Engineering The design of buildings must account for wind loads, and these are affected by wind gradient. The respective gradient levels, usually assumed in the Building Codes, are 500 meters for cities, 400 meters for suburbs, and 300 m for flat open terrain. For engineering purposes, a power law wind speed profile may be defined as follows: The wind gradient can create a large
bending moment in the shaft of a two-bladed turbine when the blades are vertical. The reduced wind gradient over water means shorter and less expensive wind turbine towers can be used in windparks which are placed in (shallow) seas. For wind turbine engineering, a polynomial variation in wind speed with height can be defined relative to wind measured at a reference height of 10 meters as:
Gliding In gliding, wind gradient affects the takeoff and landing phases of flight of a
glider. Wind gradient can have a noticeable effect on
ground launches. If the wind gradient is significant or sudden, or both, and the pilot maintains the same pitch attitude, the
indicated airspeed will increase, possibly exceeding the maximum ground launch tow speed. The pilot must adjust the airspeed to deal with the effect of the gradient. When landing, wind gradient is also a hazard, particularly when the winds are strong. As the glider descends through the wind gradient on final approach to landing, airspeed decreases while sink rate increases, and there is insufficient time to accelerate prior to ground contact. The pilot must anticipate the wind gradient and use a higher approach speed to compensate for it. Wind gradient is also a hazard for aircraft making steep turns near the ground. It is a particular problem for gliders which have a relatively long
wingspan, which exposes them to a greater wind speed difference for a given
bank angle. The different airspeed experienced by each
wing tip can result in an aerodynamic stall on one wing, causing a loss of control accident. The rolling moment generated by the different airflow over each wing can exceed the
aileron control authority, causing the glider to continue rolling into a steeper bank angle.
Sailing In
sailing, wind gradient affects
sailboats by presenting a different wind speed to the
sail at different heights along the
mast. The direction also varies with height, but sailors refer to this as "wind shear." The mast head instruments indication of
apparent wind speed and direction is different from what the sailor sees and feels near the surface. According to one source, the wind gradient is not significant for sailboats when the wind is over 6 knots (because a wind speed of 10 knots at the surface corresponds to 15 knots at 300 meters, so the change in speed is negligible over the height of a sailboat's mast). According to the same source, the wind increases steadily with height up to about 10 meters in 5 knot winds but less if there is less wind. That source states that in winds with average speeds of six knots or more, the change of speed with height is confined almost entirely to the one or two meters closest to the surface. This is consistent with another source, which shows that the change in wind speed is very small for heights over 2 meters and with a statement by the Australian Government Bureau of Meteorology according to which differences can be as little as 5% in unstable air. In
kitesurfing, the wind gradient is even more important, because the
power kite is flown on 20-30m lines, and the kitesurfer can use the kite to jump off the water, bringing the kite to even greater heights above the sea surface.
Sound propagation Wind gradient can have a pronounced effect upon sound propagation in the lower atmosphere. This effect is important in understanding sound propagation from distant sources, such as
foghorns,
thunder,
sonic booms,
gunshots or other phenomena like
mistpouffers. It is also important in studying
noise pollution, for example from
roadway noise and
aircraft noise, and must be considered in the design of
noise barriers. When wind speed increases with altitude, wind blowing towards the listener from the source will refract sound waves downwards, resulting in increased noise levels downwind of the barrier. These effects were first quantified in the field of highway engineering to address variations of noise barrier efficacy in the 1960s. When the sun warms the Earth's surface, there is a negative
temperature gradient in atmosphere. The
speed of sound decreases with decreasing temperature, so this also creates a negative
sound speed gradient. A wind speed gradient of 4 (m/s)/km can produce refraction equal to a typical temperature
lapse rate of 7.5 °C/km. Higher values of wind gradient will refract sound downward toward the surface in the downwind direction, eliminating the acoustic shadow on the downwind side. This will increase the audibility of sounds downwind. This downwind refraction effect occurs because there is a wind gradient; the sound is not being carried along by the wind. There will usually be both a wind gradient and a temperature gradient. In that case, the effects of both might add together or subtract depending on the situation and the location of the observer. In the case of transverse sound propagation, wind gradients do not sensibly modify sound propagation relative to the windless condition; the gradient effect appears to be important only in upwind and downwind configurations. For sound propagation, the exponential variation of wind speed with height can be defined as follows: because they could not hear the sounds of battle only six miles downwind. Scientists have understood the effect of wind gradient upon
refraction of sound since the mid-1900s; however, with the advent of the U.S.
Noise Control Act, this refractive phenomenon was widely used beginning in the early 1970s, chiefly in the consideration of noise propagation from
highways and resultant design of transportation facilities.
Wind gradient soaring is an expert in
dynamic soaring using the wind gradient. Wind gradient soaring, also called
dynamic soaring, is a technique used by
soaring birds including
albatrosses. If the wind gradient is of sufficient magnitude, a bird can climb into the wind gradient, trading
ground speed for height, while maintaining airspeed. By then turning downwind, and diving through the wind gradient, they can also gain energy. ==See also==