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Weather Basics

Master these keys to meteorological understanding

By Jack Williams

Buckling yourself into the left seat of an airplane for the first time can be overwhelming. Can you ever learn how to use all of those dials, buttons, levers, pedals, and even a "steering wheel" that looks somewhat like that in a car but operates differently?

The art of flight instructing includes teaching the student some very basic principles of flight—such as what the flight controls do—and then helping the student build on that knowledge. Unfortunately, a student can earn a pilot certificate, sometimes even advanced ratings, without ever learning enough basic meteorological theory to be really comfortable making decisions involving the weather. For instance, the pilot might know that areas marked with an "L" on a weather map are likely to have rain or snow, but isn't sure why this is so.

The basic principles of meteorology discussed here will help you understand how the weather works and how to make more sense of weather, whether it's described in your local television station's news or a preflight weather briefing.

Airmet: An airmet is a warning of weather that could make your flight difficult. The airmets below are warnings of common misunderstandings that could make understanding weather harder.

Now, let's turn to some basic principles, with a few airmets.

The sun is not an equal-opportunity heater. The seasons are the most-obvious example. During winter in the northern hemisphere the Earth is tilted away from the sun, which means days are shorter and the sun is lower in the sky than during the summer. It turns cold. At the same time, the southern hemisphere is tilted toward the sun, enjoying the warmth of summer. Throughout the year the sun is close to directly overhead in the tropics—the region about 1,600 miles north and south of the Equator—which means it's warm all year.

On a smaller scale, even when the sun shines with the same intensity on a region, parts of it warm more than others. For example, on a hot afternoon at the beach, you race across the sand, which burns your feet, to cool off in the water. Yet, the sun has been shining on the sand and the water the same amount of time. The top layer of sand has absorbed the sun's heat. Sunlight penetrates the water, warming a deeper layer. Also, currents and waves mix cool water with warmer water.

Airmet: The air generally cools with altitude, but there's no contradiction in places closer to the sun being cooler. Solar energy passes through the lower atmosphere without warming it.

You can trace the world's winds and weather back to unequal heating. Ground and water temperatures determine the warmth or coolness of the air right above them. As air warms, it becomes less dense and begins to rise; air becomes denser as it cools, and it begins to sink. This is why glider pilots know to look for thermals—rising air that will keep them aloft—over paved parking lots or bare ground.

On a regional scale, unequal heating causes "sea breezes" near the oceans or large lakes. As the land warms up during the day, air heated by the warmed land begins rising and cooler air moves in to replace it, creating a cooler breeze from the water to land.

On a global scale, warm air rises in the tropics and cold air tends to sink in the polar regions. If the Earth were not rotating, this would set up a relatively simple global wind pattern. Air that rises in the tropics would move toward the poles at high altitudes to replace air that sinks in the polar regions. At the same time, the cool air sinking in the polar regions would create winds blowing toward the equator to replace the rising air.

Earth's rotation makes the winds, and the weather, a lot more complicated than they otherwise would be. Since the air isn't attached to the Earth, high-altitude winds (wind is nothing more than moving air) heading away from the tropics, and low-level winds blowing from the poles toward the equator, trace curved paths across the Earth. Think of it this way: The Earth is rotating under the moving air.

Scientists call the effect of the Earth's rotation on winds and ocean currents the Coriolis force. It works with the other forces that affect the winds, including air pressure differences, to create huge wind spirals.

Airmet: Don't believe anyone who says water goes down drains in different directions in the Northern and Southern hemispheres. The Coriolis force affects large systems, such as hurricanes and global ocean currents, but not small, short-lived movements such as water going down a drain.

Some of the air that rises in the tropics descends over the subtropical regions between about 20 and 30 degrees latitude north and south. Some of the air continues toward the poles to help create the high-altitude jet streams that blow generally from west to east in both hemispheres.

From time to time, air rising above warm oceans creates the huge swirls of stronger and stronger winds known as tropical cyclones. Hurricanes and typhoons are examples of tropical cyclones.

Cold winds blowing across the Earth's surface from the polar regions help create swirls of air that move across the middle latitudes as storms.

Now, let's turn from the big picture of how winds move around the Earth to a close-up of how water acts in the air. Then, we'll put the big and close-up pictures together.

If air is cooled enough, some of the water vapor in it—the humidity—condenses into liquid water or turns directly into ice.

Molecules of any substance are always moving (unless the substance is at absolute zero, minus 460 Fahrenheit), and temperature depends on how fast the molecules are moving. They are moving slowest in a solid, faster in a liquid, and fastest in a gas. As water warms up, some of its molecules begin moving fast enough to join the air as water vapor—a gas.

Then, if the air is cooled, the molecules of all the gases in it, including the water vapor, slow down, and some of the water vapor begins turning back into liquid water. (Air's other gases, mostly nitrogen and oxygen, have to be cooled to more than minus 300 degrees F to begin condensing.) Thus, fog and clouds form if the air is cooled enough. With enough humidity in the air, and enough cooling, tiny cloud drops or ice crystals grow large enough to begin falling as rain or snow.

Airmet: You often hear that "warm air can hold more water vapor than cold air," but this can lead to misunderstandings. Adding water vapor to the air is not like stuffing more golf balls into a bag. When water vapor is added to the air it displaces some nitrogen and oxygen. Italian physicist Amadeo Avogadro explained why in the early 1800s. He found that a fixed volume of gas—say one cubic foot—at the same temperature and pressure always has the same number of molecules no matter what gas is in the container. Since water molecules are lighter than oxygen or nitrogen molecules, humid air is less dense than dry air.

The air's pressure decreases with altitude. The air's pressure depends on the weight of the air above the point where the pressure is being measured. The higher you go, the less air there is above you; thus, the lower the pressure.

Lowering the air's pressure causes air to cool. Increasing the air's pressure causes the air to warm. If air rises for any reason, its pressure decreases to match the pressure of the surrounding air, and the rising air cools. Whenever air descends it's compressed to match the surrounding air pressure and the air warms.

Rising air cools at the rate of 5.5 degrees F for each 1,000 feet gained, and descending air warms at the same rate.

The only thing that changes this rate is condensation of water vapor. Condensation releases the heat water molecules gained when they evaporated. This means that if cloud drops are forming in rising air, the heat being released by condensation slows the cooling rate.

Airmet: The cooling of rising air depends only on how far it rises and whether water vapor condenses as the air rises. The temperature of the surrounding air does not affect how cold the rising air becomes.

Air is rising in areas of low pressure at the surface—indicated with an "L" on a weather map. Air is descending in areas of high pressure at the surface—indicated with an "H" on a weather map. The rising air in the tropics creates low pressure at the surface, but nature has other ways of creating low air pressure.

Upper-atmospheric winds and patterns of air pressure are the key players in the formation of areas of high and low pressure at the surface in the middle latitudes. The details can be quite complicated, but let's look at one way how this works.

Imagine a river of high-speed air—a jet stream—blowing across the United States. Over the Great Plains, the high-altitude winds are coming together, or converging, and the river of wind is growing narrower. This increases the upper air pressure and some of the air is pushed down to the surface to create high pressure on the ground.

The descending air spirals out of the surface high pressure in a clockwise direction. (In the southern hemisphere the winds flow counterclockwise around high pressure.)

Farther east, our narrow river of wind spreads out; the air is diverging, which creates an area of low pressure aloft. Air begins rising from the ground into the area of divergence, which creates an area of low pressure at the surface. As air rises and the surface air pressure drops, air begins spiraling counterclockwise inward to the low-pressure center, where it rises. (In the southern hemisphere winds spiral clockwise into low pressure.)

Airmet: Temperatures in areas of high and low air pressure can be either warm or cold. In a hurricane, the air in the low-pressure center is warmer than the surrounding air. In a middle-latitude storm the temperature in the low-pressure center is colder than the surrounding air. High-pressure areas can also be warmer or colder than the surrounding air.

The rising air in low-pressure areas causes clouds and precipitation and descending air in high-pressure areas stifles cloud formation. Low pressure generally means poor flying weather, while high pressure generally means good weather.

But you can't always count on high pressure to create perfect flying weather. For instance, if high pressure, with its light winds, sits on an area long enough, the air can grow hazy with poor visibility.

These basic meteorological principles won't allow to you understand everything about weather. They will, however, help you make sense of what you see when you look at the sky, or listen to what a briefer has to say about the weather for your flight.

Jack Williams is the weather editor of An instrument-rated private pilot, he is the author of The USA Today Weather Book and co-author with Dr. Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.