How Wind Can Change the Danger Level of a Thunderstorm

A shelf cloud looms over the Alberta landscape during a storm in 2014. Shelf clouds are common along the leading edge of a squall line. Image credit: Jeff Wallace via Flickr // CC BY-NC 2.0

If you’re lucky, you live in a part of the world where you’ve never had to deal with damage from a nasty thunderstorm. The occasional downed tree branch or loosened piece of vinyl siding is common with even a stiff breeze, but a real bear of a storm—the kind that makes you afraid to gawk at it from your living room window—can do some serious damage in a hurry. Not all thunderstorm winds are created equal, and it’s important to know the difference between the types of winds a severe torrent can throw your way.

The two types of damaging winds you’ll experience in a thunderstorm are called straight-line winds and tornadoes. The latter doesn’t need much explanation—a tornado is a rapidly rotating column of air that stretches from the base of a thunderstorm to the ground. The United States sees about 1000 tornadoes a year, the vast majority of which touch down in the central and southern parts of the country. A decent number of those tornadoes are relatively weak, but a couple of them each year grow powerful enough to scrub even a well-built home clean from its foundation.

Odds are you’ll never see a tornado in your life, let alone find yourself in one’s path. Straight-line winds, on the other hand, are present in just about every thunderstorm, and sometimes they can cause just as much damage as a tornado.

Unlike the swirling winds of a tornado, straight-line winds all blow in the same direction. We experience these gusts every time there’s a thunderstorm; the cool burst of air you feel ahead of a storm’s arrival is the outflow produced by the storm’s downdraft, which is the rain-cooled air that sinks out of a storm and rushes out ahead of it. However, we typically reserve the term “straight-line winds” for gusts that reach severe limits, which in the United States is defined as 58 mph or stronger.

The most common types of straight-line winds you’ll encounter are downbursts, microbursts, and squall lines.

A microburst—the bulbous area of rain and hail beneath the base of the storm—speeding toward the ground during a thunderstorm. Image credit: NOAA via Flickr // CC BY 2.0

A downburst is a storm’s downdraft that’s strong enough to cause damage. Downbursts can be as large as a small town, covering an area a couple of miles in diameter. Microbursts are smaller, more localized downbursts that can be as small as a couple of city blocks. The strongest microbursts can reach speeds in excess of 100 mph when they slam into the ground and radiate away from the point of impact.

The abruptness of a microburst can rip roofs off of homes, mow down acres of trees in seconds, and cause arriving and departing aircraft to lose altitude and crash. Thankfully, like downbursts, we usually have the ability to detect them with enough time to warn folks to get out of harm’s way.

Squall lines occur almost every day during the warm season. If upper-level winds and instability levels are just right, single thunderstorms can congeal into a line that shares an updraft and a downdraft, moving as one entity that takes on the appearance of a semi-circle or an archery bow on weather radar (hence the squall line’s other common name, a “bow echo”).

A weather radar image shows an immense squall line stretching across Kansas on June 15, 2012. Image credit: Gibson Ridge

A squall line can be just as dangerous, if not more dangerous, than a tornado. These lines of storms can grow hundreds of miles long and stream along a path that stretches more than 1000 miles in extreme instances; these long-lived squall lines are called derechos. That’s a whole lot of real estate that’s exposed to destructive winds, with damaging gusts so intense that some people swear that they were hit by a tornado or a freak hurricane.

It’s important to know the difference between these types of damaging wind events because it reinforces how serious a severe thunderstorm is, even if there’s no threat of a tornado. Severe thunderstorms with destructive straight-line winds can cause extreme damage with a scope that goes beyond anything a tornado could ever hope to destroy.

Consider the ferocious derecho that ripped through the Great Lakes region on July 4, 1999, resulting in a massive forest blowdown; people caught in the path captured a small portion of the horrifying storm on video, showing the winds snapping tall pine trees with ease. Or the derecho in Kansas back in May 2009, which produced such strong winds that they punched a dent in the side of an 80,000-gallon barrel at an oil refinery. In addition to the rare derecho, the National Weather Service receives thousands of reports of wind damage every year from thunderstorms, ranging from downed trees and power lines to mobile homes tossed and structures shorn of their roofs.

Every severe thunderstorm is dangerous. If you ever find yourself in the path of one and hear that damaging winds are on their way, treat it as seriously as a tornado. While not as pretty or photogenic as a tornado, other winds can cause damage that is just as severe.