Not all the ice crystals in a precipitation cloud fall down onto the earth in the form of raindrops. If these drops pass through a layer of very cold air on their way down, their outer surface freezes. They then change into little translucent balls less than five millimetres in diameter. This phenomenon is known as sleet.
But if very strong winds blow through the clouds before falling, the crystals will be drawn up again to the top of the cloud several times by rising air currents. Each time, more ice particles will accumulate onto the crystals. Layer by layer, hailstones will be formed. When they weigh enough to withstand the rising currents, they will finally fall to the ground. A hailstone generally measures between 0.5 and 5 centimetres in diameter. But people have been known to pick up incredible ‘cannon balls’ almost 20 centimetres across, with a weight of about one kilogram, which had fallen at a speed of something like 160 kilometres an hour. These hailstones cause extensive damage to standing crops, buildings and vehicles.
If it is really very cold in the cloud and the temperatures are below freezing all the way down to the ground, the ice crystals will cluster together to form snowflakes. Seen under a microscope, they have countless variety of shapes, depending chiefly on the temperature of the air they have passed through during their fall. They can be classified into 100 separate groups. But every single flake, even if it reaches more than two centimeters across has a common characteristic: they form superb symmetrical geometric shapes, always with six sides.