The Weather Never Sleeps

A bumper crop of gusts and wind shear

Speed shear, below, involves slower winds near the ground than aloft. Directional shear, below right, occurs when winds blow in different directions at different altitudes. Combinations of speed and directional shears are common.
In a microburst, above left, the aircraft experiences a headwind, a downdraft, and a tailwind within a short period of time. Shear can weaken a hurricane, above, by pushing its cloud tops and bottoms in different directions.

March brings longer days and warmer weather, but the end of winter can't be counted on to eliminate pilots' weather hassles. For many parts of the United States, especially east of the Rockies, March brings the year's highest average winds, or winds to match those of the fading winter.

Most pilots would find wind easier to handle if it would only settle down. Instead, it huffs and puffs and bounces aircraft up and down. A fast but steady wind isn't too bad. High-flying jets routinely hitch rides on 100-mph jet streams. The ride becomes bouncy only when the airplane flies between areas in which the wind is blowing at different speeds, or from different directions.

The bumpy rides airline passengers experience as their jet flies into or out of a jet stream's fastest winds are caused by wind shear. It's also responsible for many of the other surprises wind holds in store for pilots.

Until the 1980s hardly anyone except meteorologists and a few weather-wise pilots had heard of wind shear. Forecasters rarely used the term; it was too technical and obscure for most people. That changed in the 1980s when research meteorologists figured out that microbursts, which create particularly dangerous low-level wind shear, had caused several airline accidents, including especially deadly crashes at New Orleans in 1982 and at Dallas-Fort Worth in 1985.

Press reports often used wind shear when describing the accidents or reporting on microburst research. Part of this effort included FAA educational materials on how to avoid and react to low-level wind shear, which added the term to pilots' vocabularies.

In 2004 six hurricanes hit the United States, followed in 2005 by the extremely destructive Katrina and three other hurricanes. News coverage of hurricanes and hurricane science soared, and more and more people heard about how wind shear weakens hurricanes.

What's going on? How can the same phenomenon, wind shear, destroy both airliners and hurricanes? Obviously, we're talking about different kinds of wind shear.

You sometimes hear that wind shear refers to a "rapid change in wind speed or direction." But, unless "rapid" is defined, the definition doesn't tell you much. Also, the wind shear that can weaken a hurricane isn't "rapid," although microburst wind shear certainly happens in a hurry.

Meteorologists' formal definition is that wind shear is "the local variation of the wind vector or any of its components." A vector is a quantity specified by both a magnitude and a direction, such as a "30-mph wind blowing from 280 degrees." This means that shear can refer to winds blowing at different speeds or from different directions in adjoining localities at the same time. In the shear definition, you can think of "local" as describing the part of the atmosphere in which you are interested.

Figure 1 (see p. 41) shows speed shear with slower winds near the ground than aloft, which creates up-and-down eddies in the air--vertical shear. A jet flying north to south from 40 mph westerly winds into a jet stream with 100-mph westerly winds would also encounter speed shear in the north-south direction--horizontal shear. In other words, wind shear can be horizontal or vertical.

Directional shear, as shown in Figure 2 (see p. 41), is very common because the winds normally blow in at least slightly different directions at different altitudes. Combinations of speed and directional shear are common.

Downbursts are winds that blast down from a thunderstorm, or maybe even a shower. If the winds are concentrated in an area with a diameter of 2.5 miles or less, the downburst is called a microburst. This means that a microburst's shear occurs in an area of less than 2.5 miles in diameter and within a couple of thousand feet of the ground.

An airplane might be only 100 feet above the ground while landing with a 10-kt headwind when it runs into a microburst, which shifts the wind in seconds to a 50-kt tailwind. This sudden shift slows the wind's speed over the wings, reducing lift, as Figure 3 shows. The reduced lift means the wings stall, and the airplane's nose pitches down. If the airplane is far enough above the ground, the pilot should be able to recover from the stall and fly away. The airliners that crashed in microbursts were too close to the ground when wind shear robbed them of lift.

The shear that weakens a hurricane occurs between right above the ocean and 20,000 feet or higher above the ocean; the winds involved could be blowing over an area of a few hundred miles. For instance, winds right above the ocean are likely to be the trade winds blowing over hundreds of miles of ocean from the east at around 10 kt. Winds at 30,000 feet might be blowing at 30 kt over a similar distance in the other direction. The opposing winds push the tops and bottoms of a hurricane's thunderstorms in different directions, weakening the storm, as shown in Figure 4.

On clear nights in summer or winter, the air near the ground cools off much faster than the air aloft. This is because with no clouds in the sky to block infrared radiation leaving the Earth, the ground cools quickly. The chilly ground cools the air next to it, but the air aloft stays warmer.

Such inversion, with warm air atop cooler air, cuts off convection--the up-and-down movement that brings winds down from aloft, causing gusts, and takes slower-moving air aloft. During the day, convection helps ensure that wind speed changes gradually as you go aloft.

With a nighttime inversion, however, while the surface winds are nearly calm, you might run into winds blowing 30 or 40 mph, or even faster, when you climb into the warmer air. This could be only a few hundred feet above the ground.

The shear zone, with the rapid wind speed, is likely to be turbulent. And, if the fast wind is a tailwind, your airplane will momentarily lose airspeed, potentially stalling or coming close to a stall, especially if you don't expect the wind shift. Reports of winds aloft might not catch the higher speeds aloft because weather balloons are launched in the late afternoon or early evening, probably before the inversion and faster winds aloft develop.

Fronts, which are boundaries between air masses, can also cause wind shear, especially cold fronts with a temperature difference of 10 degrees or more across the front, or fronts that are moving faster than about 30 mph.

Pilots can't escape the winds. The best way to avoid being surprised is to learn as much as you can about them and try to visualize how they are behaving when you're ready to go flying.

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.