When there is enough water vapour in the air, and when it’s cool enough.
If you take a glass of water, what do you see? You see a liquid, which is colourless and odourless. It looks like it’s standing still, but actually, water is made of molecules. A molecule is too small for you to see, but it’s there in everything.
In ice, the molecules are very tightly packed. When you warm ice, the molecules get restless, start moving around, and break free from each other to form water. When you warm water still more, the molecules move further apart, and form water vapour.
This can still happen when it is cold, if the water is left open for long enough. This is why water disappears when you leave a glass uncovered. The water molecules at the surface, which are in touch with the air above, try to break free from the liquid water below. This is called evaporation.
All around the world, water evaporates from all the water bodies – lakes, ponds, rivers, the seas and oceans. This releases water vapour into the atmosphere. The amount of water vapour in the atmosphere is called humidity.
When the humidity increases, and the temperature drops, water molecules in the water vapour in the atmosphere start getting together to form liquid drops of water. This is called condensation. Once there enough water drops fuse together, they get too heavy to be floating around in the air, and they fall down, usually as rain. This is called precipitation.
Evaporation, condensation and precipitation together form the water cycle. This is how water on the earth is moved around from place to place, and we get enough to drink and use.
Whether precipitation occurs as rain, snow, sleet or hail depends on the temperature of the place. Sometimes, changes in the weather can be very sudden, causing thunderstorms. At other times, the sun can shine and it can rain at the same time, causing a rainbow.