Eight ays to weather wisdom
Blizzards-frigid, windy snowstorms-can be especially dangerous because people can become lost only yards from warm shelter in the zero visibility created by blowing snow. Polar research camps string ropes (lifelines) between buildings to guide those who must venture out in zero visibility.
| When the temperature is cold enough, water vapor can turn directly into ice to create frost on objects or snow in clouds. |
Student pilots can feel they are getting lost in a blizzard of scientific terms and concepts as they study aviation weather, but these eight concepts-and the brief explanations following each-can be lifelines. They can keep you from wandering off course into confusion as you learn how to answer knowledge test questions about weather, and you can use this knowledge to obtain preflight weather briefings.
1. The sun does not evenly heat the Earth. Seasons are the best example. Anyone who lives in a place with large seasonal changes associates winter cold with short days and the sun being low in the sky, while summer's warmth comes with long days and the sun high in the sky. While sunlight is warming the Earth, heat is also radiating away as infrared energy, day and night. Nights are cooler than days because heat continues to radiate away after sunset and the Earth loses energy. During a year the polar regions lose more heat in winter than they gain during the summer while the tropics gain more heat than they lose. Similar, smaller-scale effects are also important. Land warms up faster than water, which means the air over land warms more than the air above water. If nothing else happened, warm places would grow increasingly warm and cold places colder and colder.
2. Air temperature differences start the winds blowing. The variations in ground or water temperatures create different air temperatures in different climate zones, such as the poles and the tropics, and to a smaller degree over land and water. Air tends to rise where the surface is warm and sink where it's cold. If the Earth were not rotating, warm air would rise in the tropics, flow aloft toward the poles, and sink in the polar regions. Cool air from the northern and southern parts of the globe would flow across the surface to replace the rising air in the tropics.
On a local scale, unequal heating causes "sea breezes" near the oceans or large lakes. As the land heats up during the day, air heated by the warm land begins rising, and cooler air moves in to replace it, creating a cooler breeze from over the water to the land.
3. The Earth's rotation creates a complex pattern of winds. The effect of the Earth's rotation, known as the Coriolis force, combines with the other forces that drive the winds to create huge wind spirals around areas of high and low atmospheric pressure. These break up a simple flow of high-altitude air toward the poles and a surface flow of air toward the tropics described previously.
High-altitude jet streams blow from west to east in both hemispheres. The twists and turns of jet streams help create areas of high and low atmospheric pressure at the surface. In some areas a flow of jet stream wind spreads out (called divergence), which reduces the atmospheric pressure at the wind's altitude. This, in turn, encourages air to rise from below, forming an area of low atmospheric pressure at the surface. The flow of jet stream winds can also squeeze together (called convergence), which forces air down to create areas of high atmospheric pressure at the surface.
Divergence and convergence aloft help explain why areas of high pressure and areas of low pressure form in the middle latitudes and follow each other from west to east, bringing weather changes. This also helps explain why some areas of low atmospheric pressure are warmer than the surrounding air-hurricanes that form in the tropics are an example-while other areas of low pressure are colder than the surrounding air, such as middle-latitude storms. In the Northern Hemisphere, the air flows counterclockwise around low-pressure areas and clockwise around high pressure.
4. Cool air can "hold" less water vapor than warm air. When the air is relatively warm, water evaporates into it, which means the water becomes an invisible gas known as water vapor. If the air is cooled, some of the vapor condenses to form dew or the tiny water droplets that make up fog and clouds. (Fog is merely a cloud that's on the ground.)
When conditions are right, cloud droplets come together to create small drops called drizzle or larger drops called rain. When the temperature is cold enough, water vapor can turn directly into ice to create frost on objects or snow in clouds. 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.
5. Air pressure decreases with altitude. The decrease of air pressure with height is important to pilots both because of the role that it plays in weather and because aircraft altimeters use decreasing pressure to indicate height above sea level. Also, the lower pressures at altitude affect aircraft performance. Air pressure decreases with altitude because 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.
6. Lowering the air's pressure causes air to become cooler; increasing air's pressure causes it to become warmer. This fact can create confusion in a couple of ways. First, this statement applies to air only when its pressure is changing. Second, as air rises into lower air pressure aloft its pressure falls to match the pressure of the surrounding air. The fall in pressure, not the temperature of the surrounding air, causes the temperature of rising air to fall.
7. Rising air causes clouds and precipitation; sinking air tends to clear the sky. As a parcel of air rises from the surface, the pressure of the surrounding air decreases with altitude. The pressure of the rising air decreases to match the pressure of the surrounding air. As a result, the rising air cools. If there is enough water vapor in the air, it begins condensing to form clouds and maybe precipitation.
When air is sinking, its pressure increases as it descends into the higher-pressure air at lower altitudes. This causes the air to warm. If there are any clouds, they begin to evaporate as the air warms. The warming air, of course, will also keep clouds from forming.
8. Air is rising from areas of low air pressure at the Earth's surface and sinking into areas of high pressure at the surface. As a result, low-pressure areas tend to bring clouds and precipitation, while the sky is normally clear in areas of high pressure.
At the Earth's surface, air spirals into areas of low air pressure where it rises. As the air rises, it cools, and the water vapor in it begins condensing to form a widespread area of clouds and precipitation. Air that rises in low-pressure areas flows into the upper atmosphere until it begins sinking to form areas of high pressure at the surface. While the sinking air keeps clouds from forming, it doesn't always bring completely clear skies. Sometimes, the high pressure can trap pollution, which can create low visibility in haze.
Understanding the weather requires more than these eight basic ideas. They can be your lifeline, however, when you begin to feel like you're wandering around in zero visibility while trying to learn all of the things about weather that pilots need to know.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic.
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