3 days ago
Straight line winds are common source of damage during thunderstorms, but are different than a derecho
If you have broken tree limbs, roof damage, or lost power during a thunderstorm, you may think your house was hit by a tornado. But the far more likely culprit is thunderstorm wind damage, often called straight-line winds.
Straight-line winds are different than tornadoes because instead of rotating, they blow in a straight line.
It all has to do with warm air rising and cool air sinking.
A thunderstorm forms when warm, moist air rises in an updraft, condensing into a large cumulonimbus cloud. Inside of that cloud, tiny droplets of water collide as violent winds clash. Some of the droplets coalesce into larger and larger water drops, eventually forming rain that is too heavy to stay suspended and falls toward the ground.
As the falling rain in a thunderstorm gets heavier and more widespread, this large column of falling raindrops cools the surrounding air. The cooling is even more pronounced if there is dry air present, as evaporating raindrops cool the air further. Since cool air is more dense and heavier than warm air, it drops to the ground as a downdraft.
When the area of rain-cooled air hits the ground, it has nowhere to go but outward in all directions. While a location directly under the column of sinking air may experience this as a downburst or microburst, areas surrounding it will experience strong straight-line winds.
Small tree limbs can break in wind gusts over 40 miles per hour. If wind gusts reach or exceed 58 miles per hour, the storm is considered severe.
If a complex of thunderstorms produces a long-lasting, large swath of damaging winds, it may be classified as a derecho.
A derecho is defined by the National Weather Service as a complex of thunderstorms that produces a relatively unbroken swath of wind damage at least 240 miles in length, with wind gusts of at least 58 miles per hour along its path. Some derechos have left damage swaths more than 1,000 miles in length.
Seventy percent of derechos occur during the hot summer months. Derechos are most common across the Plains, Midwest and Great Lakes, where communities are impacted on average once every year.
