Weather

Wind is hard to visualize—unless it is blowing around rain, snow, or dust. Yet, being able to visualize the wind is a handy skill for a pilot.

Probably the best way to create a mental picture of what the wind is doing is to imagine what water would be doing in a similar case. You could

think of a brisk wind blowing over and around trees, buildings, and other obstacles at an airport as something like water rushing down a whitewater stream and washing over and around the rocks in the stream.

The waves and swirls that the water makes as it hits rocks and squeezes through narrow openings between boulders create turbulence that’s easy to see.

As you think of water as being like wind, remember that a cubic foot of fresh water weighs approximately 62 pounds per cubic foot. At sea level, a cubic foot of air weighs approximately 0.07 pounds, and it becomes even lighter at higher altitudes. Any particular force that’s acting on the air will accelerate the air much more than it would water. The wind can give an airplane a bumpier ride than water gives a raft shooting white-water rapids.

If you plan to fly near mountains, keep in mind that wind blowing through a mountain pass will accelerate and grow turbulent.

You can feel what this is like in complete comfort by settling into a Jacuzzi, turning on the water jets, and enjoying the turbulence—soothing in a Jacuzzi, but not in an airplane.

Up, up, and away with the wind. In general, as you go aloft the wind speed increases and the direction changes. The table, on the next page, based on measurements of wind speeds and directions from a weather balloon, is typical.

This case also illustrates how the wind generally changes direction to the right, or clockwise, and the speed increases with altitude.

This happens because as you go higher the force of friction with the ground, which slows the wind, is decreasing. Why this changes the wind’s direction is explained in “The Weather Never Sleeps: Forecasting the Winds of Change” (January 2010 Flight Training), a story about the forces that cause the wind to blow.

A pilot should expect such wind changes during climb or descent, but since several factors are involved, there’s no handy rule of thumb for exactly what to expect. When the wind changes in a clockwise direction, either with increasing altitude or over time, it is said to be “veering.” A counterclockwise change is said to be “backing.”

Wind shear. Wind shear refers to winds in the same area blowing in different directions, or at different speeds, or both. This shear creates turbulence by creating rolling motions. As the graphic at right shows, differences in wind speeds or directions create a rolling motion. To see what happens, hold something round such as a pencil between your palms and move your hands in different directions. An airplane flying in the area between the two arrows in the drawing, which represent winds, would go up and down as it flies across a “tube” of rotating air. The greater the difference in wind speed or in wind direction, the stronger the turbulence will be.

The image represents wind shear caused by winds at different altitudes. Winds blowing in different directions ahead of and behind a weather front can also cause wind shear. The strongest such shear is likely along a cold front that’s moving across the ground at 30 knots or more with a temperature difference of nine degrees or more between the two sides of the front.

Winds blasting down from thunderstorms or sometimes showers, known as "microbursts," cause the most dangerous kinds of wind shear.

Near-Earth jet streams. Jet streams also impact wind patterns. The jet streams television meteorologists talk about are streams of high-speed winds up in the 30,000-foot plus altitudes where airliners fly. Jet streams sometimes occur much closer to the ground, however, and when they do they can catch pilots by surprise with wind-shear turbulence.

During the day, when the sun is heating the Earth, warm air rises and cooler air from aloft sinks. This mixes up the air and decreases the differences between winds near the surface and a couple of thousand feet up. After sunset on a clear night, however, the ground cools off faster than the air aloft until the air at the ground is a few degrees colder than the air a few hundred feet above—this is called an "inversion ."

When this happens, meteorologists say the air above the inversion is “decoupled” from the air below. If other conditions are right, a low-level jet stream forms.

Forecasting wind shear. When potentially dangerous wind shear, which will not be caused by thunderstorms, is expected up to 2,000 feet above the ground the National Weather Service adds a wind-shear alert to the terminal aerodrome forecasts (TAFs) for the airports to be affected. (Any time thunderstorms are forecast near an airport you should expect wind shear and turbulence.)

Forecasters denote a wind shear alert by “WS” with figures for the height of the shear level and the wind direction and speed at that level at the end of a TAF. A TAF with a low-level wind shear forecast could read: FM0800 07012KT 2SM -RA BR BKN008 OVC015 WS020/13045KT. The boldface type is the wind shear alert.

This forecast translates as: From 0800 Zulu, the surface wind will be from 70 degrees at 12 knots, the visibility will be two statute miles in light rain and mist, the ceiling will be broken clouds at 800 feet above the ground with an overcast layer at 1,500 feet above the ground. Wind shear is expected up to 2,000 feet where the wind will be from 130 degrees at 45 knots.

In this case the forecast difference in wind speed between the surface and 2,000 feet is 33 knots and the wind direction difference is 60 degrees. Forecasters can use charts to quickly see the wind shear dangers of different combinations of wind speeds and directions.