Why Does it Rain?

Article Updated: 24 Dec , 2015

Many people love to ask, why does it always rain on me? There are those who would like to know why it rains so much when they are sad, when they feel like going out, or only when they decide to jog or take their pet for a walk. There are no easy answers for these arguably subjective questions. However, if one to ask Why Does it Rain, the answer would be much simpler.

For starters, rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. It begins with the vaporization of water near the Earth’s surface, in the form of rivers, lakes, oceans or ground water, provided there are atmospheric temperatures above melting point of water (0°C), and is followed by the condensation of atmospheric water vapor into drops of water heavy enough to fall, often making it to the surface.

Rain occurs when two basic processes occur: Saturation and Coalescence. The first process happens when “invisible” moisture in the air (water vapor) is forced to condense on microscopic particles (i.e. pollen and dust) to form tiny “visible” droplets. The amount of moisture in air is also commonly reported as relative humidity; which is the percentage of the total water vapor air can hold at a particular air temperature.

How much water vapor a parcel of air can contain before it becomes saturated (100% relative humidity) and forms into a cloud (a group of visible and tiny water and ice particles suspended above the Earth’s surface) depends on its temperature. Warmer air can contain more water vapor than cooler air before becoming saturated.

Condensation then occurs when the air is cooled down to its dew point temperature, the point at which it becomes saturated. Coalescence occurs when water droplets fuse to create larger water droplets (or when water droplets freeze onto an ice crystal) which is usually the result of air turbulence which forces collisions to occur. As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain.

Rain is the primary source of freshwater for most areas of the world, providing suitable conditions for diverse ecosystems, as well as water for hydroelectric power plants and crop irrigation. Rainfall is measured through the use of rain gauges and amounts are estimated actively by weather radar and passively by weather satellites.

Precipitation is also a major component of the water cycle, the continuous movement of water on, above and below the Earth, and is responsible for depositing most of the fresh water on the planet. Human activity, in the form of Global Warming, is also causing changes in the global precipitation pattern, including wetter conditions across eastern North America and drier conditions in the tropics.

We have written many articles about rain for Universe Today. Here’s an article about the evidence of rain on Mars, and here’s an article about cloud formations.

If you’d like more info on rain, check out Visible Earth Homepage. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.


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