Cumulonimbus clouds bring thunderstorms because they contain powerful rising air currents that lift moist air high into the atmosphere. As the air rises, it cools, water vapor condenses into droplets and ice, and the released heat helps the updraft grow stronger. Inside the cloud, collisions among ice particles help separate electrical charges. Lightning and thunder can follow.
How a thunderstorm cloud forms
A typical thunderstorm begins when warm, humid air becomes buoyant and rises. It cools as air pressure falls with height. Once the air cools enough, water vapor condenses, forming a growing cumulus cloud. If the atmosphere is unstable and moisture is plentiful, the cloud can build into a tall cumulonimbus cloud, sometimes with a flat, spreading anvil top.
Why does it rain so hard?
The cloud’s strong updrafts can hold droplets and ice aloft while they grow. Eventually, precipitation becomes heavy enough to fall through the upward-moving air. Downdrafts carry cool air and rain toward the ground. Sudden downpours, gusty winds, hail and, in some storms, tornadoes are possible.
Where do lightning and thunder come from?
Within the storm, collisions among graupel, ice crystals and other particles help separate electric charges. When the electrical difference grows large enough, a lightning discharge occurs. Lightning heats surrounding air extremely rapidly; the air expands and produces the sound wave we hear as thunder.
How can you recognize one?
Look for a towering cloud with a dark base and a high, often flattened anvil. The anvil is not proof that severe weather will occur, but thunder is a clear warning that lightning is close enough to be dangerous. Move inside a substantial building or fully enclosed vehicle when thunder is heard. Wait at least 30 minutes after the last thunder before resuming outdoor activities.
Frequently asked questions
Are all cumulonimbus clouds severe?
No. Cumulonimbus clouds produce thunderstorms, but only some storms meet severe-thunderstorm criteria such as damaging wind gusts or sufficiently large hail.
Can a thunderstorm happen without rain where I stand?
Yes. Rain can fall elsewhere in the storm or evaporate before reaching the ground. Lightning can also strike outside the heaviest rain.
For weather alerts and lightning safety, check the U.S. National Weather Service at weather.gov.
Leave a Reply