where we hope you learn about the fascinating weather that affects us day to day. This is a great learning resource for parents and teachers.

  • A rainless desert
    The next time you feel annoyed because it is raining, again, just think what life would be like if there were no rain. No grass, no flowers, no trees, no birds, no bees, no life.

  • Hedgerow after rain
    Many places in the world cannot support any life because they have no rainfall.

What is rain?

Evaporation and condensation

  • The atmosphere on the planet Earth (the air around us) is full of water vapour, which you cannot see.  
  • Vapour is what water turns to when it is heated, like steam out of a boiling kettle. This process is called evaporation.   
  • Most of the water vapour in the atmosphere has evaporated from the world’s oceans.  Of course the oceans do not boil like a kettle but the heat of the planet earth means that there is constant evaporation of moisture from its surface. 
  • When the air is warm, it can hold a lot of water vapour.  However, as the warm air rises it meets colder air.  Cold air cannot hold as much water vapour as warm air.  
  • As the warm air meets the cold air, the water vapour condenses, which means that it turns into tiny drops of water or ice. 
  • These particles (tiny drops of water or ice) join together to form clouds. 
  • As the clouds grow in size the water drops join together and become heavy.  They then fall to the earth and it rains. 

What makes it rain?

Before clouds can form, the air has to rise.

  • As we have seen, warm air naturally rises.  This is because as the air heats, it expands and rises upwards. This is known as convection.  
  • Air is carried upwards when winds meet obstacles such as  mountains.  This is why we often see clouds around mountain tops.
  • A mass of cold air can act just like a mountain if it meets warmer air.  We call this mass of cold air a cold front. The warmer air is carried above the cold air. As the warm air cools, the water vapour in it condenses and turns into rain. 

The wettest parts of the Earth

The wettest areas are those reached by winds which have blown across the oceans for very long distances.  The rain is heaviest in warm, tropical climates where the heat increases the rate of evaporation. 

For example, in parts of Burma more than 390 inches of rain fall in a year (the average rainfall in London is 32 inches and in New York 49 inches). 

The monsoon describes a great rush of moist air from the ocean to the land.  Many tropical countries have monsoons, but the best known monsoon is the Indian summer monsoon.  In June the Trade Winds reach India.  Since they have crossed the Indian Ocean, they are full of water vapour.  When they meet the hot dry air over India they cause tropical storms, which lead to heavy rainfall over much of the country. 

The rainfall can cause rivers to overflow and there is widespread flooding.

How big is a raindrop?

Raindrops are really very small, usually about one hundredth of an inch. Even the largest raindrops are only one tenth of an inch in diameter (from side to side).  

Rain falls from the clouds at a speed of between 2 miles per hour, for very fine drops, and 18 miles per hour for the largest drops. 

More information - Why raindrops are different sizes

Acid rain

Not all rain gives life.  Acid rain kills plants and poisons the fish in rivers. 

We have seen how warm air rises and the vapour in it condenses and turns to rain. 

  • The warm air from cities, from factrories and from car exhausts also rises.
  • The rain that forms from this warm air is often pollutred with sulphur and nitrogen.
  • We call this polluted rain ‘acid rain’.
  • Acid rain can kill trees and other plants.

More information on acid rain

library.thinkquest.org - Good information

epa.gov - fun and interactive activities about acid rain for kids