Weather

A common complaint among student pilots and even many experienced pilots is that weather is hard to understand. Before a flight lesson the instructor makes sure the student understands some basic principles about the skills to be worked on in the upcoming flight, such as how to coordinate use of the ailerons, elevator, rudder, and power before going out to do steep turns.

Unless the student is in a college flying program that requires meteorology courses, however, a student can earn a pilot certificate—and sometimes even advanced ratings—without ever learning enough meteorological theory to be comfortable making weather-related decisions. A pilot might know that areas marked with an “L” on a weather map are likely to have rain or snow, but isn’t really sure why.

While they won’t take the place of a meteorology course, these nine basic principles of meteorology can help you see the big picture of how the weather works. They also can help you answer some of the weather questions on the FAA knowledge tests.

1. The sun heats the Earth unevenly. The sun’s heat supplies the energy that drives the weather. At the most basic level all weather is the result of the flow of this energy. An important part of Earth’s energy flow, in turn, is dependant on the sun’s uneven heating of the Earth on both the global and local scales. As two FAA knowledge-test questions put it, all weather is the result of a “heat exchange” and “unequal heating of the Earth’s surface” causes variations in altimeter settings (and other weather factors). In these questions none of the other answer choices include any of the details of how the heat exchange and unequal heating work to cause weather changes.

The changes in heating caused by the seasons are big and create global weather patterns, but local changes are also important. For example, since land heats faster than water—you’ll learn why in a meteorology course—a “sea breeze” begins blowing inland as land warms during the day.

2. Air rises over warm areas, lowering the pressure of the air at the Earth’s surface. Warmed air rises because it becomes less dense; that is, “lighter” than surrounding air. This not only explains such phenomena as the sea breeze, but also large-scale weather patterns. When you see an “L” on a surface weather map it shows that the air is rising from the surface in that area. As we’ll see below, this is a very important weather maker.

3. Pressure differences and other forces cause wind to blow. The pressure differences that cause wind at any level of the atmosphere are at that level. The higher air pressure at the surface does not cause air to rise toward lower pressure aloft because the weight of the air above is pushing down, balancing the pressure force pushing up. However, air does rise or sink when this balance is upset, such as when lower-level air is heated.

Once a higher pressure begins pushing air toward lower pressure, other forces come into play to determine the wind’s direction and speed. The Earth’s rotation creates the Coriolis force. It and other forces cause the wind to follow a counterclockwise, curving path from high toward low pressure in the Northern Hemisphere and a clockwise path in the Southern Hemisphere.

4. The air’s pressure at a particular altitude depends on both its pressure and temperature. In general, the air’s pressure and temperature decrease with altitude. While the pressure always decreases with altitude above any particular location, the temperature is sometimes warmer at a higher elevation than below. The air’s pressure decreases with altitude because pressure at any particular place depends on the weight of the air above that’s pressing down.

In general, air is cooler aloft because most of the sun’s energy passes through the air without warming it. The Earth’s surface, which has absorbed heat from the sun, warms the air that’s touching it; this warming decreases with altitude.

Some weather conditions can create a layer of air aloft that’s warmer than the air below. Such a warmer layer is called an “inversion” and can be an important weather factor.

Newcomers to weather are sometimes confused because they think colder air is “heavier” than warm air and wonder why cold air aloft doesn’t sink. Colder air aloft doesn’t sink because the lower pressure decreases the air’s density enough to offset any increase in density caused by the air aloft being colder.

However, as we’ll see in number 6, some conditions upset this balance and cause air aloft to sink.

5. Cold air can “hold” less water vapor than warm air. Note the word “hold” is in quotation marks. “Hold” is shorthand for what really happens. But, it does make the point that the amount of humidity—water in the form of a gas known as water vapor—that can be in the air is limited by the air’s temperature. If the air is cooled to the temperature where it can no longer “hold” the water vapor in it, the vapor will begin condensing into tiny droplets to form a cloud. If the cloud’s bottom is touching the ground or very near the ground, it’s called fog.

Air becomes saturated when it’s cooled to the point that it can no longer “hold” the water vapor in it. The temperature at which this happens is its dew point.

“Hold” is in quotation marks because the air doesn’t “hold” water vapor in the way that a basket holds apples and the use of “hold” has led to downright wrong explanations of what happens. For an explanation of what does happen see “Weather: Water in the Atmosphere” (September 2009 Flight Training).

6. Air rises from surface areas of low pressure, and sinks into areas of surface high pressure. As we saw in number 1, when warmed air rises, it decreases the surface pressure. Areas of high and low air pressure aloft create winds, including high-speed jet streams. In some places streams of upper-air winds converge (come together) to increase the amount of air over some areas. The resulting increase in pressure aloft causes air to sink to the surface where it forms areas of surface high pressure.

7. Air cools as it rises and warms as it sinks, which is a key to creating or suppressing the formation of clouds and precipitation. As warm air rises it cools at a rate of 5.5 degrees Fahrenheit per 1,000 feet until it reaches the dew point temperature at which water vapor begins condensation. Condensation, in turn, releases heat that slows the rate at which the rising air cools. The result is that low surface pressures create clouds and precipitation as the humidity in the rising air condenses. When air sinks toward the surface it warms at this same rate of 5.5 degrees per 1,000 feet.

Mostly clear skies are found above areas of surface high pressure because the warming of sinking air keeps clouds from forming or evaporates existing clouds.

The temperature changes in rising and sinking air do not depend on the temperature of the surrounding air. In other words, rising air does not grow colder because it moves into colder surrounding air.

8. Where and when air rises and how far and fast it rises depends on the atmosphere’s stability, which in turn is determined by the air’s temperature profile. When air is given an upward shove, such as being warmed by the ground, blowing across a mountain with the wind, or being shoved up by an advancing weather front, it will continue rising as long as it remains warmer than the surrounding air.

The atmosphere is unstable when its temperature decreases at a rate greater than 5.5 degrees per 1,000 feet. In such a case rising air cooling at the rate of 5.5 degrees per 1,000 feet will remain warmer than the surrounding air and thus continue rising. In such conditions lumpy, cumulus clouds will form. The atmosphere is unstable.

When the temperature of air that’s not moving up or down decreases at a rate of less than 5.5 degree per 1,000 feet, rising air will cool to or below the temperature of the surround air and thus stop rising. The atmosphere is stable.

9. The atmosphere’s stability is a major factor determining the kind of weather that occurs. An unstable atmosphere encourages air to rise, and cool clouds will form if the air is humid enough. If the atmosphere is unstable to high altitudes and if the rising air has a good supply of water vapor, powerful thunderstorms towering more than 60,000 feet into the sky can form. If the air is too dry for clouds to form, an unstable atmosphere will create turbulence. When the atmos-phere is stable and has enough water vapor, the clouds that form will tend to be flat and widespread.

For more information on concepts number 8 and 9, see “Weather: Atmospheric Stability Is Key” (March 2010 Flight Training).

While these nine keys to understanding the weather are far from all a pilot needs to know about the weather, they could help you master aviation meteorology.