Weather

Most flight instructors wouldn’t be comfortable sending a student off on a solo cross-country flight if the student didn’t have a good grasp of navigation. Maybe the student could answer all of the knowledge questions about navigation and could do a decent job of plotting a course before a flight.

But if the instructor suspects the student doesn’t have a good practical sense of how the wind affects a flight and what do to if he doesn’t see a planned checkpoint when he should, remedial instruction is in order before the instructor signs off on the student flying over the horizon alone.

In a similar way, students can memorize the answers to knowledge questions about weather and how to obtain a preflight weather briefing, without a good understanding of basic concepts of meteorology they need in order to understand possible weather effects on their flights.

For example, the student might know that areas marked with an “L” on a weather map are likely to have rain or snow, but can’t explain why.

The concepts here should help student pilots—and maybe some instructors—better understand how weather works and how to make more sense of weather reports and forecasts, as well as the actual weather encountered on a flight. Some of them will also help answer FAA knowledge questions about weather.

WEATHER BEGINS WITH UNEQUAL HEATING. Places near the equator receive more direct sunlight than areas around the North and South poles. The atmosphere responds to this unequal heating with warm air rising over the tropics and starting to flow aloft toward the North and South poles.

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 with rising tropical air flowing directly to the polar regions, sinking and flowing directly back toward the tropics.

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.

If you could see the air from space you’d see it moving in straight lines while the Earth rotates under it. From our position on the rotating Earth we see the air following curved paths.

Scientists call the effect of the Earth’s rotation on winds and ocean currents the Coriolis force. It works with other forces that affect the winds, including air pressure differences, to create huge wind spirals we see in large storms, and global-scale winds both at the surface and aloft. These global-scale winds include the generally west-to-east flow of winds in the middle latitudes, as well as the high-altitude jet stream winds.

Big, swirling storms and upper air winds that swoop toward the equator and back toward the poles carry warm air toward the poles and cold air toward the equator.

AIR PRESSURE COMES INTO PLAY. Rising warm air creates areas of low atmospheric pressure at the surface. Surrounding air, where the pressure is higher, flows into the low pressure area as wind, replacing the rising air. Sinking air creates an area of high atmospheric pressure at the surface, where air flows outward toward areas of lower pressure as wind.

Rising and sinking air, combined with winds—which are affected by Earth’s rotation, create areas of high and low pressure aloft—which in turn affect the paths and speeds of winds at the surface and aloft and help create areas of surface high and low pressure.

These centers of high and low pressure are keys to the kind of weather you will experience on a flight. An area of low atmospheric pressure is at the center of any large storm. The lower the pressure, the stronger the storm.

AIR PRESSURE DECREASES WITH ALTITUDE. The air’s pressure, which creates forces acting in all directions, depends on the weight of all of the air above it that’s being accelerated by gravity toward the Earth’s center. If air isn’t moving up or down, the air pressure at any altitude represents a balance between the weight of the air above and the pressure of the air below acting upward.

Pilots need to understand the decrease of air pressure with height not only because of its effects on aircraft performance, but also because aircraft altimeters use the generally regular decrease of air pressure with altitude to indicate height above sea level.

RISING AIR COOLS, SINKING AIR WARMS. Any time air moves into an area of lower pressure—such as by rising because it’s warmer than the surrounding air—its pressure decreases, which cools the air. If air moves into an area of higher pressure, such as by sinking from aloft, 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—such as being heated by the warm ground or forced up by wind blowing over a mountain—the pressure of the rising air decreases at a regular rate that is not affected by the temperature of the surrounding air. As the pressure decreases, the air cools. When air descends into higher air pressure below, it’s compressed and warmed, also at a regular rate.

RISING AIR BRINGS CLOUDS AND PRECIPITATION. Unless it’s very dry, rising air creates clouds and maybe rain or snow. As air grows colder, the water vapor in it begins condensing to form clouds. If the air rises high enough, ice crystals begin forming. When conditions are right, including enough water vapor in the air, clouds can produce precipitation such as rain or snow.

Sinking air, on the other hand, warms— which prevents clouds from forming or evaporates clouds.

Air is rising from surface areas of low pressure, which is why areas on a surface weather map marked with an “L” (for low) tend to be cloudy, possibly with rain or snow. Sinking air creates high atmospheric pressure at the surface. Since sinking air tends to clear the sky, areas on surface weather maps marked with an “H” tend to have 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. 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.

TEMPERATURES IN LOW PRESSURE AREAS. Although warm air rising from the ground is one way that areas of surface low pressure form, don’t assume that the centers of all low-pressure areas are warm.

In fact, tropical cyclones, such as hurricanes, are the only kinds of large storms with warm centers. These storms form over warm oceans and begin to weaken and die when they move over cool water or land. They are called “warm-core” storms because the temperatures in the center at all altitudes are warmer than the air surrounding the storms.

Extratropical storms, which can form and thrive over land or cold oceans, are called “cold core” storms because the center is generally colder than at least some of the air surrounding the storm’s center. These storms have fronts separating warm and cold air.

One practical consequence for pilots is that parts of extratropical cyclones all of the way down to the surface can be cold enough for dangerous aircraft icing to occur. Another consequence is that when thinking about the weather, you shouldn’t let a basic concept—such as warm air rises—lead you to conclude that all low-pressure areas are warm.