Wicked, Wicked Wind Shear

You are flying along on a beautiful clear day with a nice little tailwind. Suddenly, as though a freight train had just barreled past, your flight takes on a new dimension in the struggle to be straight and level. Up becomes down, your seat belts barely restrain you from connecting with the ceiling and bolts creak as the fuselage metal stresses against opposing forces. This is wind shear.

According to Webster’s definition, wind shear is a difference in wind speed and direction over a relatively short distance in the atmosphere. Wind shear can be broken down into vertical and horizontal components. Horizontal wind shear is primarily seen across weather fronts and near the coast. Vertical shear typically occurs near the surface, though it can also be generated at jet stream altitudes and near the tops of strong upper level fronts.

A combination of both occurs during microburst activity wherein a strong downdraft hits the earth’s surface and spreads out laterally.

According to the National Weather Service (NWS) wind shear is categorized by speed and direction. Shear speed is the component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 mph at 6,000 feet increasing to 50 mph by 12,000 feet. Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.

Directional shear is the component of wind shear which is due to a change in wind direction with height, for example southeasterly winds at the surface and southwesterly winds aloft. A wind which veers dramatically with height in the lower part of the atmosphere is a type of directional shear often considered important for tornado development. We’ve seen this quite often in the last few months as tornadoes have ripped through the eastern half of the US.

The NWS keeps a close eye on charts and weather conditions in order to provide pilots with accurate forecasts through Terminal Aviation Forecasts (TAF). If wind shear is anticipated it is encoded with “WS” and always follows the forecast surface winds.

If the airport surface winds are forecast to be 14012KT and a thunderstorm is in the area, you may see “VRB45KT” included. This may be encoded as OCNL.

If the same forecast shows a possible wind shear zone at 1,500 feet in which the wind will change to 240 degrees at 20 knots, then it would be encoded in a TAF as “WS015/24020KT”. The wind shear group would immediately follow the surface wind group, thus providing a clear indication of how dramatic the wind change (shear) is expected to be. If it is uncertain as to what the wind direction and speed might be above the shear zone, or the height itself is in question, then the group may only include “WS015” or “WS”, respectively.

Wind shear may not always be specifically forecasted at an airport by the NWS, but the TAF, Area Forecasts and Radars may indicate that thunderstorms are expected. Thunderstorms are assumed to contain strong gusty winds, both surface and aloft, as well as sudden downdrafts and vertical wind shear.

Back when I lived in Indiana I was standing in a parking area where the surface winds were dead calm. I looked straight up at black clouds tinged with green frothy tips whirling incredibly fast at an altitude that looked to be only a few hundred feet above me. As I watched the clouds formed a sideways rotating cone. The cone became a funnel and touched down about half a mile south of where I stood.

You can often observe wind shear by observing formations of clouds or by debris bouncing around the airport. Shear experienced during landing or climbout can be attributed to wind deflected from hangars and other buildings, terrain features such as hills and trees along the runway or from the vortices of other aircraft (wake turbulence).

Rotor clouds and standing lenticular cloud formations are visual indications of areas of strong windshear activity aloft. Rotor clouds appear to be long horizontal fluffly cotton balls at low to mid altitudes. They appear to be rotating slowly, but you are only seeing the outside edges. Inside the winds are much stronger.

Standing lenticulars clouds are also seen primarily at mid and higher altitudes, but they appear to have a long smooth cigar shape, or can appear saucer shaped like the classic UFO. These are encoded in METAR’s as either ACSL (Alto Cumulus Standing Lenticulars) or CCSL (Cirro Cumulus Standing Lenticulars).

Frequently both the Rotor clouds and the lenticulars will form on the lee side of mountains. If you seen them, you may want to avoid flying into or near to them as they contain rapidly rotating horizontal tunnels of air.

Winds Aloft Forecasts or FD’s are good indicators of expected wind shear. Recently I briefed a pilot at Double Eagle Airport in Albuquerque. He told me the winds at the airport were very light, and the nearby TAF at ABQ also indicated surface winds were expected to be light, but the ABQ FD indicated winds of 230045kt at 9,000 – which is only about 3,500 AGL. You see this condition frequently in southern California during the Santa Ana windflow.

The FD may also predict winds to be relatively light at two succeeding forecast altitudes, but the direction may be as much as 180 degrees different! For example, the FL060 winds are forecast to be 09015 (east at 15 knots), while the FL090 winds are forecast as 27015 (west at 15 knots). The shearing effect on your aircraft will happen somewhere between the two altitudes, sometimes in as small as 500 feet elevation.

Another way of confirming wind shear is to have your briefer check his VAD wind readout. The Vertical Azimuth Display (VAD) is a function available on some radar sites showing current winds at 1,000 foot increments above the surface. The data is updated every 6 minutes.

Pilot reports are always appreciated if you experience any wind shear. Low level wind shear (wind shear within 2,000 feet of the surface) is classified as an urgent pilot report if air speed fluctuates 10 knots or more or if the air speed fluctuation is unknown.

Low Level Wind Shear (LLWS) is entered as the first remark in the pilot report. LLWS may be reported as minus (-), plus (+) , or plus and/or minus (+/-) depending on how it effects the aircraft. If your aircraft suddenly speeds up during landing, then just as you have compensated it just as suddenly slows or drags, this would be considered +/-.

As summertime causes uneven heating of the earth’s surface, and the jet stream looks like ocean waves across the upper air charts, you can expect to deal with wind shear in many different forms. Whether you brief yourself or call Flight Service, keep a sharp eye out for conditions that will cause it.

References:

http://www.srh.noaa.gov/jetstream/tstorms/windshear.htm

http://www.faa.gov/airports_airtraffic/air_traffic/publications/atpubs/FSS/fss0902.html#Section%202.%20Pilot%20Weather%20Report%20(UA/UUA)

Rose Marie Kern is a Flight Service Specialist at Lockheed Martin’s DCA Flight Service. This article was written with the assistance of Lockheed Martin specialist Nancy Causey. For questions and comments Rose can be reached at author@rosemariekern.com.