farther away from protons, creating an environment that allows electrons to move around freely. In other words, the air becomes a good conductor.

Once the air is conducive to the movement of electrons, those electrons continue to increase the distance between their positively charged counterpart, protons. This is ionization, and plasma is simply ionized air. The phenomenon that causes St. Elmo's Fire is a dramatic difference in charge between the air and a charged object, like the mast of a ship, the tip of an airplane wing or the 30-foot steeple of a church -- things we often think of as potential lightning rods.

When the voltage gets high enough, usually around 30,000 volts per centimeter of space, the charged object will discharge its electrical energy [source: Scientific American]. The reason why St. Elmo's Fire occurs most often on pointed objects is that a tapered surface will discharge at a lower voltage level. The tip of a steeple, mast or airplane wing presents something like a condensed surface charge.

When the air molecules tear apart, they emit light. In the case of St. Elmo's Fire, the discharge is continuous -- sometimes lasting several minutes -- and creates a constant glow. The glow is blue because different gasses glow different colors when they become plasmas. Earth's atmosphere has nitrogen and oxygen in it, and this particular combination happens to glow blue.

St. Elmo's Fire is exactly what's happening in neon tubes -- essentially a continuous spark. If Earth's atmosphere were made up of neon, St. Elmo's Fire would glow orange instead of blue. A neon tube is simply St. Elmo's Fire contained in glass. St. Elmo's Fire also behaves something like a plasma globe. One pilot described the phenomenon occurring on the windshield of her small plane while flying through a storm cloud; when she touched the inside of the windshield, blue streaks reached toward the tips of her fingers [source: USA Today].

Ball Lightning
St. Elmo's Fire and "ball lightning" are two different things. The scientific community can't agree on what ball lightning is, but it's definitely not St. Elmo's Fire. Ball lightning can float around the air, while St. Elmo's Fire stays put.

Why is the sky blue? Why sky is blue?

Here is s­omething interesting to think about: When y­ou look at the nighttime sky, it's black, with the stars and the moon forming points of light on that black background. So why is it that, during the day, the sky doesn't remain black with the sun acting as another point of light? Why does the daytime sky turn a bright blue and the stars disappear?

The first thing to recognize is that the sun is an extremely bright source of light -- much brighter than the moon. The second thing to recognize is that the atoms of nitrogen and oxygen in the atmosphere have an effect on the sunlight that passes through them.

­The­re is a physical phenomenon called Rayleigh scattering that causes light to scatter when it p­asses through particles that have a diameter one-tenth that of the wavelength (color) of the light. Sunlight is made up of all different colors of light, but because of the elements in the atmosphere the color blue is scattered much more efficiently than the other colors.

When you look at the sky on a clear day, you can see the sun as a bright disk. The blueness you see everywhere else is all of the atoms in the atmosphere scattering blue light toward you. Because red light, yellow light, green light and the other colors aren't scattered nearly as well, you see the sk­y­ as blue.

Keep reading for more links to questions that have been puzzling you, such as whether smog makes for beautiful sunsets.

Why would someone fly an airplane into a hurricane?

On Halloween Day 2007, a tropical storm named Noel by the National Oceanic and Atmospheric Administration (NOAA) escalated. The storm was seated over Haiti, the Dominican Republic and Cuba, before picking up enough strength from the warm Caribbean air to develop into a Category 1 hurricane as it approached the northwestern Bahamas on Nov. 1, 2007. HurricaneUnited States and finally disintegrated in Nova Scotia. Noel's winds reached 80 mph in t­he Caribbean before it moved up the Atlantic coast of the

In its wake, Noel left at least 160 people dead in the Caribbean islands [source: NOAA]. It didn't turn out to be the strongest storm of the 2007 Atlantic hurricane season, but Noel was the deadliest. And the impression it left on meteorologists was enough for the World Meteorological Organization to retire Noel from the list of storm names [source: NOAA].

But in addition to leaving destruction and death in its wake, Hurricane Noel also left behind a breakthrough in scientific research. It was the first hurricane to host the remote controlled airplane called the Aerosonde. This unmanned aerial system (UAS) was developed by NASA and NOAA and launched from Wallops Flight Center in Delaware as the storm approached on Nov. 2. Aerosonde made history as the first unmanned vehicle to fly into the eye of a hurricane [source: WFC]. Eighty miles off the coast of the United States, Aerosonde explored the hurricane wall at altitudes as low as 300 feet (91 meters), sending real-time information about temperature, wind speed and atmospheric pressure back to hurricane researchers.

Like tornadoes, much of how hurricanes work remains a mystery to meteorologists. The information provided by Aerosonde will help researchers better predict what tropical depressions and storms will develop into full-blown hurricanes. Aerosonde represents a huge leap forward in hurricane research. It also signals the beginning of the end of another type of research: Aerosonde and other UASs will eventually replace the people who risk life and limb by flying airplanes into hurricanes.

Flying into the Storm

OK, so people today fly airplanes into hurricanes to gather data. That much is understandable. But why would anyone fly into a hurricane before the Weather Bureau or the National Oceanic and Atmospheric Association (NOAA) ever existed to accept weather data?

"Just for fun," was the answer given by Col. Joe Duckworth [source: Coleman and McCloud]. He and Lt. Ralph O'Hair, both flyboys for the Army Air Corps were among the first people to fly an airplane into a hurricane. In July 1943, Duckworth and O'Hair flew a small AT-6 prop plane into the eye of a hurricane with 132 mph winds off the coast of Galveston, Texas [source: Old Farmer's Almanac]. While the pilot and navigator won highballs at the officer's club after safely returning that day, the prize for science was much more pronounced: The thermometers aboard the plane recorded a 25-degree Fahrenheit (14 degrees Celsius) difference in temperature between the eye of the hurricane and the air circling it.

­Flight Image Gallery

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Hulton Archive/Getty Images
A 1945 photo of an AT-6 two-passenger fighter plane, the first to be flown into a hurricane. See more flight pictures.

Duckworth and O'Hair's flight into the Texas hurricane proved two things: It's possible to fly into hurricanes and survive, and such flights could provide valuable scientific information. Following that excursion, manned flights into some of nature's most severe storms became more frequent.

The next year, Navy and Army flights successfully tracked an Atlantic hurricane along the United States' Eastern seaboard. The coordinated flights reported on the hurricane's path and were credited with saving lives; a surprise storm had killed 600 people in New England six years before, while the 1944 storm (about which residents were forewarned) took only 50 lives [source: USA Today].

The advent of satellites in the 1960s made it virtually impossible for a hurricane to surprise anyone. Land-based researchers use satellite imaging to track the development and movement of every storm as it forms at sea. While these images provide information about the size and direction of a hurricane, there's still plenty of data associated with these meteorological phenomena that can't be culled from photos. Images provide overviews of a storm; to get the details, one must go inside.

Today, most manned flights into hurricanes are undertaken by the Air Force's 53rd Weather Reconnaissance Squadron (popularly called the Hurricane Hunters) and the NOAA. NOAA mans 8-hour flights into storms, going from one side into the eye, back into the storm and out the other side several times per flight [source: National Science Foundation]. NOAA drops a Dropwindsonde device into the storm to gather real-time data about the characteristics of a storm from top to bottom. The 53rd Weather Reconnaissance Squadron flies out of Keesler Air Force Base, Miss., and keeps track of Atlantic hurricanes with a flight crew operation of 20 people [source: 403rd].

Together, NOAA and the Air Force (and occasionally NASA) provide in situ (on location) data about hurricanes as they unfold. But the airplanes used by both groups have drawbacks. These are large, lumbering transport planes, like the C-130, and they don't fly quite as fast as necessary to provide the data needed to truly map the minute-to-minute changes in a hurricane [source: Henning]. The need for this kind of information still exists; until we fully understand all of the processes that create and direct a hurricane, we'll never be able to confidently model and predict future storms. With the advent of unmanned vehicles that can fly into hurricanes, it looks like this data will be provided without maverick humans flying into the storms.

Will the U.S. be a desert in 50 years?

What happens when climate variations and human actions such as land clearing, urbanization and unsustainable agricultural habits mix? The combination results in an ecological crisis: land degradation. When land degrades, it can no longer properly absorb, store and recycle water, nutrients and energy. Land degradation in arid, semi-arid and dry, subhumid areas (drylands) is called desertification.

Water Image Gallery

Joel Sartore/National Geographic/Getty Images
Poor agricultural habits can lead to dust and desertification. See more water pictures.

Seventy percent of drylands worldwide are degraded, and one-third of the world's land surfaces are threatened -- lands become more prone to flooding, wind damage and erosion, the quality of soil becomes poor, and productivity plummets [sources: UNCCD, UNESCO]. Areas affected by desertification are faced with not only environmental consequences but also economic effects. The World Bank estimates areas affected by desertification lose income greater than $40 billion annually [source: UNESCO].

­In dryland a­reas, soil is fragile, vegetation is thin and the climate is merciless. Drylands cover more than 40 percent of our planet, and their susceptibility to desertification puts more than 1 billion people at risk for poverty and starvation [source: USDA].

Desertification Threat

­While deserts expand and contract naturally, it's important to understand that desertification is different from this natural cycle. Desertification is also far from being a new global crisis: Ancient empires, including Sumeria, Babylon and Rome, were dealt its debilitating effects. Unfortunately, there is no one reason we can point to as the cause of desertification, but rather a combination of climate and our own mistakes, ignorance and actions.

Arthur Rothstein/Resettlement Administration/Time & Life Pictures/Getty Images
While the dust bowl ravaged the Great Plains, many farmers stayed on to try to make a go of it.

Desertification is triggered mainly by the overuse of land and unsustainable agricultural practices (overcropping, overgrazing, poor irrigation and deforestation) which lead to­ a loss of natural vegetation and healthy soil. An expanding human population and urban living area as well as the low level of priority placed on environmental protection efforts also contributes.

Currently, the most endangered regions are near the world's five main deserts, including the Sonoran Desert which lies in Northwest Mexico and the Southwest United States; the Atacama Desert in South America; the Kalahari Desert in Southern Africa; most of Australia; and the large desert mass made up by the Sahara, Arabian, Great Indian, Taklimakan, Gobi and the deserts of Iran and the former Soviet Union.

But areas not on this list are still of concern. For example, desertification is a looming crisis in Africa where almost 70 percent of the continent is arid or semi-arid land. More than 30 percent of North America is comprised of arid or semi-arid lands, with about 40 percent of the continental United States at risk for desertification [source: U.N.].

Seventeen states in the Western U.S. can be categorized as arid, semi-arid or dry subhumid based on climate and soil type; this characterizes them as vulnerable to desertification. The most severely affected areas in the U.S. are in New Mexico, Texas and on the Navajo Indian reservation in New Mexico and Arizona. Overgrazing, which has led to wind and water erosion, and poor irrigation are the leading causes of desertification in the U.S., and scientists believe it's unlikely these areas will mend naturally from the damage in the next 100 years.

The 1930s Dust Bowl As a result of poor farming practices and drought, the U.S. became intimately acquainted with desertification in the 1930s. Farmers set the stage for the dust bowl period as they plowed the natural prairie grasses in the Great Plains to make room to farm wheat. Unlike the natural grasses, wheat wasn't hearty enough to endure drought. Crops died and the bare land was left exposed to the elements. Catastrophic dust storms whipped across the plains, eroded soil­, reduced visibility and caused lung illnesses. Three million people abandoned their farms on the great plains, most headed out West. As a result of the dust bowl, the U.S. government created the Soil Conservation Service a group for researching and funding ways to fight the threat of desertification in America.

With the establishment of the Soil Conservation Service after the damaging 1930s dust bowl, the U.S. recognized desertification as a national problem and the government began providing financial support and resources to put appropriate agricultural practices into place. Together, state and federal agencies and nongovernment organizations work with private ranchers, farmers and developers on technologies that help reduce soil loss, improve irrigation and draw on renewable energy sources.

Land health in the U.S. is an ongoing and considerable ecological problem. In 2007, scientists noted the disappearance of green oases that used to dot the desert landscape as well as an increase in temperature and drought in the Southwest.

How far does ultraviolet light penetrate into the body? What depth ultraviolet light penetrate into the body?

In human skin, UV penetration depth is actually very slight -- less than 1 millimeter. Ultraviolet radiation usually doesn't make it past the epidermis, the top layer of skin.

This is why UV-initiated skin cancers are generally in the epidermal layer.

If the temperature is 30 degrees F, why do we sometimes get snow and other times get freezing rain?

When you watch the local weather report on the evening news, you always hear the "current temperature." It may be, for example, 32 degrees Fahrenheit (0 degrees Celsius) outside. That is useful information, but if it is precipitating, it is only one part of the puzzle.

It turns out that the atmosphere is layered, and these layers control the form that precipitation will take. The local weather report only gives us the ground-level temperature. In order to understand sleet, snow and freezing rain, what we would need is perhaps four to six different temperature readings at different altitudes.

Precipitation starts in the cloud as snow. As it falls, it may travel through a layer of air that has a temperature greater than 32 F (0 C). This layer melts the snow into rain. If the temperature at ground level is below freezing, then the water may refreeze in the air, and we get sleet. Or, if the layer of sub-freezing air at ground level is thin, the precipitation falls as rain but then freezes once it touches a freezing object on the ground.

For snow to fall, all of the layers of air that the snow falls through once it leaves the cloud must be sub-freezing.

The warm middle layers are normally caused by the movement of warm fronts or cold fronts through the area. In the Southeast, temperatures often hover around 32 F, so the form of precipitation can change all the time. In more northern areas, the temperature is well below freezing, so snow is a sure thing.

What is relative humidity and how does it affect how I feel outside? What is relative humidity?

Humidity is something we hear about daily in weather reports. Humidity is to blame for that muggy, steam-room feeling you experience on certain summer days.

Karl Weatherly/Getty Images If the air is at 100-percent relative humidity, sweat will not evaporate into the air. As a result, we feel much hotter than the actual temperature when the relative humidity is high.

Humidity can be measured in several ways, but relative humidity is the most common. In order to understand relative humidity, it is helpful to first understand absolute humidity.

Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature. The hotter the air is, the more water it can contain.

Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain. This doesn't mean that the relative humidity must be 100 percent in order for it to rain -- it must be 100 percent where the clouds are forming, but the relative humidity near the ground could be much less.

Humans are very sensitive to humidity, as the skin relies on the air to get rid of moisture. The process of sweating is your body's attempt to keep cool and maintain its current temperature. If the air is at 100-percent relative humidity, sweat will not evaporate into the air. As a result, we feel much hotter than the actual temperature when the relative humidity is high. If the relative humidity is low, we can feel much cooler than the actual temperature because our sweat evaporates easily, cooling us off. For example, if the air temperature is 75 degrees Fahrenheit (24 degrees Celsius) and the relative humidity is zero percent, the air temperature feels like 69 degrees Fahrenheit (21 C) to our bodies. If the air temperature is 75 degrees Fahrenheit (24 C) and the relative humidity is 100 percent, we feel like it's 80 degrees (27 C) out.

People tend to feel most comfortable at a relative humidity of about 45 percent. Humidifiers and dehumidifiers help to keep indoor humidity at a comfortable level.

Why are there more thunderstorms during the summer?

According to the National Oceanic and Atmospheric Administration (NOAA) National Weather Service, approximately 1,800 thunderstorms are occurring at any given time, resulting in about 16-million thunderstorms each year. Most thunderstorms last about 30 minutes and are typically about 15 miles (24 km) in diameter. The two biggest threats associated with most thunderstorms are lightning and flash floods. To understand why thunderstorms occur more often during the warm months requires some understanding of thunderstorm basics.

Stockbyte/Getty Images High humidity and warm temperatures create prime conditions for a thunderstorm.

Thunderstorms thrive under certain conditions. The two most basic elements that cause a thunderstorm to develop are:

• Moisture
• Rapidly rising warm air

Because moisture and warmth are crucial to thunderstorms, it makes sense that they would occur more often in the spring and summer, particularly in humid areas such as the southeastern United States. The high humidity, in conjunction with warm temperatures, creates massive amounts of warm, moist air rising into the atmosphere, where it can easily form a thunderstorm.

Where does the thunder (and lightning) come from? The basic idea is that thunder clouds can become giant Van de Graaff generators and create huge charge separations within the cloud. Let's look at how it works.

Clouds contain millions and millions of water droplets and ice particles suspended in the air. As the process of evaporation and condensation occurs, these droplets collide with other moisture that is condensing as it rises. The importance of these collisions is that electrons are knocked off of the rising moisture, creating a charge separation. The newly knocked-off electrons gather at the lower portion of the cloud, giving it a negative charge. The rising moisture that has lost an electron carries a positive charge to the top of the cloud.

As the rising moisture encounters colder temperatures in the upper cloud regions and begins to freeze, the frozen portion becomes negatively charged and the unfrozen droplets become positively charged. At this point, rising air currents have the ability to remove the positively charged droplets from the ice and carry them to the top of the cloud. The remaining frozen portion either falls to the lower portion of the cloud or continues on to the ground.

The charge separation has an electric field associated with it. Like the cloud, this field is negative in the lower region and positive in the upper region. The strength or intensity of the electric field is directly related to the amount of charge build-up in the cloud. As the collisions and freezing continue to occur, and the charges at the top and bottom of the cloud increase, the electric field becomes more and more intense -- so intense, in fact, that the electrons at the Earth's surface are repelled deeper into the Earth by the negative charge at the lower portion of the cloud. This repulsion of electrons causes the Earth's surface to acquire a strong positive charge.

All that is needed now is a conductive path so the negative cloud bottom can conduct its electricity to the positive Earth surface. The strong electric field creates this path through the air, resulting in lightning. The lightning is a high-voltage, high-current surge of electrons, and the temperature at the core of a lightning bolt is incredibly hot. For example, when lightning strikes a sand dune, it can instantly melt the sand into glass. The combination of the rapid heating of the air by the lightning and the subsequent rapid cooling creates sound waves. These sound waves are what we call thunder. There can never be thunder without lightning.

Why do bridges ice before the rest of the highway?

On a cold, rainy day, ice forms more quickly on bridges and overpasses for two reasons:

• The freezing wind strikes the bridge above and below and on both sides, so it's losing heat from every side. The road is only losing heat from its surface. Even while the temperature on the road surface is dropping, the heat underneath the road keeps it warm enough to prevent icing as temperatures in the atmosphere drop below freezing. Bridges have no way to trap any heat, so they will continually lose heat and freeze shortly after temperatures in the atmosphere hit the freezing point.
• Most bridges today are built with steel and concrete, both of which are good heat conductors. Because these materials conduct heat, any heat that the bridge has moves through the bridge to the surface where the heat is lost through the air flow around it. Roads are mostly made from asphalt, which is a poor conductor of heat, and that lessens the rate of heat loss from the road.

The bottom line is that a bridge will follow the air temperature very closely. If the air temperature falls below freezing, a bridge's surface will fall below freezing very quickly. Rain or snow, therefore, will freeze and stick to the bridge.

The Oregon State Department of Transportation advises motorists to slow down when approaching bridges and overpasses while traveling in cold weather. Often times you can't see the ice on the road, and it may be hidden beneath a thin blanket of snow. If, while traveling over a bridge, your car feels like it's floating, you should gradually slow down to maintain control of the vehicle. Never slam on your brakes.

Why do they use salt to melt ice on the road in the winter?

If you live in a place that has lots of snow and ice in the winter, then you have probably seen the highway department spreading salt on the road to melt the ice. You may have also used salt on ice when making home-made ice cream. Salt lowers the freezing/melting point of water, so in both cases the idea is to take advantage of the lower melting point.

Ice forms when the temperature of water reaches 32 degrees Fahrenheit (0 degrees Celsius). When you add salt, that temperature drops: A 10-percent salt solution freezes at 20 F (-6 C), and a 20-percent solution freezes at 2 F (-16 C). On a roadway, this means that if you sprinkle salt on the ice, you can melt it. The salt dissolves into the liquid water in the ice and lowers its freezing point.

If you ever watch salt melting ice, you can see the dissolving process happen -- the ice immediately around the grain of salt melts, and the melting spreads out from that point. If the temperature of the roadway is lower than 15 F or so, then the salt really won't have any effect -- the solid salt cannot get into the structure of the solid water to start the dissolving process. In that case, spreading sand over the top of the ice to provide traction is a better option.

When you are making ice cream, the temperature around the ice cream mixture needs to be lower than 32 F if you want the mixture to freeze. Salt mixed with ice creates a brine that has a temperature lower than 32 F. When you add salt to the ice water, you lower the melting temperature of the ice down to 0 F or so. The brine is so cold that it easily freezes the ice cream mixture.

Why is it colder at the top of a mountain than it is at sea level?

You may already know about the relationship between temperature and pressure: When you pressurize air (or any gas), it gets hotter, and when you release the pressure on air it gets colder. So a bicycle pump gets hot when you pump up a tire, and a spray paint can or a C0₂refrigerator puts both of these processes together, pressurizing gas on the outside of the refrigerator to release heat and decompressing it inside the refrigerator to absorb heat (see How Refrigerators Work for details). cartridge gets cold as you release the pressurized gas. A

John Foxx/Getty Images Lower pressure at higher altitudes causes the temperature to be colder on top of a mountain than at sea level.

You may also know that air pressure decreases as altitude increases. This table shows the pressure (in pounds per square inch) at different altitudes:

Altitude	Air Pressure
Sea level	14.7 PSI
10,000 feet	10.2 PSI
20,000 feet	6.4 PSI
30,000 feet	4.3 PSI
40,000 feet	2.7 PSI
50,000 feet	1.6 PSI

As air rises, the pressure decreases. It is this lower pressure at higher altitudes that causes the temperature to be colder on top of a mountain than at sea level.

Why is snow white? What is snow white?

One of the reasons that so many people love snow is that it coats everything in a clean, "pure" white blanket. We even talk about snow in these terms -- weatherman say that we'll be getting "some of the white stuff" and every December you're likely to hear the song "White Christmas" over and over again. Snow wouldn't be snow if it wasn't white. But if you think about it, it seems weird that snow is white at all, since it's just a bunch of ice crystals stuck together. So where does it get its distinctive color?

Joseph Devenney/Photographer's Choice/Getty Images
Snow is a whole bunch of individual ice crystals
arranged together.

To understand where the whiteness comes from, we need to back up and look at why different things have different colors in the first place. Visible light is made up of many different frequencies of light. Our eyes detect different frequencies as different colors. Different objects have different colors because the particular particles (atoms and molecules) that make up the object have different vibration frequencies. Basically, the electrons of the particle will vibrate a certain amount in response to energy, depending on the frequency of the energy. In the case of light energy, the molecules and atoms absorb a certain amount of light energy depending on the frequency of the light, and then emit this absorbed energy as heat. This means that objects absorb certain frequencies of light more than others.

A couple of different things can happen to the light frequencies that are not absorbed. In some material, when a particle re-emits the photons, they continue to pass through to the next particle. In this case, light travels all the way through the material, so the material is clear. In most solid material, the particles re-emit most of the unabsorbed photons out of the material, so no light, or very little light, passes through and the object is opaque. The color of an opaque object is just the combination of the light energy that the object's particles did not absorb.

So, since snow is frozen water, and we all know that frozen water is clear, why does snow have a distinctive color? To understand this, we need to back up and look at an individual piece of ice. Ice is not transparent; it's actually translucent. This means that the light photons don't pass right through the material in a direct path -- the material's particles change the light's direction. This happens because the distances between some atoms in the ice's molecular structure are close to the height of light wavelengths, which means the light photons will interact with the structures. The result is that the light photon's path is altered and it exits the ice in a different direction than it entered the ice.

Snow is a whole bunch of individual ice crystals arranged together. When a light photon enters a layer of snow, it goes through an ice crystal on the top, which changes its direction slightly and sends it on to a new ice crystal, which does the same thing. Basically, all the crystals bounce the light all around so that it comes right back out of the snow pile. It does the same thing to all the different light frequencies, so all colors of light are bounced back out. The "color" of all the frequencies in the visible spectrum combined in equal measure is white, so this is the color we see in snow, while it's not the color we see in the individual ice crystals that form snow.

Can you calculate how far away lightning struck by how long it takes for the thunder to arrive?

Sound travels through air at "the speed of sound." Officially, the speed of sound is 331.3 meters per second (1,087 feet per second) in dry air at 0 degrees Celsius (32 degrees Fahrenheit). At a temperature like 28 degrees C (82 degrees F), the speed is 346 meters per second.

As you can see, the speed of sound changes depending on the temperature and the humidity; but if you want a round number, then something like 350 meters per second and 1,200 feet per second are reasonable numbers to use. So sound travels 1 kilometer in roughly 3 seconds and 1 mile in roughly 5 seconds.

When you see the flash of a lightning bolt, you can start counting seconds and then divide to see how far away the lightning struck. If it takes 10 seconds for the thunder to roll in, the lightning struck about 2 miles or 3 kilometers away.

What causes the seasons? How seasons are caused? What is season?

It turns out that the elliptical orbit of the Earth has little effect on the seasons. Instead, it is the 23.45-degree tilt of the planet's rotational axis that causes us to have winter and summer.

The diagram below demonstrates what happens.

In this diagram, you can see the axis of rotation and the equator. The Northern Hemisphere (at the top) is currently experiencing winter, and the Southern Hemisphere is experiencing summer. By looking at how sunlight is landing on the planet in the diagram, you can clearly see two things:

• The Southern Hemisphere is getting about three times as much sunlight as the Northern Hemisphere.
• The North Pole is getting zero sunlight, which is why it experiences 24 hours of darkness in January.

That huge difference in the amount of sunlight reaching the ground in the different hemispheres is what causes the seasons.

How could it rain for 45 straight days?

For three years, ending in the spring of 2007, the state of Texas experienced a terrible drought, the effects of which were felt throughout the state. In 2006, two million acres burned in wildfires. That same year, the drought caused $4.1 billion losses in state agriculture [Source: Time]. When a large storm system settled over the state in late May of 2007 and began to unleash rain, many Texans rejoiced that the drought was over. But then the rain continued. And continued. For 45 days, it didn't stop.

Weather Image Gallery

Image courtesy of Tony Gutierrez/Associated Press The torrential rainstorms in Texas made entire roads disappear under water.

The record rainfall created rampant flooding and devastation in many parts of the state. All of the state's water basins filled to capacity, and many rivers and reservoirs flooded. The governor declared disaster conditions in 37 counties and dispatched the Texas National Guard. President Bush declared 17 counties federal disaster areas and dispensed $3.8 million in federal aid to house, feed and clothe residents as they repaired their homes and recovered belongings. Texas launched the largest search-and-rescue operation in the state's history. Hundreds of vehicles, National Guard troops, helicopters, boats and professional search-and-rescue workers (some from other states) scoured for survivors. At least 13 people died -- most of them drowned.

Some people compared the damage to that from Hurricane Rita. Ninety mile-per-hour winds struck Seminole, Texas. In parts of the state, trucks and trains were turned over. Mobile homes, bridges, entire front yards and small shops washed away. Businesses lost their inventories, and city infrastructure suffered significant damage. Many communities were evacuated, while hundreds of homes were completely submerged in water. People took to the nearest high ground they could find, scrambling onto roofs, trees and the tops of cars or into boats.

For many parts of the state, it was the worst weather in half a century. In some areas, eight inches of rain fell in an hour [Source: NPR]. In Marble Falls, northwest of Austin, what's called a rain bomb fell, dropping an estimated 18 inches of rain in three hours. A creek that was normally a few feet across exploded into a 100-foot-wide raging river. Officials raised concerns about contaminated water as well as surly fire ants and snakes that were flushed out of their homes.

Despite the devastation, the drought problem was solved. Water rationing ended. Boats that had been moored on dry lake beds now floated on lakes filled to the brim. Nutrient-rich waters and blooming vegetation began to provide a hardy supply of plankton for re­covering fish populations. Troublesome algae that thrived in low-water conditions started to decline. ­Finally, ranchers and farmers rejoiced as their pastures soaked in much-needed water.

Video Gallery: Extreme Weather Watch this video to learn why New Orleans' levees failed during Hurricane Katrina and about some problems with the concept of levees. Ever wondered how meteorologists predict hurricanes? Find out in this video about hurricane forecasts from NASA Connect. Rescue teams often have to battle dangerous currents and rapidly changing conditions to save people from flood waters. Learn how rescue crews helped to save people from flooding in Texas.

After more than 40 days of rain, the scene seemed like something out of a disaster movie. What could cause such terrible storms to persist for more than a month and a half? Is this an isolated event or a sign of things to come? We'll consider these questions and look at other recent extreme weather events

45 Days of Rain and Other Severe Weather in 2007

Several weather patterns converged to create the devastating rainstorms in Texas. A low-pressure system came from the Caribbean and absorbed warm water from the Gulf of Mexico. That then combined with an "upper-level trough" and another weather front. This moisture-filled system stayed in place and refused to budge, especially over south and west Texas. Stuck in place, the weather system dropped its moisture on the state in a deluge of rain.

While the Texas situation was dramatic and parts of the state experienced record rainfall, the event was not unique. Tropical weather patterns often cause massive rain storms, especially in parts of the world that go through monsoon seasons. The rain bomb that dropped on Marble Falls produced an estimated 18 inches of rain, but even that amount is far from a record. Dr. Steve Lyons, a weather consultant at the Weather Channel, blogged about the theoretical maximum rainfall possible for an area, producing the following numbers [Source: Weather Channel]

• 2.4 inches in one minute
• 16.6 inches in one hour
• 28 inches in three hours
• 54 inches in 12 hours
• 75 inches in 24 hours

These are theoretical maximums, but Dr. Lyons' point is that the Texas rainstorms must be considered in perspective and that even the Marble Falls rain bomb is not a singular event.

Image courtesy Tony Gutierrez/Associated%20Press">Associated Press Floodwaters caused significant devastation across Texas. Much of the town of Gainesville was under water.

Drought conditions have affected large parts of the United States in 2007, and average temperatures increased nearly everywhere in the U.S. from 1976 to 2005 [Source: Climate Prediction Center]. But it's interesting to note that average rainfall actually increased around much of the country during that same 30-year period -- by more than 1.5 inches in the areas experiencing the greatest increases.

Whether climate change is responsible for this event is hard to say, but global warming is believed to be linked to an increase in extreme weather events such as drought and massive storms. The warming of the waters of the Gulf of Mexico is likely a factor. A study published by Indian scientists in December 2006 offers a clue. In that study, researchers found an increase in so-called extreme rain events in India during monsoon season from 1952 to 2000. However, the number of "moderate" rain events decreased, meaning that, overall, the average rainfall did not change significantly. Their research linked the increase in extreme rain events to "rising global surface temperatures" [Source: Science magazine].

In case you were wondering, the wettest place on Earth is under dispute, but several areas, such as the Indian village of Cherapunjee, can receive two months or more of nonstop rain. Some of Hawaii's volcanoes rarely emerge from rain clouds. Lloro, Colombia, has the highest annual rainfall, averaging 523.6 inches a year. In comparison, the wettest area of Texas, the Upper Coast region, averages about 46 inches of rain a year.

Miguel Angelo Silva | Dreamstime Climate change will likely cause lake beds to dry up one year and refill the next.

In addition to Texas, other areas of the U.S. and the world experienced severe weather in the first seven months of 2007. Floods coursed through OklahomaKansas. Drought conditions caused fires throughout the country, including in Alaska, which is warming more quickly than anywhere on Earth [Source: The Independent]. A massive heat wave in the western U.S. contributed to the driest year in Los Angeles' history and to a fire in Lake Tahoe that burned 250 homes. A cyclone in Pakistan killed hundreds and left hundreds of thousands homeless. and

The consequence of this severe weather may be that Americans are becoming more aware of the climate change problem. In late June 2007, the Pew Center in Washington, D.C., published a survey showing that the percentage of Americans who think that climate change represents a "major global threat" is 37 percent, up from 23 percent in an earlier poll [Source: The Independent].

How do trees affect the weather? Whether trees affect weather?

Trees make o­ur lives more pleasant. They're beautiful to look at, and they can provide sh­ade or a good hideout. Forests cover 30 percent of our planet, concentrated mostly in 10 countries: the United States, Canada, the Russian Federation, Brazil, China, Australia, the Democratic Republic of Congo, Indonesia, Peru and India [source: United Nations Environment Programme].

Marcel Mochet/AFP/Getty Images
Forests, like this one in France, cover 30 percent of the Earth.

That may sound like a lot of forest, but worldwide, our forests are disappearing. About 13 million hectares of forest -- an area roughly the size of Greece -- disappear every year. Deforestation happens because of population growth and increasing agricultural and industrial demands; trees are cut down to make room for new developments.

Why does it matter if trees are planted or cut down? Trees provide us with wood, fuel, food, medicine, latex and other products used in our daily lives. They also affect our climate. No, they can't make snow on a sunny summer day, but their very existence -- or removal -- makes a difference.

Climate isn't the same as weather. While weather is short-term, like the five-day forecast on the evening news, climate is the weather pattern over a long period of time, usually 30 years. And the climate of our planet is warming -- heating up by approximately 1 to 1 1/2 degrees in the past century [source: The Weather Channel].

In the next section, let's look at the ways trees positively and negatively affect our climate, and, in turn, our weather.

Trees and our Climate

Trees affect our climate, and therefore our weather, in three primary ways: they lower temperatures, reduce energy usage and reduce or remove air pollutants. Each part of the tree contributes to climate control, from leaves to roots.

Leaves help turn down the thermostat. They cool the air through a process called evapotranspiration. Evapotranspiration is the combination of two simultaneous processes: evaporation and transpiration, both of which release moisture into the air. During evaporation, water is converted from liquid to vapor and evaporates from soil, lakes, rivers and even pavement. During transpiration, water that was drawn up through the soil by the roots evaporates from the leaves. It may seem like an invisible process to our eyes, but a large oak tree is capable of transpiring 40,000 gallons of water into the atmosphere during one year [source: USGS].

Evapotranspiration.

The outdoor air conditioning provided by trees reduces the energy used inside your home or office. Shade provided by strategically planted deciduous trees cools buildings during the warm months, allows the sun's warming rays to shine through its branches in the winter and also protects buildings from cold winds. With some planning, urban trees can help minimize the heat island effect that saddles many cities.

Heat islands are cities that are often several degrees warmer than the suburbs because the urban areas generate and trap heat. Studies of Atlanta found that temperatures downtown were 5 to 8 degrees hotter than those in the suburbs. This, in turn, increased the number of local storms [source: NASA]. Phoenix is also warmer than its outlying areas. In 1950, Phoenix was 6 degrees warmer than the nearby Casa Grande Monument. By 2007, however, the temperature difference increased to 14 degrees [source: Christian Science Monitor].

When trees grow throughout urban areas, both surface and air temperatures are reduced. Researchers have found that planting one tree to the west and one to the south of a home can significantly reduce energy consumption. In the Environmental Protection Agency's study, annual cooling costs were reduced by 8 to 18 percent while annual heating costs were reduced 2 to 8 percent [source: EPA].

Leaves also filter particles from the air, including dust, ozone, carbon monoxide and other air pollutants. Through the process of photosynthesis, trees remove carbon dioxide (a greenhouse gas) and release oxygen into our air. Trees store the carbon dioxide, called carbon sequestration, and -- depending on the size of the tree -- can hold between 35 to 800 pounds of carbon dioxide each year [source: EPA].

Trees aren't our saviors from smog, though. Photochemical smog is smog caused when sunlight and chemical compounds such as car exhaust combine. Trees contribute to this when they release organic gases.

Additionally, planting trees as a solution to global warming -- a practice commonly linked to carbon offsets -- may have a positive impact on global temperature control only when planted in the tropics, a thin geographical belt around the equator. Normally, trees help cool the planet by absorbing carbon dioxide as part of the photosynthesis process and by evaporating water into the air. In the tropics, water evaporates naturally from trees, increasing cloud cover and keeping temperatures cooler. Outside of the tropics, however, researchers are finding that forests trap heat because their dense, dark canopies absorb sunlight.

How Meteorology Works? What is Meteorology? How Meteorology functions?

Imagine for a sec­ond you weren't a human reading an Internet article and instead were a flea -- and not just any flea, but a flea living its entire life in the fur of a grizzly bear. Given such a hostile environment, y­ou'd need to nail down a few facts to stay alive. Where does the bear scratch or bite most often? When does it sleep? Where's the best blood? How ferociously has it gone after its parasites in the past, and how might it behave in the future? If this sounds like a stressful existence, just consider the bear of an environment we live in every day.­

­Human beings make their home in the outermost region of Earth's crust, a 5.5-quadrillion-ton (4.99-quadrillion-metric-ton) layer of gases we call the atmosphere. As with our exemplary flea, our lives depend on exactly how the environment around us behaves. In its tamer moments, the weather nurtures our crops, gives us clean air to breathe and provides generally comfortable surroundings. But at its worst, the atmosphere, which is covered in depth in How Weather Works, delivers lethal frosts, searing heats, destructive floods and storms capable of laying whole cities to waste. Like any survival-savvy parasite, it pays to know what your host is liable to do next.

If you've ever rushed off to school or work in the morning without first checking the weather forecast, you probably didn't experience the equivalent of being raked with a bear claw. Maybe you slipped in the snow or the commute was a bit sluggish, but oh well, right? At the end of the day, you're probably still going to eat a hot dinner in a warm home.

Our lives haven't always been so weatherproof, though. In developing nations and harsher environments, daily survival still depends on Mother Nature. Despite modern safeguards against drought and flood, the weather can still hurt food and water supplies even in the most advanced nations. But in the ancient world, the ability to use past and present observations to predict weather patterns made you more than a weatherman -- it made you a shaman, a prophet and even a leader. Today, meteorologists augment observations with modern technology, physics and statistics to perform a no-less vital service, but we don't necessarily regard them as prophets.

In this article, we'll examine meteorology, the scientific study of the atmosphere and the myriad phenomena that keep it constantly swirling and raging all around us.

Weather of the Past: Relying on Animals, Plants and Local Lore

Pattern recognition is one of the key properties of intelligence. Your dog knows she'll get a treat if she does a trick because you always feed her after she rolls over. You know your aunt will probably give you pajamas for Christmas because your closet's stuffed with hideous, plaid pajamas. Our minds learn and act based on the patterns we perceive all around us. By knowing what came before, we know what the future will hold.

Burazin/Photographer's Choice/Getty Images
Early humans observed local animals and vegetation to help them determine the weather. Who needs Al Roker when you have migratory birds?

­Early humans observed patterns in the weather and learned to anticipate changes that affected their food supply and well-being. They created names for the seasons and even forms of calendars to guide their wanderings and, later on, the growing cycles of their crops. People knew what the weather would be based on their own cumulative experiences, as well as those passed onto them by their ancestors. For instance, certain Australian Aboriginal tribes carry with them 18,000 generations worth of local weather observations [source: BBC]. Outside of their territory, their knowledge gradually fizzles out, but their understanding of weather incorporates a great deal of local detail. A tribe may recognize as few as two or as many as six seasons, depending on local precipitation and temperature factors.

Early humans knew that cooling temperatures meant the coming of winter. They knew the sights and smells that preceded a rainstorm. And where their own senses failed them, they turned to those in nature: the life cycles of various vegetation and the migration of other animals. Plus, many animal species are far more attuned to changes in air and water pressure that often signal storms and other atmospheric changes.

Long before humans invented devices to measure these conditions, they simply looked to the skies and fields. Animals recognized subtle patterns in the atmosphere, and we recognized patterns in their responsive behavior. These traditions continue to this day in the pages of The Old Farmer's Almanac and with practices such as Groundhog Day. To learn more about animals and weather, read Can animals predict the weather?

Following the weather in a region or tribal territory is one thing, but charting atmospheric patterns on a global scale is a different undertaking altogether. Over the last few centuries, synoptic meteorology, or the idea of comprehensively charting the weather over a large area, has emerged. By comparing simultaneous weather conditions in adjacent areas, scientists were able to better understand prevailing conditions over a larger area and to provide the kinds of weather maps we view on TV and the Internet every day.

­How do meteorologists record current weather conditions?

Weather of the Present: Barometers, Thermometers and Hygrometers

You've probably heard the saying "too many cooks spoil the broth," most likely in reference to an album, business venture or even a sports team. It means that when too many people work on a project, the results are often confusing, lackluster or just plain awful. In a way, the atmosphere is a broth stirred by way too many cooks: gravity, sunlight, rotation, conflicting pressure zones, cool oceans, hot deserts, mountain ranges and colossal air currents, just to name a few. These forces constantly stir the atmosphere to movement, and just figuring out what it's doing at any given second requires a great deal of study and observation.

Bennett Barthelemy/Aurora/Getty Images
A scientist checks local conditions at a weather station in Arizona.

Three of the key atmospheric properties are air pressure, air temperature and humidity. To really understand what's going on, you have to be able to measure these conditions. For this reason, meteorology didn't really emerge as a science until the 17th-century inventions of the barometer, which measures air pressure, and a reliable thermometer, which gauges temperature. Before the 1600s ended, scientists also developed reliable hygrometers to measure humidity. These instruments, along with rain gauges, allowed for improved agricultural planning and sea travel.

But to gain a truly synoptic view of current weather conditions, you need a way to communicate with other observers in other regions. The 1837 invention of the telegraph made this possible. By the mid-19th century, meteorologists at various weather stations were able to communicate quickly with each other and assemble the big picture.

Toward the end of the 19th century, meteorologists used weather balloons to study the upper atmosphere. In doing so, they made crucial discoveries about upper-air pressure and wind patterns. Through this, meteorologists were able to discover the role low-pressure centers play in the weather. You've probably seen these centers pointed out on a map in weather forecasts. Cooler, denser air spirals into warmer, low-pressure areas from the surrounding regions. This, in turn, pushes the warm air up into the upper atmosphere, where the air spirals out in all directions. These formations are called cyclones (not to be confused with hurricanes and typhoons, which are called cyclones in some regions).

But this elevation of air doesn't just happen at low-pressure centers. It also happens when two air masses meet each other in a front. In either case, elevated air often forms clouds and storm sy­stems. With these discoveries, meteorologists were better prepared to forecast weather. They weren't just making educated guesses based simply on pattern recognition; they were understanding how the atmosphere works.

In the 20th century, advancements in aviation made it possible for us to better study the upper atmosphere, and new radio technology allowed meteorologists to send sensitive equipment up with balloons to high altitudes -- a practice that continues today. Similarly, radio weather buoys communicated back conditions at sea, including water temperature, wind speed and wave height. Following World War II, scientists began using radar to study the weather since the technology made it possible to detect rainfall in addition to aircraft.

In 1960, another advancement added to our ability to observe and measure Earth's atmosphere: the weather satellite. By placing these automated observatories in north-to-south polar orbitsgeostationary orbits, humans were able to view their atmosphere from the outside and observe storm systems from a truly synoptic vantage point. Weather satellites provide more than just an extraterrestrial view of weather; they also carry sensors to measure temperature, moisture and solar radiation. and east-to-west

­It's one thing to know what's happening now, but how do meteorologists transform all this data into an idea of what tomorrow's weather will bring?

Weather of the Future: Numerical Weather Prediction Models

Modern technology allows meteorologists an unprecedented understanding of Earth'sweather. But how do meteorologists translate this into a reasonable prediction of what the weather will continue to do? atmosphere and an excellent vantage point from which to observe its

Joe Raedle/Getty Images News/­Getty Images
A meteorologist at the National Hurricane Tracking Center in Miami studies the movements of a powerful storm system in an effort to forecast its movements.

­Instead of simply looking at current conditions and estimating based on past observances, meteorologists create numerical weather prediction (NWP) models. These models are objective, physics-based calculations which, when processed through a computer, predict what current weather will look like in the future. The equations involved in these models are complex and involve multiple atmospheric variables. These variables leave room for error, so the further out meteorologists attempt to forecast, the greater the potential for error becomes.

Look at any hour-by-hour weather forecast: Each hour's forecast is a step into the possible future. An initial forecast (such as what the weather will be an hour from now) is the result of running a computer model based on what the weather is doing now. Then, to run a model of what the weather will be doing in two hours, you apply the various equations to the model that came before it. So while the first forecast was based on actual conditions, the second was based on forecasted conditions that might have been less than accurate. Each subsequent forecast compounds the possibility for error. For this reason, NWP models become increasingly flawed the further ahead you try to look.

Meteorologists have steadily improved NWP models since the 1980s. By constantly tweaking them, they've created more accurate equations with fewer errors. Another technique, called Model Output Statistics, improves weather forecasts by taking the NWP model, which is based on current conditions, and extrapolating it by comparing it to past surface conditions for a particular region. This method essentially uses past weather readings to balance some of the errors inherent in an NWP model.

Weathermen vs. Meteorologists What's the difference between a TV weather anchor and a meteorologist? While a meteorologist is a scientist who has earned a degree in meteorology or atmospheric sciences, the TV weather anchors you see daily aren't necessarily scientists. In fact, neither Willard Scott nor Al Roker is a meteorologist, despite making their names in front of a weather map.

­Despite continued advancements in meteorology, don't expect a flawless weather forecast anytime soon. When considering the numerous variables in an NWP model, it's important to realize just how much difference even a small discrepancy can make. In 1961, meteorologist and chaos theory founder Edward N. Lorenz took a close look at the drastically different models a difference of a single decimal point could make. Based on these findings, Lorenz coined the term butterfly effect, in which the question is asked, "Does the flap of a butterfly's wings in Brazil set off a tornado in Texas?"

But while forecasting the weather is far from flawless, meteorology has saved countless lives by allowing scientists to predict where destructive weather will strike and warning people ahead of time. Your five-day forecast may not be perfect, but neither is our understanding of the complex array of movements that fill the atmosphere we live in.

What is Rainbow? How Rainbows Work? How Rainbows forms? How Rainbows Functions?

Rainbows are one of the most beautiful spectacles nature has to offer -- so beautiful, in fact, that they've inspired countless fairy tales, songs and legends. It's a good bet that most of the artists behind these tales were totally mystified by the rainbow phenomenon -- just like most people are today.

But the science of rainbows is really very simple. It's just basic optics! In this article, we'll find out how rain and the sun align to put color in the sky.

Light Bends

The fundamental process at work in a rainbow is refraction -- the "bending" of light. Light bends -- or more accurately, changes directions -- when it travels from one medium to another. This happens because light travels at different speeds in different mediums.

Photo courtesy National Oceanic and Atmospheric Association

To understand why light bends, imagine you're pushing a shopping cart across a parking lot. The parking lot is one "medium" for the shopping cart. If you're exerting a constant force, the cart's speed depends on the medium it's traveling through -- in this case, the parking lot's paved surface. What happens when you push the shopping cart out of the parking lot, onto a grassy area? The grass is a different "medium" for the shopping cart. If you push the cart straight onto the grass, the cart will simply slow down. The grass medium offers more resistance, so it takes more energy to move the shopping cart.

But when you push the cart onto the grass at an angle, something else happens. If the right wheel hits the grass first, the right wheel will slow down while the left wheel is still on the pavement. Because the left wheel is briefly moving more quickly than the right wheel, the shopping cart will turn to the right as it moves onto the grass. If you move at an angle from a grassy area to a paved area, one wheel will speed up before the other and the cart will turn.

Similarly, a beam of light turns when it enters a glass prism. This is a simplification, but think about it this way: One side of the light wave slows down before the other, so the beam turns at the boundary between the air and the glass (some of the light actually reflects off the prism surface, but most passes through). The light turns again when it exits the prism, because one side of it speeds up before the other.

In addition to bending light as a whole, a prism separates white light into its component colors. Different colors of light have different frequencies, which causes them to travel at different speeds when they move through matter.

A color that travels more slowly in glass will bend more sharply when it passes from air to glass, because the speed difference is more severe. A color that moves more quickly in glass won't slow down as much, so it will bend less sharply. In this way, the colors that make up white light are separated according to frequency when they pass through glass. If the glass bends the light twice, as in a prism, you can see the separated colors more easily. This is called dispersion.

A prism separates white light into its component colors. For simplicity's sake, this diagram shows only red and violet, which are on opposite ends of the spectrum.

Drops of rainwater can refract and disperse light in the same basic way as a prism. In the right conditions, this refraction forms rainbows. In the next section, we'll find out how this happens.

Making a Rainbow

An individual raindrop has a different shape and consistency than a glass prism, but it affects light in a similar way. When white sunlight hits a collection of raindrops at a fairly low angle, you can see the component colors red, orange, yellow, green, blue, indigo and violet -- a rainbow. For simplicity's sake, we'll only look at red and violet, the colors of light on the ends of the visible light spectrum.

The diagram below shows what happens when the sunlight hits one individual raindrop.

When the white light passes from air into the drop of water, the component colors of light slow down to different speeds depending on their frequency. The violet light bends at a relatively sharp angle when it enters the raindrop. At the right-hand side of the drop, some of the light passes back out into the air, and the rest is reflected backward. Some of the reflected light passes out of the left side of the drop, bending as it moves into the air again.

In this way, each individual raindrop disperses white sunlight into its component colors. So why do we see wide bands of color, as if different rainy areas were dispersing a different single color? Because we only see one color from each raindrop. You can see how this works in the diagram below.

When raindrop A disperses light, only the red light exits at the correct angle to travel to the observer's eyes. The other colored beams exit at a lower angle, so the observer doesn't see them. The sunlight will hit all the surrounding raindrops in the same way, so they will all bounce red light onto the observer.

Raindrop B is much lower in the sky, so it doesn't bounce red light to the observer. At its height, the violet light exits at the correct angle to travel to the observer's eye. All the drops surrounding raindrop B bounce light in the same way. The raindrops in between A and B all bounce different colors of light to the observer, so the observer sees the full color spectrum. If you were up above the rain, you would see the rainbow as a full circle, because the light would bounce back from all around you. On the ground, we see the arc of the rainbow that is visible above the horizon.

Sometimes you see a double rainbow -- a sharp rainbow with a fainter rainbow on top of it. The fainter rainbow is produced in the same way as the sharper rainbow, but instead of the light reflecting once inside the raindrop, it's reflected twice. As a result of this double reflection, the light exits the raindrop at a different angle, so we see it higher up. If you look carefully, you'll see that the colors in the second rainbow are in the reverse order of the primary rainbow.

And that's really all there is to rainbows. Light and water happen to combine in just the right way to paint a beautiful natural picture.