Quick Answer
Indicators of imminent thunderstorms include tall, dense cumulonimbus clouds, towering cumulus clouds with flat bases and rounded tops, and low-hanging stratus clouds with virga, indicating heavy precipitation.
Reading Clouds for Storm Indicators
When observing clouds for signs of impending thunderstorms, focus on cumulus and stratocumulus clouds. A tall cumulus cloud can grow into a towering cumulus or even a cumulonimbus cloud, which may indicate a severe thunderstorm. If you see a cumulus cloud with a flat base and rounded top, be aware that it may develop into a thunderstorm within 30 minutes.
Identifying Cumulonimbus Clouds
Cumulonimbus clouds are the most severe and intense storm clouds. They are dark and towering, extending up to 10 km (6.2 miles) into the atmosphere, with vertical growth exceeding 1,000 meters (3,300 feet) per hour. These clouds are characterized by towering vertical growth, an anvil-shaped upper surface, and a flat base. If you spot a cumulonimbus cloud, be prepared for heavy precipitation, hail, lightning, and strong winds.
Fronts and Low-Hanging Stratus
Low-hanging stratus clouds with virga (precipitation visible in the air but evaporating before reaching the ground) can indicate the approach of a front, which may bring thunderstorms. Virga is a sign that precipitation is falling, but it’s being evaporated before reaching the surface. This can be a sign that the air is unstable, and a thunderstorm may develop.
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