Precipitation

Under certain conditions, the liquid cloud particles may grow in size and fall into the ground as precipitation. For precipitation, there are two important processes. The collision-coalescence process and the ice crystal process.

1) Collision-coalescence process

When the temperature of a cloud is above 0°C or the temperature of top cloud is warmer than -15°C, the cloud is called a warm cloud. In that kind of clouds, collision between droplets can play a important role in producing precipitation. To produce the many collisions necessary to form raindrop, some cloud particles must be larger than others. They can be formed on large condensation nuclei, or through random collision of droplets.

As some large cloud particles fall, they pass smaller droplets and collide with them in their path. This merging of cloud droplets by collision is called coalescence. But collision does not always guarantee coalescence. A significant factor influencing cloud particle growth by coalescence is the amount of time that the droplet spends in the clouds. So rising air currents do some important role. Because they slow the rate of cloud droplet fall, and the time that cloud droplet spends in a cloud is longer, and it's size grows larger.

It grows until it reaches a size of about 1 mm. At this point, the updraft and the pull of gravity on the droplet is balanced. Then the droplet grows just little bigger, it slowly descends. As the droplet falls, it grows larger and larger because of the other smaller droplets. By the time this droplet reaches the bottom of the cloud, it will become large raindrop with a diameter of over 5 mm. And then, raindrops fall faster and become rainfall to the ground.

2) Ice crystal process

The ice crystal process proposes that both ice crystals and liquid droplet must co-exist in clouds at temperatures below 0 °C. That clouds are called cold cloud. Surprisingly, in the cold clouds, almost all of cloud droplets are liquid. And that kind of droplets are referred to as supercooled. The reason why there are few ice crystal in cold cloud is that liquid cloud droplets need lower temperature to freeze. And to forming ice-crystal, they need ice-forming particles called ice nuclei. And it does not abound in nature.

So in cold, supercooled, saturated cloud, many supercooled liquid droplets will surround each ice crystal. Since the air is saturated, there are more vapor molecules above the droplet. The difference in vapor pressure cause water vapor molecules move from the droplet toward the ice crystal. The removal of molecules reduces the vapor pressure above the droplet and it is now out of equilibrium. So more evaporation occurs. This process provides a continuous source of moisture for ice, and ice crystal grow larger and larger

Ice crystals can also grow to a size where they can break the buoyancy and fall out of the cloud's bottom, And when they pass through a temperature of 0 °C, they will melt. And they become rain drops, also providing the ground with rain.