Supercell thunderstorms are severe

In the last couple of articles, we looked at the ingredients needed to produce a severe thunderstorm: heat, humidity, lift and strong winds aloft to vent the storm.

This week, we’ll look at what can take a severe thunderstorm and turn it into one that people remember for years.

Let’s start with a hot, humid air mass.

The air several thousand feet above the ground is much colder, providing strong lift, and a jet stream overhead is helping remove air from the top of the storm.

All the ingredients are in place for severe weather, but what can Mother Nature add to make things even more intense?

The first, and probably most important, extra ingredient is wind shear.

Wind shear occurs when the wind changes direction with height. Since the atmosphere is three-dimensional, winds at the surface can blow from one direction while winds higher in the atmosphere blow from another.

Why does this matter?

Imagine a rising column of air. Near the ground the wind is blowing from the south, but a few thousand feet higher it shifts to the east, and higher still it blows from the northwest.

As the air rises through these different wind layers, it begins to twist and rotate. This rotation can dramatically strengthen a thunderstorm.

First, it helps create a localized area of low pressure within the storm. Since air naturally flows toward areas of lower pressure, this encourages even stronger upward motion and helps fuel the thunderstorm.

Second, rotation helps separate the storm’s updraft and downdraft.

In a typical thunderstorm, rain-cooled air eventually falls back to the ground and cuts off the storm’s supply of warm, moist air. Once this happens, the storm begins to weaken.

A rotating thunderstorm can avoid this problem.

The updraft remains separated from the downdraft, allowing warm, moist air to continue feeding the storm. As a result, these storms can survive for several hours and often become much stronger than ordinary thunderstorms.

The longer lifespan allows the storm to produce larger hail, stronger winds, heavier rainfall and in some cases tornadoes.

While scientists are still working to fully understand tornado formation, we do know that most significant tornadoes develop from rotating thunderstorms known as supercells.

It is believed that as the rotating column of air becomes concentrated into a smaller area, wind speeds increase dramatically, eventually producing a tornado — more on this in upcoming articles.

Of course, thunderstorms rarely behave exactly like they do in textbooks.

Sometimes all the ingredients appear to be in place and very little happens. Other times, a key ingredient seems to be missing and severe weather develops anyway. That uncertainty is part of what makes weather so fascinating.

In the next issue, we’ll take a look at the much more common garden-variety thunderstorm and examine why most summer storms live fast, die young and rarely become severe.