The process in which any form of water that falls from the cloud and reaches the earth surface is called Precipitation.
To have precipitation, cloud droplets or ice crystals must grow heavy enough to fall through the air. On reaching the threshold size, the droplets tend to fall. While moving down, they may collect some more small droplets and become heavy enough to fall out of the cloud as raindrops.
Forms of Precipitation
Depending on the elements of weather or climate of the region, the form of precipitation varies. It drizzle or rains in the warmer parts of the world. But, it may be snow or ice in the colder regions. Common types of precipitation are rain, sleet, freezing rain, hail and snow.
Rain is liquid precipitation. It is the most common kind of precipitation. If the size of raindrops is <\(0.5mm \)in diameter, then it is called a drizzle. If the size is >\(0.5mm \) in dia, then it is known as rain.
Generally, drizzle takes place from stratus clouds.
Sleet is partially frozen rain, i.e., a mixture of water droplets and tiny particles of ice (5mm in
diameter) known as sleet.
When raindrops fall through a layer of air below \(0\)°C, i.e. freezing point of water, the raindrops freeze into solid ice particles. So, the mixture of water droplets and ice particles would fall on the earth surface. Sleet or ice pellets are the rain that falls from clouds but freezes before it reaches the ground.
Freezing Rain
This kind of precipitation occurs when the air near the ground and the ground is at freezing temperature. Here the raindrops do not get enough time to freeze in the air, so the raindrop touches the cold surface as a liquid, but freezes on touching the surface. This is called freezing rain, and the drops of water are usually greater than\( 0.5 \)mm in diameter.
Here, the precipitation occurs as round pellets of ice larger than \(5mm \)in diameter called hail or hailstones.
Hail forms only during thunderstorms in cumulonimbus clouds. A hailstone starts as an ice pellet inside a cold region of a cloud. Strong updrafts in the cloud carry the hailstone up and down through the
cold region many times.
Often, during the fall of temperature below freezing point, water vapour in a cloud is converted directly into snow pieces. The precipitation in the form of a powdery mass of ice is known as snowfall.
It is common in the high altitude of mountains and polar regions.
Snowfall occurs when many individual snowflakes fall from the clouds. Unlike a hail storm, snowfall is usually calm. Hailstones are hard, while snowflakes are soft.
Depending on the temperature and humidity of the air, Snowflakes develop different patterns.