Wind shear is defined as a change in wind speed and/or direction over a short distance, either horizontally or vertically. Low-level wind shear (LLWS) can affect any type of aircraft, but is especially hazardous to helicopters as they often operate closer to the surface. Wind shear is also one of the most common causes of turbulence. This is when the air flow becomes irregular, with vertical eddies and currents.
The National Weather Service (NWS) defines LLWS as “a wind shear of 10 knots or more per 100 feet in a layer more than 200 feet thick (air speed loss or gain of 20 kts or more) which occurs within 2,000 feet of the surface.” The amount of change in wind speed or direction with height will determine the severity of the impact.
The LLWS and turbulence associated with convective activity is well documented and can be usually avoided by staying clear of convective clouds. It can also be detected by Doppler radar with cloud or precipitation particles providing sufficient radar targets. But there are other causes of LLWS (and turbulence) that can produce dangerous flying conditions that are not readily apparent in clear air.
There are a number of situations in which non-convective LLWS is common. Whenever there is a front in the vicinity, you can expect LLWS. Pronounced temperature contrasts typically produce strong winds, and frontal passages are associated with changes in wind direction. Fronts are three dimensional, with the frontal surface above the ground sloping upward towards colder air. They can be accompanied by low-level jet streams, which add to the wind speeds.
Inversions — areas where the temperature increases with height — often have stronger winds or even low-level jet streams at the top of the inversion layer. Frontal surfaces have such inversions, and nocturnal inversion layers can form on clear, calm nights when air near the surface is cooled and becomes colder than the air above.
Another cause for LLWS is an impediment to the wind or normal airflow, such as buildings or topographic features.
How do we know where LLWS is occurring? Pireps (pilot reports) are a major source of LLWS conditions. Of course, this relies on an aircraft encountering the area of LLWS and the pilot reporting it. The Aircraft Meteorological Data Relay (AMDAR) program is another source of such data. Many commercial aircraft are now equipped with meteorological sensors that can report on wind speed and direction during ascent and descent. Major airports typically have wind shear detection systems including Terminal Doppler Weather Radar, but mainly for convective LLWS.
Terminal Area Forecasts (TAFs) can include non-convective LLWS forecasts. WS will be the indicator. The upper height of the wind shear layer (above the surface) will be given as will the wind speed and direction at the top of this layer. This can be compared to the forecast surface conditions. Of course, TAFs will only cover terminals and the immediate vicinity.
If LLWS is expected over a larger area, an AIRMET TANGO can be issued by the NWS Aviation Weather Center. These are separate from, but may be included with, AIRMET TANGOs for turbulence and/or strong winds. AIRMETs, however, cover large areas that may experience LLWS for only a limited time. I’ve talked in the past about the Graphical Forecasts for Aviation tool (GFA) found at the NWS Aviation Weather Center website. Clicking the low altitude icon will give you access to current and hourly forecast wind conditions from the surface to 5,000 feet in 500-foot increments from surface to 2,000 feet above ground level (AGL), and 1,000-foot increments from 2,000 feet AGL to 5000 feet AGL. In particular, G-AIRMETs for LLWS are shown as highlighted polygons. Clicking on the polygon will show you valid times, severity of the shear, and base and top heights of the shear layer.