Why is it Raining? Snowing? Hailing?
Clouds are made of water droplets. But what causes these droplets to rain, snow, or hail down on us? The water droplets that make up clouds are extremely tiny — so tiny that even the most gentle air currents will keep them floating in the sky.
What causes these droplets to fall from a cloud?
Much rain begins inside a cloud as bits of ice, not drops of water. The upper parts of a very tall cumulonimbus, or thundercloud, are cold enough for water vapor to freeze and form tiny ice crystals.
Cumulonimbus (from Latin cumulus, «heaped» and nimbus, «rainstorm») is a dense, towering vertical cloud, forming from water vapor carried by powerful upward air currents.
Like seeds, these crystals grow in the cloud. As more water vapor condenses on the seeds, they grow bigger. When the crystals grow, little pellets of ice are formed. When the pellets become too heavy for air currents to hold up, they fall. As they fall down through the tall cloud, the pellets pass through warmer air. The warm air melts them and they turn into tiny raindrops. As they fall, the tiny raindrops bump into each other and form bigger drops. The average raindrop has about a million times as muchwater as a tiny water droplet in a cloud.
Can it rain if a cloud is too warm for ice crystals to form?
Only if the cloud is over the ocean. The droplets that form clouds always condense on microscopic particles in the air. Seed crystals must be formed in clouds before it rains. When it is warm, ice crystals can't form, but clouds that are over or very near the ocean make a different kind of seed. This time, the seeds for the raindrops are tiny crystals of salt from the ocean. Water droplets soon condense around the salt crystals. The water droplets bump. intoeach other and form bigger drops. Before long the drops are big enough and heavy enough to fall as rain.
What happens when the air both high in the sky and close to the ground is very cold?
The tiny ice crystals that form inside clouds do not melt and turn to rain. Instead, they become crystals of ice. As they fall through the cloud, they gather more ice crystals and grow bigger. By the time they reach the bottom of the cloud, they have beautiful, intricate shapes. Can you guess what these are? Snowflakes!