Eight concepts for better understanding
Figure 1. The curved motion of winds aloft and at the surface help break the Earth's overall wind patterns into separate regions. Extreme cold surface chills air that warms as it sinks (1). The polar front includes an ever-shifting boundary between cold and warm air (2). Some air sinks to form large high pressure areas (3). Air rises in the tropics, creating large thunderstorms (4). |
Figure 2. In the Northern Hemisphere, converging air aloft forces air down. Descending air warms. Air flows out at the surface. Diverging air aloft causes air to rise. As rising air cools, clouds form. Surface air flows into a low pressure system. |
How would a student would handle her first attempts at turns, climbs, and descents if instead of talking about the ailerons, elevators, and rudder during the preflight walkaround, the instructor instead spent 10 minutes explaining the trim tab and using the trim wheel without mentioning the other controls?
Unfortunately, a student's introduction to weather can be somewhat like a trim-tab-only introduction to aircraft control. New pilots are often expected to handle some rather complex weather concepts before learning a few basics about how the atmosphere works.
One way to address this problem is to think of these eight basic weather concepts as an equivalent of a good walk-around discussion of an airplane's control surfaces.
1. The sun heats the Earth unequally. Solar radiation supplies almost all of Earth's heat that makes life possible and drives the weather. While the sun warms our planet, the Earth is always giving up heat that radiates into space as infrared energy. The balance between incoming and outgoing heat energy keeps the Earth in a temperature range that allows life as we know it.
Earth has seasons because its axis is tilted in relation to the planet's yearly path around the sun. During the Northern Hemisphere winter the North Pole is tilted away from the sun; in summer it's tilted toward the sun, causing the hemisphere's long summer days with the sun higher in the sky. This additional heating makes summer warmer than winter for places outside the tropics.
If the Earth were not tilted in relation to the sun we wouldn't have seasons, but the sun would still heat the Earth unequally because it would still be almost directly overhead in the tropics all year and lower and lower in the sky as you move toward either pole. Solar energy reaching the polar regions does not replace all of the infrared energy escaping into space. If nothing else happened, the Arctic and Antarctic would continue growing colder and colder until the temperature reached absolute zero and all molecular motion stopped. The tropics, on the other hand, would continue growing warmer and warmer until everything vaporized because arriving solar energy more than makes up for escaping infrared energy.
Fortunately, neither happens. Monthly average temperatures on Antarctica's polar plateau, Earth's coldest area, range from a high of minus 15 Fahrenheit to a low of minus 80 F. Extremely warm places such as large deserts in and near the tropics see monthly low and high averages ranging from around 50 to 100 degrees F.
Earth's global weather patterns, with the help of ocean currents, keep global temperatures in rough balance by moving heat into the polar regions and cool air and water into the tropics. The weather part of these movements begin with solar energy warming land and water, which in turn warm the layer of air next to them. As air warms, it becomes less dense and begins to rise. In the polar regions, the land and oceans grow colder as infrared energy radiates away, cooling the air next to them, making it more dense and causing it to sink and spread out.
2. Air temperature differences start the winds blowing. If the Earth were not rotating, warm air would rise in the tropics and flow as high-altitude winds to the north and south, where it would descend in the polar regions as the frigid surface cools it from below. Cool air from the northern and southern parts of the globe would flow across the Earth's surface as winds heading directly toward the equator to replace the rising air in the tropics.
3. Air rises from areas of low surface air pressure and sinks into areas of high surface pressure. When air rises from the surface and flows aloft, it creates an area of surface low air pressure. Where air sinks toward the surface, it creates an area of surface high air pressure. Air flowing from high pressure toward low pressure creates winds.
4. The effect of the Earth's rotation, known as the Coriolis force, disrupts the simple flow of air aloft toward the polar region and on the surface back to the tropics. The Coriolis and other forces cause winds to spiral around areas of high and low air pressure. The curved motions of winds aloft and at the surface help break Earth's overall wind patterns into separate tropical, middle latitude, and polar regions, as shown in Figure 1 (p. 33). This figure shows the long-term average of air movements. In the middle latitudes large areas of high and low air pressure, moving generally from west to east in both hemispheres, transport warm air toward the poles and cool air toward the equator.
The Coriolis force, created by Earth's rotation, causes air to flow counterclockwise around low air pressure and clockwise around high pressure in the Northern Hemisphere. It flows in the opposite ways in the Southern Hemisphere. Storms are areas of low air pressure.
5. Cool air can "hold" less water vapor than warm air, which is why cooling the air creates clouds and precipitation. 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 aloft cools, some of the vapor condenses to form the tiny water droplets that make up fog and clouds. When conditions are right, cloud droplets come together to create small drops called drizzle or larger rain drops.
When the temperature is cold enough, water vapor can sublimate directly into ice to create ice crystals in clouds. Freezing rain and sleet are created when rain falls from air that's above 32 degrees into freezing air.
6. Air pressure decreases with altitude. This occurs because the air's pressure, the force that it exerts in all directions, depends on the weight of the air above the point where you are measuring the pressure. At sea level, for example, the air's average pressure is 14.7 pounds per square inch. At an altitude of around 18,000 feet the air's pressure is approximately 7.3 pounds per square inch.
The decrease of air pressure with height is important to pilots because of the role it plays in weather, and because aircraft altimeters use decreasing pressure to indicate height above sea level. Lower pressures at altitude also affect aircraft performance.
7. If air moves into an area of lower pressure, its pressure decreases, and the air cools. If air moves into an area of higher pressure, its pressure increases, and the air warms. The pressure of moving air adjusts to match the pressure of the air surrounding it. Thus, when anything forces air to rise--it's heated by warm ground or forced up by wind blowing over a mountain--the pressure of the rising air decreases. As the pressure decreases, the air cools. When air descends into higher air pressure below, it's compressed and warmed.
8. Rising air causes clouds and precipitation; sinking air tends to clear the sky. Since cool air can hold less water vapor than warm air, the moisture in rising air begins condensing to form clouds and precipitation. This is why areas of low pressure generally bring clouds and wet weather. As descending air warms it tends to evaporate clouds back into water vapor and keep clouds from forming. This is why areas of high pressure generally bring clear weather. Figure 2 shows what happens in areas of high and low air pressure at the surface in the Northern Hemisphere with air rising to form clouds and precipitation over the surface low, and air flowing aloft to sink to Earth in a high pressure area with generally clear skies.
While rising warm air is one cause of low air pressure at the Earth's surface, not all low-pressure areas form over warm land or oceans. The global patterns of rising and sinking air that begin with warm air rising in the tropics create upper-air winds, including jet streams of high-speed winds aloft. As upper-air winds bend and converge and diverge they can affect air pressures below, as shown in Figure 2 (above). For example. If upper-air winds aloft spread out, or diverge, air from below will rise to replace the air that's spreading out. This helps create low air pressure at the surface. When upper-air winds converge air is pushed down, creating high pressure at the surface below.
These eight basic weather principles are far from the whole story in the same way that knowing what happens to the ailerons, elevator, and rudder when you move the controls is far from all you need to know to safety fly an airplane. Both are only the beginning of mastering the skills that a pilot needs.
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, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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