Wind gusts – why doesn't the air flow smoothly?
Wind gust speed is the maximum wind speed measured in a very short time period. In meteorology wind gust is defined as the 3 second average wind speed.
In an isolated gust the wind speed is typically 1,5–2,5 times greater than the 10 minute average wind speed. In thunderstorms the difference between gusts and average wind speeds can be even greater.
Typically the difference between wind speed averages and gusts is smaller over large open water surfaces than over land. Over land wind gusts can sometimes be quite unexpected, and they may cause damage.
Wind gusts are a chaotic phenomena, so in isolated gusts wind directions can seem quite arbitrary. Because of this, wind direction is not mentioned when talking about gusts, but only when talking about wind speeds averaged over several minutes.
Buildings and terrain obstacles increase wind gustiness
Wind gusts are formed, when small vortices appear in smooth air flow. These vortices can either locally weaken or strengthen the larger scale air flow.
Vortices are formed especially easily close to the ground, where friction and vertical airflow caused by solar heating enable their formation. Because of this, speed differences between gusts and average wind speeds are smaller.
Strong winds are always somewhat gusty, because terrain shapes and other obstacles disturb the wind flow. The greater wind speeds are, the more easily obstacles disturb the flow.
Friction caused by the ground is smaller on an open field than over a forest, let alone human-built environments with lots of buildings. Winds can locally increase in valleys and on streets between buildings, if their shape coincides with the direction of the wind.
When do gusts appear?
Gusts are especially common in areas where the air mass is fairly cold, but the sun heats the surface. In these cases the air starts to rise up as it is heated by the warm ground. The air rises in pillars.
To conserve mass, air starts to descend between the pillars of rising air. This descending air carries windier air from aloft to closer to the ground, and gustiness at ground level increases.
In mid- and high latitudes the windiest and gustiest days inland are often sunny and cool spring days. Similar situations can occur over large water bodies in fall and early winter, when water temperatures are higher than the ambient temperature.
Sometimes the vertical temperature profile of the atmosphere can prevent the formation of gusts. On very cold winter days in high latitudes the coldest air stays close to ground and instead of the temperature falling with height, it actually rises.
These so called inversion layers are very stable and resistant to vertical air flow. Close to the ground there is often practically no wind, even if it was windier at the height of a few hundred feet.
Also in hot high pressure weather vertical temperature profiles of the air can often prevent strong wind gusts from forming.
Article last updated 8/6/2021, 8:10:00 AM