Wind Shear
4
The other wind problem mentioned previously, the downburst, is also downdraft related. It is an extremely
intense, localized downdraft from a thunderstorm. This downdraft exceeds 720-feet-per-minute vertical velocity
at 300 feet AGL. The power of the downburst can actually exceed aircraft climb capabilities, not only those of
light aircraft, but, as is documented in one case, even a high-performance Air Force jet.
The downburst is usually much closer to the thunderstorm than the first gust, but there is no absolutely reliable
way to predict the occurrence. One clue is the presence of dust clouds, roll clouds, or intense rainfall. It would be
best to avoid such areas.
Wind Shear From Temperature Inversions
Pilots who have flown in the Southwest, Southern California, or Colorado are familiar with this weather pattern.
Overnight cooling creates a temperature inversion a few hundred feet above the ground. When coupled with high
winds from what is known as the low-level jet stream, this inversion can produce significant wind shear close to
the ground.
Figure 3. Temperature inversions.
Turbulence at boundary between
calm, cold air and a low-level warm
air jet stream.
One particularly bothersome aspect of temperature inversion shears is that as the inversion dissipates, the shear
plane and gusty winds move closer to the ground. In some areas of the Southwest, a 90-degree change in direc-
tion and 20- to 30-knot increases in surface winds in a few minutes are not uncommon. Obviously, such a shift
would make an approach difficult at best.
Wind Shear From Surface Obstructions
Wind shear from surface obstruction is generally associated with hangars or other buildings near the runway. The
sudden change in wind velocity can seriously affect a landing.
Another type of surface obstruction—mountains—can also affect wind shear. Some airfields are close to moun-
tain ranges, and mountain passes are close to the final approach paths. Strong surface winds blowing through
these passes can cause serious localized wind shear during the approach. The real problem with such shear is that
it is almost totally unpredictable in terms of magnitude or severity. A pilot can expect such shear whenever strong
surface winds are present.
Types of Wind Shear
Wind shear can be divided into horizontal and vertical shears. Although both components can affect an aircraft
simultaneously, it is easier to discuss each separately.
Horizontal Wind Shear
Horizontal shear occurs when the flight path of an airplane passes through a wind shift plane. Figure 4 shows
how such a penetration would appear as an aircraft crosses a cold front.