Q:

How is rain created?

A:

Quick Answer

Rain is the most visible part of the water cycle, which is a global engine for distributing heat. Water that is above freezing is almost always subject to some evaporation. As water evaporates, it rises into the atmosphere. Normally, this water vapor is invisible, though it can be felt as mist or humidity.

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Full Answer

When conditions are right, the atmosphere in a given place is capable of holding a great deal of water. This usually takes the form of cloud cover. Sometimes, clouds are made mainly of tiny ice particles, and sometimes they are warm enough to support liquid water droplets. As the molecules of water vapor coalesce, usually described as condensation, they form larger and larger drops that are carried in the cloud by unpredictable thermal updrafts.

Eventually, the drops of water grow too large and heavy to be supported by the internal uplift of air currents inside the cloud. This point can be reached fairly quickly if the relative humidity is at 100 percent and the local temperature falls rapidly to match the dew point. When these conditions are met, the cloud can no longer hold its water, which then precipitates out as rain. If the temperature inside the cloud is close to freezing, this precipitation can also fall as hail.

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