You can have an interesting conversation with a child along the following lines:
Where does the rain come from?
The sky!
But where in the sky? / What do you notice about the sky when it is raining?
From the clouds.
How do you think the water gets into the clouds?
This can lead on to the fact that water evaporates - especially when it is sunny. You could get them to do a little experiment, by wetting a non-porous surface (a plate or tile, for example) and seeing how long it takes to dry out if left in the sun. Where has the water gone? It disappears into the air as water vapour, which is invisible.
How does it form a cloud? On a cold day they can form their own cloud simply by breathing - the water vapour in their breath condenses into tiny water drops when it hits the cold air. Notice how much it looks like a small cloud. That's all a cloud is - a lot of little water droplets. If it's not a cold day, then you can produce a cloud by boiling a kettle. If you place a cool surface in the steam you can show them that water condenses onto it. It gets colder with height (by about 1C every 150 metres) so water vapour condenses if it rises high enough, forming clouds. When enough water builds up in a cloud the droplets grow heavier and fall as rain.
Clouds can be divided into different types and older children could try to identify them. You can find a useful identification guide from the Met Office here. Which types of clouds produce rain? Younger children could draw a picture (e.g. of your house) and stick on some cotton wool for clouds.
Wind is another important feature of the weather. If you have a weather vane in the garden then children can use this to see which way the wind is blowing. If not, they could make a simple weather vane of their own - see this WikiHow page for instructions. This vane will not be weather-proof, but they can carry it outside to make a measurement. You will need a compass to orient the vane, unless you already know which way is north in your garden, or you could use the fact that the sun is in the south at midday (or at 1pm in British Summertime).
Here are some questions for them to explore with their weather vane. From which direction does the wind blow most of the time? Does it feel colder when the wind is in the north and warmer when it comes from the south? Does it ever blow from the east, and how does it feel then? Do clouds always seem to move downwind? (It is possible for the wind at height to differ in direction from the wind on the ground.) You can judge which clouds are highest by seeing how fast they move in the wind - the higher the cloud, the slower it will seem to move. This is especially striking on a day when high cirrus and low cumulus are present in the sky together. Of course, investigating the wind could naturally lead on to a bit of kite flying!
