A wind gust is a pronounced temporary increase in wind speed. When the wind is blowing outside it is common to feel temporary changes in the wind speed. The gusts typically last from 5 to 15 seconds. Why is the wind speed not constant? The answer to this question is because of surface friction. As air collides with objects at ground level it causes the wind to slow down and also for the wind to have turbulence. The air higher above the ground does not experience as much of this friction thus the wind speed tends to be higher in velocity and smoother flowing aloft. The layer of air near the ground surface is called the friction layer. This is where the wind is slowed and turbulent due to friction. This slowing and turbulence is not uniform thus it will cause the wind to have varying velocities. Another reason for gustiness of wind is when air is brought down to the surface from higher aloft since the wind speed is typically greater aloft. Gustiness of wind is more noticeable when the overall wind speed is higher. This is because the force felt by the wind increases at an increasing rate as wind speed increases. For example a change from 5 to 10 miles per hour is less noticed than a change from 15 to 30 miles per hour even though both represent a doubling in wind speed. The 30 mile per hour wind produces more than twice as much force as the 15 mile per hour wind, thus 30 mph feels far stronger than 15 mph. The 5 and 10 mph winds both feel fairly weak since the wind speed is fairly light. Suppose wind gustiness caused the wind speed to increase by a factor of multiplying by 1.5. A 10 mph wind would have gusts of 10*1.5 = 15 mph, but a 30 mph wind will have gusts of 30*1.5 = 45 mph. Thus, higher winds tend to produce gusts that are greater in magnitude above the initial overall wind speed. Since the force of the wind increases at an increasing rate with wind speed also, the gusts produced by initially strong winds will feel much more powerful. |