How a Thunderstorm Forms

Thunderstorms are common occurrences in many areas around the world, and provide vital precipitation for survival on Earth.  They are often common in late spring and summer.   The storms often bring strong winds, lightning, and rain, but normally peak in less than 30 minutes.

Thunderstorms begin to form when warm water vapor rises in an unstable portion of the atmosphere.  In a process similar to that which forms fluffy cumulus clouds, masses of water moisture travel upward, expanding until they begin to cool off, approaching the temperature of the colder air high in the atmosphere.  As water droplets form from the cooling vapor, they release latent heat from the reaction, and the warmed air continues to rise higher and higher.

The mass of air rises quickly, and a cloud is formed as more water vapor condenses.  This cumulonimbus cloud may be up to 9 miles high, depending on the strength of the updrafts.  At a certain height, the cooling air cannot rise any further, and it spreads out in the shape of an anvil.  The water vapor condenses into droplets, which join together and become larger.  Eventually the particles grow too big and the updrafts cannot support their weight.  When they fall, they melt to become rain.

If the updrafts are strong enough, the particles that form in the cumolonimbus cloud will continue to accumulate water in a process known as riming.  The particles will keep falling and being pushed by the updrafts until they are heavy enough to fall under their own weight.  Hail and freezing rain falls when these particles do not completely melt before hitting the ground. 

As particles fall to the ground, they create downdrafts that can be very powerful.  Moving updrafts and downdrafts that occur simultaneously lead to strong winds on the surface.  Lightning occurs when the positively charged air at the top of the cloud meets the negatively charged air in the middle of the cloud.  When this discharge happens, the air molecules rapidly expand and contract, causing the sound commonly recognized as thunder.

Thunderstorms begin to dissipate when their supply of moist, warm air is cut off.  Downdrafts carry the cool air back to the surface, often resulting in strong winds and rain in what is known as a downburst.  A downburst normally begins 20-30 minutes into the thunderstorm, and causes it to dissipate quickly. 

Thunderstorms are a common but potentially dangerous component to weather on Earth.  They form from warm masses of air and moisture that rise to create large cumulonimbus clouds high in the atmosphere, and produce a variety of weather conditions, including rain, hail, wind, and lightning.