Wind gradient is the reduction of the speed of the
prevailing wind as one descends through the air to the
surface (of the Earth). The cause of the gradient is simply
friction with the surface or objects on the surface. The wind gradient is usually found to start within 60 meters of the ground, though it can be perceived at up to twice that height or may be confined to 30 meters or even a little less, from the surface (vis a vis
light aircraft)
. There is often a small
direction change to the wind along with the slowing. Wind gradient affects aircraft that are landing or descending to fly close to the surface, especially motorless aircraft that operate at low speeds, such as
sailplanes and
hang gliders.
A wing depends on
airspeed to produce
lift, and that airspeed can be seen as the
vector sum of the wing's motion
relative to the surface and the air's motion relative to the surface. In in simplest terms, a wing moving 15
MPH North, relative to the surface, against a 10 MPH South wind has an airspeed of 25 MPH. Suppose the wing has a
stall speed of 22 MPH, and the aircraft is some type of
glider, such as a hang glider. If the pilot were to make a
control input that caused the wing to slow to 20 MPH and then cease that input, the wing would stall and the aircraft would drop its nose and
dive, gaining speed relative to the air and the ground, until it reached an airspeed and
angle of attack such that it would begin producing lift again and
stabilize (hang gliders are
convergent in
pitch). This
recovery usually takes at least 30 to 60 meters of vertical displacement. If a glider were flying in conditions as described above (against a wind) and the wind were to suddenly drop 5 MPH the recovery process would be exactly the same. This is all fine as long as the glider is more than 60 meters from the ground
Now suppose that same hang glider is preparing to
land, and is
flying as first described, but at an
altitude appropriate for maneuvering to setup a
landing (
approach altitude), say 80 meters, and there is a wind gradient such that the wind speed on the ground is only 3 MPH from the North, instead of the 10 MPH up higher. The pilot flies the glider at some speed just above stall and descends through the gradient, losing airspeed as he concentrates on his landing target. At some point, say at 40 meters, the wing's airspeed drops below stall speed, and the wing dives to
compensate. This is a problem, since the
pilot and glider will
impact the ground before the wing starts flying again, and when the wing is
not flying the pilot has virtually no control over the system's behavior.
This is one of many reasons why it is important that one should set up a landing approach at an airspeed well above the aircraft's stall speed.
Wind gradient, as stated, is primarily due to friction between the air and the surface - air is a
fluid, after all, and the rules of
fluid dynamics apply. As one might imagine,
brush covered fields can enhance wind gradient as compared to large fields of short grass. Tree lines, small hills, buildings, and other obstructions, especially if fairly densely distributed, all contribute to gradients.
Micro- or macro-
meteorological conditions, such as
catabatic winds, can also produce strong (and suprising) wind gradients.