What is rain, and how does it come? When we put a wet shirt out in the sunshine on a hot day, it is quite dry in a short time. Where has all the water gone that made the shirt so wet? It has been sucked up by the warm air as water vapour. As a rule we cannot see this vapour but when we dry our shirt before a fire, we sometimes see it steaming off like thin mist, and then it disappears in the air.
Warm air is like a sponge, and sucks up water as vapour, and can hold a lot, though we cannot see it. But cold air cannot hold much, and very cold air scarcely any. So if warm air full of moisture, or water vapor, is suddenly chilled, it is like squeezing a sponge when it is full of water. The sponge has to let its water go, and the warm air when made cold has to give up a lot of the water vapour it holds. This vapour first turns into very tiny water drops that float in the air and which we can see; and in that form, we call it mist, fog, or cloud. If the air is made still colder, the tiny drops of the mist flow together and make large drops, till these become so heavy that the air can no longer hold them up, and they fall to the ground as dew and rain.
Now we can understand how the rain comes. When the sun shines on the sea, rivers, lakes, and damp land, it warms the air over them, which sucks up a lot of water in the shape of unseen vapour. Now warm air is lighter than cold air, and so it rises up, while cold air falls to the earth. The warm air full of moisture rises till it gets high up in the sky, where the air is colder. Then it is chilled and so can riot hold all the vapour it has, some of which condenses and can be seen as mist. When the mist is high up in the sky, we call it cloud. Now, these clouds are blown along by the wind over the land; and if they come up against cold mountains, or colder air, the tiny drops that make the clouds run together, and become big drops; and these fall to the earth below as rain.
If we could follow a drop of water from the sea, up to the clouds, down in rain on to the mountains, down streams and rivers back to the sea, we would know how wonderful God’s way of watering His great garden, this world is.