What is a dry thunderstorm?

01

How can a thunderstorm produce no rain?[1]

The rain falls but never lands. When the cloud base sits high above a deep, dry layer, droplets evaporate on the way down. The storm above is fully electrified and producing lightning while the ground below stays dry.

The visible signature is virga: streaks of precipitation hanging below the cloud that fade out before reaching the surface. On a dry thunderstorm day those streaks can extend for thousands of feet.

Evaporation also cools the air it falls through, which makes it dense and drives it downward. That is why dry thunderstorms are associated with strong, gusty surface winds arriving with no rain, a combination that spreads fire rather than dampening it.

02

Where do dry thunderstorms happen?[1][2]

Mostly over arid and semi-arid interiors: the western United States, interior Australia, the Mediterranean margins and high plateaus. They need moisture aloft for the storm and dry air below it, a combination common over hot, dry terrain.

In the American West they are a recognised fire-weather threat with their own forecast products, because the lightning arrives over cured fuels with wind and without rain. Fire agencies watch specifically for that setup.

03

Why do they start so many wildfires?[2][3]

Because they deliver ignition without suppression. An ordinary thunderstorm wets the fuel it drops lightning onto. A dry one leaves fuels as dry as before, adds gusty outflow winds, and can scatter dozens of ignitions across a landscape in one afternoon.

Lightning-caused fires burn a disproportionate share of total acreage in the US despite being a minority of ignitions, largely because they start in remote terrain and are often not detected quickly.

Holdover fires make it worse. A strike can smoulder in duff or a root system for days before conditions push it into open flame, so a dry lightning outbreak produces fires long after the storm has gone.

04

How do you spot one on radar?[4]

Poorly, which is the problem. Radar shows the precipitation aloft, so a dry thunderstorm can display healthy returns while nothing reaches the ground. The beam is sampling rain thousands of feet up that will evaporate before landing.

This is a case where lightning detection is more informative than radar. Strike positions record what actually happened at the surface, while the reflectivity picture describes a rain shaft that may never arrive.