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Posted by on Jan 17, 2016 in Tell Me Why |

What Causes Rain?

What Causes Rain?

You know that rain falls from clouds. Water is constantly evaporated from the earth’s surface by the warmth of the sun. The invisible water vapor is carried upward by the warm, rising air.

When the air cools, some of its moisture condenses into tiny droplets of water, which collect to form the clouds we see in the sky.

As the air continues to cool, the clouds become thick rain clouds that darken the sky. The air movement in the cloud causes the droplets to bump into others. Some of the bumping droplets join together and form drops big enough to fall as rain.

There are three different ways of turning moist air into cloud, so that it rains or snows. Rain – and other forms of precipitation – occurs when warm moist air cools and condensation occurs. Since warm air can hold more water than cool air, when the warmer air is cooled the moisture condenses to liquid – and it rains.

Frontal rain

Frontal rain occurs when two air masses meet. When a warm air mass meets a cold air mass, they don’t mix as they have different densities (a bit like oil and water). Instead, the warm less dense air is pushed up over the cold dense air creating the ‘front’. As a result, much like when air is forced up over mountains, the warm less dense air cools, and the water vapor condenses into water and falls as raindrops.

Orographic rain

Orographic rain is rainfall produced as a result of clouds formed from the topography – or shape – of the land. Where there is high ground moist air is forced upwards producing cloud and potentially, precipitation.

Mountainous areas close to prevailing westerly winds are most likely to experience this type of rainfall.

Convective rain

Convective rain is produced by convective cloud. Convective cloud is formed in vertical motions that result from instability of the atmosphere.

One way that the atmosphere can become unstable is by heating from the sun. The ground warms up, causing moisture in the ground to evaporate and rise, and the hot ground also heats the air above it.

As the water vapor rises, it cools and condenses into clouds and eventually rain. When you heat the air from below like this, much like in a boiling kettle, you tend to get “bubbles” of rising air, known as up-draughts.

These are much smaller than the large-scale lifting of air that occurs at fronts and over mountain ranges. Sometimes, you can get all three types of rain at once, and this can lead to severe flooding.

Content for this question contributed by Lori Freeland, resident of Arlington, Gilliam County, Oregon, USA