The term “straight-line winds” is used to describe ground-level winds that come out of a thunderstorm and do not have rotation. If these winds travel at speeds that exceed 57 mph, then the storm is classified as a severe thunderstorm. Storms with severe straight-line winds can also have hail and tornadoes.
Straight-line winds can cause considerable damage because these winds often do not let up. Straight-line winds will push objects over, all in the same direction as the wind is blowing.
The lack of rotation, or spin, in these straight-line winds allows meteorologists to differentiate damage from tornadic winds. Tornadoes scatter objects all over because they rotate so quickly.
Straight-line winds can be hazardous as they can push over objects that land on top of people, causing injury and death. On July 1, straight-line winds blew down trees in forested areas of northwest Wisconsin. Many areas had wind speeds of greater than 60 mph, with some wind speeds greater than 100 mph.
Thunderstorms have upward air motions, called updrafts. These supply warm moist air to the storm and help to form the precipitation. There are also downdrafts, or sinking air in a storm. Such downdrafts carry air from high elevations in the atmosphere rapidly to the ground. Since wind speed is nearly always much larger at high elevations, the downdrafts carry very high momentum air to the surface creating the straight-line winds.
Downdrafts also carry liquid water with them. When these downdrafts hit a region of dry air, such as below the cloud base, the drops evaporate. This cools the air in the downdraft, making it denser and thus causing the air in the downdraft to fall to the ground faster. You can sometimes notice this blast of cool air at the surface, often before it rains.