Elizabeth Clautic6, age 13, of Ottawa, Ont., for her questions
How do the clouds make rain?
This is one of those problems which looks very simple and turns out to be quite complicated. Clouds, of course, are made of mist hnd mist is a floating mass of tiny droplets of water. Rain is large drops of water. At first, you might think that there is no Ir oblem in making large drops of water from an abundance of tiny droplets. But when we look closely at that misty cloud, we wonder how in the world those tiny droplets of moisture ever gel into raindrops.
Let's scale ourselves down until the misty droplets in a cloud are as big as baseballs. A cloud is made from countless numbers of these dewy baseballs, floating in the air and drifting around. Strange to say, very few of them crash into each other. Why? Because there is so much space between them. On this enlarged scale, there might be ten or twenty of them drifting around in an area as big as a baseball park.
It will need many thousands of these droplets to make one raindrop. So we begin to wonder how they gather together in the misty cloud. If we could really scale ourselves down, we should see that there are other things floating around in the misty cloud besides dewy droplets. There are fragments of dust and soot, fragments of ice and salt. The salt is taken up into the air from tossing sea spray. The ice is frozen vapor or water and the dust and soot come from below.
These solid particles play a vital role in the making of raindrops from misty droplets. As they travel about through the misty cloud, tossed this way and that, they crash into the dewy droplets and gather themselves jackets of moisture. Each one acts as a nucleus, or core. The more droplets it gathers, the bigger it becomes.
The bigger it becomes, the more droplets it crashes into and adds to the growing raindrop.
A cloud, we believe, cannot turn into raindrops without the help of these tiny nuclei. When this was discovered, we were able to help certain clouds to turn into raindrops. This is what cloud seeding is all about. Fragments of ice or other chemicals are sprayed into the clouds and, when conditions are just right, these fragments act as the nuclei to form raindrops from cloud droplets.
The misty droplets in a cloud are light and small enough to float in the air for a long time. But when numbers of them gel into raindrops, the moisture becomes heavier, too heavy to float aloft in the air. When there is a strong updraft from below, the cloud may go on floating even after it becomes heavy with raindrops. But finally the load of raindrops becomes so heavy that it must fall then down tumbles the rain.