What Causes Rain?

causes-rain Credit: Jose Luis Pelaez Inc/Blend Images / 360/Getty Images

Any time the droplets in clouds grow and come together to form drops that are large enough to create a speed of falling that is greater than the speed at which the cloud is blowing upward, then they head downward, and if they make it down without evaporating, people on the ground experience them as rain or snow. Sometimes rain evaporates again and heads back up into the clouds. The greater the amount of water vapor under the cloud and the stronger the winds on the updraft, the more likely precipitation becomes.

If there are clouds out and about but no rain, then the cloud lacks enough water vapor to form the precipitation or the cloud doesn't have enough rising motion inside of it. When air masses are warmer, rain usually takes place inside localized clouds featuring harder updrafts. When air masses are cooler, rain or snow happens within low pressure zones hosting larger cloud systems, generally appearing at the edge of cold and warm air masses.

Because the Earth is a closed system, the precipitation that takes place each year equals the amount of evaporation. Rain or snow always begins as water vapor that evaporates upward from the planet's surface.