How Weather Works

When it comes to discussing the weather, most people can hold up their end of the conversation. After all, who doesn't have an anecdote about how a rainy day disrupted their plans? Weather is all around us, affecting every aspect of our lives. It's no wonder discussion of it fills our awkward pauses and doomed first dates.

This view of the weather, as something that happens around our lives -- is the first concept you have to abandon to gain a clear understanding of how Earth's atmosphere works. Forget that cloudy days make you sad or that you hate shoveling snow. Even put aside the idea of weather as something that happens to a city or region. The weather is simply the state of the atmosphere, the gaseous layer that serves as the outermost barrier between Earth and the rest of the universe.

While water covers 71 percent of Earths' surface, the atmosphere envelopes all of it. But this layer of gases doesn't just sit there, it's subject to influence from a host of terrestrial and extraterrestrial forces. Think of the atmosphere as a lucky man or woman who has just won the lottery. Suddenly, everyone seems to have a few suggestions on how he or she should spend his or her time and money. Uncle Joe says one thing, Aunt Clara another. Before you know it, everyone seems to have some sort of input into the winner's daily life.

For Earth's atmosphere, gravity, sunlight, oceans and topography all dictate certain cycles of air movement -- some very localized, others concerning vast portions of the planet. In addition, a number of these various cycles affect each other, spinning off new cycles and brewing clouds, precipitation and an unending torrent of storms. All of these various atmospheric responses are what we know as the weather.

With all of these various influences, Earth's atmosphere is quite an intricate system. No wonder it's so hard to predict the weather. In this article, we'll unravel that intricate system, starting from space with the big picture and moving steadily back down to the level you experience every day.

Evolution of the Atmosphere

Go back about 4.6 billion years and you wouldn't find the Earth. You'd find molecules and particles slowly forming a gaseous mass inside a nebula. Over time, these gases eventually condensed into liquid and solid forms. Some of it cooled to form the continents and oceans, but much of Earth's center still burns with furious heat. The atmosphere sits on the surface of this sphere.

Scientists think Earth's original atmosphere escaped from within the planet, where it formed in the heat of radioactive decay. By today's standards, this air was utterly unbreathable; rich in methane, ammonia, water vapor and neon. There was no free oxygen (O₂) at all. You might think this had to change before organisms could evolve on the planet, but it was actually the steady evolution of unicellular organisms that produced oxygen and brought about the change in the atmosphere's makeup. Over hundreds of millions of years, this evolved into the air that fills your lungs today.

Currently, the atmosphere is composed of 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon and 0.03 percent carbon dioxide. The remaining 0.07 percent consists of water vapor, hydrogen, ozone, neon, helium, krypton and xenon [source: Vogt]. Is this the finished recipe for Earth's atmosphere? Probably not, considering that the process of evolution that created it continues to this day. Plus, there's another agent of change to consider: human beings.

While some date human influence on global climate back to the industrial revolution of the 1800s, others look back several thousand years to the agricultural revolution. Environmental scientists such as William F. Ruddiman argue that carbon dioxide concentrations began to rise 8,000 years ago due to early slash-and-burn agriculture practices in Asia, India and Europe. To learn more about humanity's role in climate change, read How Global Warming Works.

So we've covered how the atmosphere developed and what it's made of, but we're still looking at the Earth from the outside. In the next section, we'll move in a little closer and explore the major physical properties at work in it.

Under (Atmospheric) Pressure

We've discussed the origins and chemical composition of the air we breathe, so it's time to go ahead and actually enter the Earth's atmosphere. As we slowly slide toward that sphere of swirling clouds, passing the occasional satellite, the obvious question is, "Where does outer space stop and the atmosphere begin?" There's no set boundary between atmosphere and space -- the thin air in the upper atmosphere just eventually thins to nothing at approximately 600 miles (1,000 km) above sea level.

This entire atmosphere sits on Earth's surface, held in place -- like everything else on the planet -- by gravity. Despite the phrase "light as air," the atmosphere is anything but, weighing in at a whopping 5.5 quadrillion tons (4.99 quadrillion metric tons). With 14 zeros trailing after it, that's a lot of mass, and it's the driving force behind air pressure.

Imagine a squad of cheerleaders forming a human pyramid. The girls on the bottom row have to bear the weight of all the other girls above them, while the girl on the top doesn't have to bear any of the weight at all. A similar situation exists in the atmosphere. The air is least pressurized at the edge of space, where there's little or nothing pressing down on it. The air at sea level, however, is weighed down by all the air on top of it -- like those poor girls shoring up the pyramid. The pressure also presses the molecules in the lower atmosphere closer together. This means that the higher the air pressure, the greater the air density. For this reason, 50 percent of Earth's air exists below an altitude of 3 miles (5 km).

Standing at sea level, the atmosphere exerts, on average, a pressure of 14.7 pounds (6.7 kg) against every square inch (2.5 cm) of your skin [source: Vogt]. If you venture above sea level, air pressure and its corresponding density will decrease. This is why it's more difficult to breathe at higher altitudes. The molecules of oxygen your lungs require are spaced farther apart, so you have to inhale more air to get what you need.

Gravity is just one force at work on the atmosphere. The primary mover and shaker is none other than the fiery ball of gas at the center of our solar system.

Can You Feel the Heat? Solar and Terrestrial Radiation

The sun emits a vast amount of energy, which travels across space in the form of short-wave radiation. Only a tiny portion of this power actually reaches the surface. But most of the atmosphere isn't directly heated by solar radiation, but rather by the terrestrial radiation that the planet itself emits.

Ever see a video of someone frying an egg on the pavement on a hot day? It's the heat emitted by the pavement that's doing the frying, not the sun, despite the fact that the pavement was heated by the sun to begin with. Earth's surface absorbs solar radiation and emits terrestrial radiation.

Why does the air absorb this home-brewed radiation on the rebound instead of the fresh solar energy? Well, a solar-charged Earth emits long-wave radiation. While water vapor and carbon dioxide molecules merely allow the passage of incoming short waves, they absorb Earth's long waves, heating the atmosphere from the ground up. This is why a mountain climber will encounter increasingly colder conditions as he or she ascends, despite effectively moving closer to the sun.

Scientists divide the atmosphere into four layers based on temperature.

Troposphere: With the exception of satellites and some aircraft, our entire world resides within this bottom layer. Even the tallest mountains don't scrape its upper boundary, called the tropopause, at roughly 7 miles (11 km) above sea level (the thickness of the troposphere varies with latitude and season). At this point, the steady drop in temperature that occurs as elevation increases stops. The troposphere contains all our weather and 80 percent of the planet's air mass. Remember, the lower the altitude, the higher the air pressure. Even though this layer isn't as thick as higher altitude layers, the molecules are more tightly packed.

Stratosphere: This layer extends another 23 miles (37 km) into the sky, terminating 30 miles (48 km) above the planet's surface at the stratopause. If you ascended through the atmosphere, the steady decrease in temperature you experienced throughout the troposphere would halt at the tropopause and remain constant for the first 12 miles (20 km) of the stratosphere. At this point, the temperature would begin to climb again, thanks to the ozone, which absorbs ultraviolet radiation from the sun. The temperature would keep rising until you reached the stratopause.

Mesosphere: Above the stratopause, the atmosphere's third layer begins to gradually get colder as you get closer to the mesopause, located more than 50 miles (80 km) above Earth's surface. The atmosphere's coldest temperatures occur here, dipping down as low as -130 degrees Fahrenheit (-90 degrees Celsius) [source: Tarbuck and Lutgens].

Thermosphere: The final layer of the Earth's atmosphere extends from the mesopause up to the very edge of space. The air molecules in this low-density layer are literally few and far between. As the molecules have less mass, they absorb solar radiation much faster. Temperatures in the thermosphere can reach higher than 3,100 degrees Fahrenheit (1,700 degrees Celsius). It wouldn't feel as hot, however, due to the low density. Think of a 3,100-degree nitrogen molecule as a smelly dog. If you were surrounded by a dozen of them spread out over a football field, you'd hardly notice the stink. But pack yourself into a broom closet with them, and you'd soon be gasping for breath.

Now let's examine how the forces behind air temperature and air pressure affect the weather.

A Recipe for Wind

Two key properties govern the atmosphere: air pressure, dictated by gravity, and air temperature, dictated by solar and terrestrial radiation. But all these gases making up the atmosphere don't just stay in one place. As you've certainly observed, air moves. The troposphere, the region of the atmosphere we experience every day, is constantly churning with cycles of vertical and horizontal movement.

Vertical air currents result from changes in temperature and pressure. When air heats up, its molecules move around more rapidly, pushing each other farther apart. The air becomes less dense and rises up through the troposphere toward thinner air. In doing so, however, it moves into colder regions and begins to cool. It eventually cools back to a denser state and sinks back down. This is why the troposphere is thickest in hot, tropic regions and narrowest near the icy poles.

If the air were all the same temperature and the entire atmosphere experienced the exact same heating and cooling, the troposphere would simply swell during the day and compress back down at night. But in reality, different temperatures persist across the globe, mainly because the sun doesn't provide the same heat to every part of the planet, nor does it shine everywhere at once. While it's daytime on one side of the world, it's nighttime on the other. While one city receives sun filtered down vertically through one atmosphere's worth of air, sunlight travels to other areas at a more horizontal trajectory. In these cases, the solar radiation is forced to filter through the equivalent of several atmospheres. This is why the sun appears far less bright at sunset than at high noon.

Temperature also varies from place to place due to the unequal cooling and heating of land and water. Under a blistering, noonday sun, which is hotter: the water in a swimming pool or the cement patio surrounding it? As your feet can attest, the cement is much hotter, which means it's absorbing more heat. This also means it's reflecting more heat back into the air above it. Now imagine this on a scale of oceans and continents. Altitude, geographic location, cloud cover and ocean currents also affect temperatures around the world.

When the air in one area heats up faster than the air in an adjoining area, the pressure differential generates wind. For a simple example of this, look no farther than a large modern city. All that concrete and steel absorbs much more heat than the surrounding countryside. As such, the air in the city grows hotter during the day, becomes less dense and rises in a vertical movement known as an updraft. Meanwhile, the cooler air in the countryside is under far more pressure and begins to flow into the city in the form of surface wind to fill the low-pressure area. Once it enters the hot city, however, it too heats up and begins to rise in an updraft. The air above it cools, but can't settle back into place due to all the rising hot air underneath it. Instead, the cooling air simply pushes out to the sides in the form of upper air wind heading back to the countryside. This wind cycle continues until nightfall sends everything into reverse, as the city cools faster than surrounding areas.

This, however, is just a localized example of the basic principles at work. On the next page, we'll examine how a similar cycle of airflow applies to the entire planet.

A World of Wind

The cycle of rising and falling air demonstrated in last section's city example illustrates a basic convection cell. Convection occurs when mass movement or circulation transfers heat through a substance. A product of changing temperature and pressure, this process is one of the central components of global weather.

Imagine an Earth that doesn't rotate and doesn't experience night. In this example, let's also pretend the sun still heats the areas around the equator the most and the poles the least. This is a lot like our city example, except the entire equatorial belt would be the "city" in this scenario, and the land and sea cooling toward the poles would be the "countryside." This would result in two massive, bowl-shaped convection cells, one for each hemisphere. Surface flows of cool air would sweep toward the equator heating up along the way. Upon arrival, this air would ascend in an updraft. Then it would sweep back toward the poles in a cooling upper air wind.

But of course, our planet does rotate, and when we apply rotation to the hypothetical two-cell model of the world, things get complicated quickly. Besides altering periods of night and day heating and cooling the Earth, you also have three other key factors at work in global atmospheric circulation:

Pressure-gradient force: While the equator and the poles represent major areas of air pressure differences, the planet is covered with areas of high and low pressure. These natural gradients generate additional wind, as high-pressure air flows into low-pressure areas. Meteorologists log these differences by drawing lines called isobars on charts to connect areas of equal air pressure. These typically appear as swirling layers and concentric circles around key high- and low-pressure areas. Again, this is the same principle we explored in the city example -- only imagine low- and high-pressure systems dotted throughout any given hemisphere. We call these low-pressure centers cyclones (not to be confused with hurricanes). These rotate in the familiar vortex pattern seen in hurricanes, where high-pressure winds spiral into the low-pressure center and then ascend in an updraft. We call the high-pressure centers anticyclones and, as the name implies, they're the opposite of a cyclone. High-pressure air descends in a downdraft and then spirals out along the surface into lower-pressure areas.

The Coriolis force: All free-moving objects and fluids on Earth are subject to this force. In the Northern Hemisphere, winds are deflected to the right. In the Southern Hemisphere, they're deflected to the left. This force is weakest at the poles and strongest near the equator. How does this affect our model of a nonrotating Earth? It means the wind doesn't merely blow north and south from high to low pressure. Instead, the Coriolis effect forces these airflows to take an easterly or westerly direction. This breaks the hemispheric convection cells into three distinct types of cells: two Hadley cells, two Ferrel cells and two Polar cells. Hadley and Ferrel cells are named for the meteorologists who discovered them.

Friction with Earth's surface: Wherever surface winds meet the Earth, there's the potential for friction, which slows and redirects the flow of air. Upper air winds, however, don't encounter this resistance and travel at much higher speeds as a result. This is especially evident in the jet streams, great snaking rivers of fast-moving air that exist at between 20,000 and 45,000 feet (6 and 14 km) and travel at speeds as fast as 200 miles per hour (322 kph).

These three forces dictate the power and direction of Earth's winds. But there are still localized conditions to consider wherever high- and low-pressure areas meet. These can include coastlines, mountains, valleys and areas near volcanic activity.

The Cycle of Rain

Water plays a major role in weather, despite making up such a small fraction of the atmosphere. In some areas, the local atmosphere may contain as much as 4 percent water, while other regions have no atmospheric water at all. As water can exist as a solid, liquid or gas under normal atmospheric conditions, it participates in the hydrologic cycle. In this cycle, water evaporates from the ocean in the form of water vapor and eventually returns to land and sea in the form of precipitation.

You can't see water vapor, but it quickly becomes visible when it cools and condenses against something. If you've ever noticed moisture beads on the windows of a warm car on a cold day, you've seen condensation in action. Warm air vapor touches the cold window and the vapor turns back to a liquid. Clouds form along similar lines. The atmosphere is full of tiny dust particles called condensation nuclei, which come from volcanic eruptions, dust storms, fires and pollution. When water vapor condenses, it clings to these microscopic specks. If there's enough cooling water vapor in the air, these accumulate by the trillions to form clouds. If temperatures are cold enough, the water turns to ice around the condensation nuclei. For a more in-depth look at clouds, read How Clouds Work.

In a windless world, these water droplets would descend right back down to the surface, but Earth's complex upper air winds keep the clouds afloat, moving them across vast distances and altering their shape in the process. If too much water condenses around a particle or if the air temperature drops, the water will fall back to the surface. Liquid particles fall in the form of rain, while frozen particles fall as snow. If the rain freezes as it falls, it becomes freezing rain. In some cases, rain ascends to higher, chilly altitudes by an updraft; the particles freeze, then return to Earth in the form of a hailstone.

Clouds come in various shapes and sizes and occur at varying altitudes. They can even gather on the ground in the form of fog. This occurs when warm, moist air close to the ground either cools rapidly or becomes oversaturated with water vapor.

But as you know, Earth's most substantial cloud formations occur in the air. On the next page, we'll look at how all that water vapor gets up so high.

When Air Masses Collide

Cloud formation occurs when humid or water vapor-filled air rises to the point where cooler temperatures force condensation. This often involves the movement of air masses, which are large bodies of air with similar temperatures and moisture content. Air masses are typically at least 1,000 miles (1,600 km) wide and several miles thick.

Four naturally occurring mechanisms on Earth cause air to rise:

Orographic lifting: This phenomenon occurs when an airflow encounters elevated terrains, such as mountain ranges. Like a speeding car heading toward a hill, the wind simply powers up the slope. As it rises with the topography, water vapor in the airflow condenses and forms clouds. This side of the mountain is called the windward side and typically hosts a great deal of cloud cover and precipitation. The other side of the mountain, the leeward side, is generally less lucky. The airflow loses much of its moisture in climbing the windward side. Many mountain ranges virtually squeeze incoming winds like a sponge and, as a result, their leeward sides are home to dry wastes and deserts.
Frontal wedging: When a warm air mass and a cold air mass collide, you get a front. Remember how low-pressure warm air rises and cold high-pressure air moves into its place? The same reaction happens here, except the two forces slam into each other. The cold air forms a wedge underneath the warm air, allowing it to basically ride up into the troposphere on its back and generate rain clouds. There are four main kinds of fronts, classified by airflow momentum. In a warm front, a warm air mass moves into a cold air mass. In a cold front, the opposite occurs. In a stationary front, neither air mass advances. Think of it as two fronts bumping into each other by accident. In an occluded front, a cold front overtakes a moving warm front, like an army swarming over a fleeing enemy.
Convergence: When two air masses of the same temperature collide and neither is willing to go back down, the only way to go is up. As the name implies, the two winds converge and rise together in an updraft that often leads to cloud formation.
Localized convective lifting: Remember the city example? This phenomenon employs the exact same principle, except on a smaller scale. Unequal heating on the Earth's surface can cause a pocket of air to heat faster than the surrounding air. The pocket ascends, taking water vapor with it, which can form clouds. An example of this might be a rocky clearing in a field or an airport runway, as both absorb more heat than the surrounding area.

And now the stage is set for the part you've all been waiting for: storms.

Stormy Skies

The troposphere is in constant motion. Air masses race across the oceans and continents, and great rivers of upper-air wind surge overhead. The atmosphere is like a battlefield, thick with the maneuvers of innumerable armies. When these forces clash, the atmosphere gives birth to the terrible beauty of storms.

Often, these storms are what we classify as severe storms. Few other natural occurrences demonstrate the raw and untamed power of nature as well as these powerful atmospheric occurrences.

Thunderstorms form like many other clouds, when a warm, moist air mass rises up and cools, causing the water vapor to condense into clouds. However, if the updraft continues, this cloud mass will continue to grow and rise 40,000 feet (12,000 m) or more up into the troposphere. Large raindrops or ice crystals form in this updraft, but they eventually grow too large and plummet back down, dragging air with them. This creates a powerful downdraft, pouring strong winds out in every direction.

As for the rumbling and crashing you hear during a thunderstorm, that also comes down to a matter of atmospheric pressure and temperature. A flash of lighting typically heats the air around it by a staggering 55,000 degrees Fahrenheit (30,000 degrees Celsius). This causes the molecules in the air to expand so rapidly that the air expands in the form of a shock wave powerful enough to break the sound barrier.

Scientists have more than one theory on how lightning forms. The most popular theory is that the falling rain and ice transfer a positive charge to even colder cloud particles. This creates a positive electric charge in the upper portions of the cloud, a negative charge in the center and a slight positive charge in the lower regions. The ground also has a positive charge. All this built-up electricity has to go somewhere, and if it builds up enough, it can jump across typically non-conductive air to reach positively charged areas within the cloud, in other clouds or on the ground. For a complete explanation of this process, read How Lightning Works.

Between their powerful winds and potentially lethal lightning strikes, severe thunderstorms can present quite a danger on their own. Occasionally, however, they transform into something even more destructive: a tornado. These storms consist of either a powerful single vortex or multiple suction vortices revolving around a tornado's center.

Tornados occur in less than 1 percent of thunderstorms, and scientists still aren't completely sure what triggers their formation. The key component, however, seem to be the powerful updrafts associated with thunderstorms. Scientists estimate that the pressures inside a tornado's vortex can be as much as 10 percent lower than surrounding air pressures. It works along the same lines as a typical updraft (such as the one in our city example), only more extreme. To learn all about tornado activity, read How Tornados Work.

Finally, there's the hurricane. Also known as typhoons and cyclones, these storms are low-pressure zones that spin out of the tropics due to the Coriolis effect, building up speed and growing to enormous size. The low-pressure area sucks in spiraling torrents of surface wind, which then ascend into the sky in a column. A warm-air downdraft fills this hollow column. This unique middle area of relative calm is called the eye of the hurricane. These massive storms boast wind speeds in excess of 74 miles (119 km) per hour and an average diameter of 375 miles (600 km). To learn much more about these powerful storms, read How Hurricanes Work.

Weather affects our lives from day to day. You can feel it on your skin and glimpse its movements in the air outside your home. Yet it's the product of a massive atmospheric system -- 5.5 quadrillion tons (4.99 quadrillion metric tons) of gases stirred to life by one burning star.

Explore the links on the next page to learn more about weather and the titanic forces that make a simple afternoon shower possible.