Why is there a calm before a storm?

Sometimes there’s calm before a storm, and sometimes there isn’t.

Here’s how a calm before a storm might happen, in the simplest kind of storm – a single-cell thunderstorm. In this type of thunderstorm, warm, humid air near the ground rises. As it rises, the water vapor in it cools and condenses into tiny droplets that form clouds. These droplets clump up and build on larger particles like dust, until they grow large enough to form raindrops. The rising warm air forms a partial vacuum, which pulls cold air from high above. That helps drive the rain down. But this partial vacuum also pulls in air from all sides of the storm front. Air moving away from the partial vacuum gets pulled back – so the area in front of the storm experiences a calm. Hence the calm before the storm.

Most thunderstorms, though, don’t start with a calm. That’s because most are actually clusters of storms with complex wind patterns. There’s so much air moving up and down in the vicinity of these storm clusters that the calm before the storm never happens. And instead, before the storm, it might be really windy!

However, there are certain factors – namely, wind and atmospheric pressure – that can combine to produce calm before even a complex or “supercell” thunderstorm. One such scenario could involve, for example, cyclone-like winds and low pressure in the rear quadrant of an approaching supercell. If there are very strong high pressure surface winds moving ahead of the storm, these winds can calm as the rear of the storm moves closer.

Our thanks to:

Dr. Dev Niyogi
Associate Professor
Department of Earth and Atmospheric Sciences
Purdue University

Dr. Ernest Agee
Department of Earth and Atmospheric Sciences
Purdue Universitya

May 24, 2010

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