The first flying lesson is the best time for a student to begin learning that a pilot has to always pay close attention to the weather. Even though the word weather might not be mentioned, an instructor is offering a first weather lesson when she directs the student to taxi to the runway that allows a takeoff into the wind. This is a good time for an instructor to plant the idea that learning which way the wind is blowing before taking off is just the beginning of the weather awareness all pilots need to cultivate.
Learning more about wind than how to use it for takeoffs and landings is a good beginning of a pilot’s weather education because wind is often a part of other weather phenomena. It’s one of the dangers of storms. Wind can push a pilot who is not paying attention off course. Wind transports the humid air that supplies water to fall as rain, snow, or ice, and also brings the dry air that shoves out clouds and precipitation. Wind can cause disconcerting and even dangerous turbulence, and quick wind shifts caused by thunderstorms can cause airplanes to crash while taking off or landing.
Since differences in air pressure cause the wind to blow, learning about wind begins with understanding air pressure. The air’s pressure at any point, whether on the ground or in the upper atmosphere, is determined by the weight of the air above that point. This is why air pressure decreases as you gain altitude.
At sea level on an average day the air is pushing in all directions with a pressure of 14.7 pounds per square inch. Instead of using square inches to measure pressure, meteorologists use a metric pressure unit called the millibar (most of the rest of the world calls this same unit a hectopascale). The average surface pressure of 14.7 pounds per square inch is the same as 1025.25 millibars (or hectopascals).
In the United States, the National Weather Service uses inches of mercury for the reports that pilots use to set their altimeters and for reports of surface pressure intended for the general public. This measurement tells how high the air pressure will push mercury up in a tube. Meteorologists prefer a direct measurement of pressure, such as millibars, because it can be plugged directly into their weather calculations.
As anyone who follows the weather has noticed, areas of high and low air pressure at the surface are always forming, moving, and dying out all over the Earth. The factors that influence air pressures are complex, but the differences begin with heated air rising over relatively warm parts of the land or oceans, making the surface pressure lower over some areas than in surrounding areas.
Air that rises from areas of surface low pressure feeds upper-air winds. These winds generally flow from west to east—in both the Northern and Southern hemispheres—usually in curvy paths that can take the wind toward the south and back toward the north. The curving paths of upper-air winds cause air to sink in some areas, creating high pressure at the surface, and to rise in other areas creating or strengthening areas of low pressure at the surface.
Differences in air pressure at each particular level supply the power that drives the winds at that level. To see how this works, let’s look at a very simplified picture of what could be happening at two points approximately 18,000 feet above the Earth. The pressure at one point is 500 millibars, which means the air is pushing in all directions with that pressure. At another point, say 30 miles away, the pressure is 496 millibars pushing in all directions. If a pipe full of air ran between the two locations, the four-millibar difference in pressure between the ends of the pipe would create a force pushing the air from the 500-millibar end of the pipe toward the 496-millibar end.
In the atmosphere, air begins moving straight from higher pressure toward lower pressure. But the Earth is turning under the moving air, which means the moving air—wind—follows a curved path in relation to the Earth. This effect of the rotating Earth is called the Coriolis force, for Gustave-Gaspard Coriolis, the French scientist who first described it mathematically in 1835.
Both the pressure difference and the distance between the high and the low pressure determine the wind’s speed. For instance, if the two ends of the “pipe” were 20 instead of 30 miles apart, the air would move faster because the four-millibar pressure difference would have less air to push. The combination of pressure difference and distance create what meteorologists call the pressure gradient force, or PGF.
If the lines on a weather map showing air pressures of 500 millibars and 496 millibars were straight and parallel, the PGF and Coriolis forces would soon balance and the wind would flow parallel to the lines of equal atmospheric pressure. When the isobars curve, as they often do, the PGF and Coriolis forces combine to keep the wind blowing parallel to the isobars in the upper atmosphere. In the Northern Hemisphere the winds blow counterclockwise around low pressure, clockwise around high.
Closer to the surface, the picture is more complicated. The strength of the Coriolis force weakens as wind speed decreases. Below approximately 5,000 feet, friction with the Earth’s surface slows the wind, which weakens the Coriolis force. The weaker Coriolis force allows the wind to spiral across isobars toward the center of an area of low pressure instead of circling the low-pressure center as winds aloft do.
Understanding a few basics about wind will help a pilot make more sense of the weather. For example, if the clockwise wind around a large area of high pressure is coming from over a warm ocean it will bring in humid air that could supply moisture for showers, thunderstorms, or steady rain. This is common during summer when winds around the Bermuda High, which is centered over the Atlantic Ocean, bring high humidity that lowers visibility and feeds thunderstorms.
On a small scale it helps to visualize wind like water. Just as water in a fast-moving stream creates turbulent whitewater conditions, when it hits rocks and other obstructions, wind also begins moving up and down or swirling when it flows over mountains or over and around trees and buildings upwind from a runway. When conditions are right, wind blowing across mountains can create up-and-down waves that generate turbulence for 300 or more miles downwind from the mountains. The downward-moving part of such a wave can push an airplane down faster than it can climb.
On a warm day when hot air is rising from the ground and cool air is descending from aloft to replace it, the descending air brings blasts of the stronger winds aloft down to the surface as gusts.
After sunset, as the ground cools off and air stops rising, the surface winds could die to nearly zero while the winds aloft continue blowing at relatively high speeds. When rising air is no longer carrying slow winds aloft and sinking air is bringing down faster winds, meteorologists say the boundary layer—the bottom 1,000 to 5,000 feet of the atmosphere where friction affects winds—has become “decoupled” from the higher atmosphere. The winds right above the boundary layer can increase to form what’s called a low-level jet.
The boundary between nearly calm air at the surface and winds blowing as fast as 50 knots—or faster—is turbulent. If you take off on a calm evening or early morning and climb into this layer you could encounter violent turbulence and a sudden shift in wind direction.
Most new pilots, unless they are sailors, have never had to think much about the wind. As a pilot you’ll always have to be concerned with the wind.
Jack Williams, a freelance science writer specializing in weather and climate, is an instrument-rated private pilot. The latest of his six books is The AMS Weather Book: The Ultimate Guide to America’s Weather. He answers questions about weather on his Web site.
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