The water droplets that come together to form clouds are very small。 They are usually less than 0。02 mm in diameter。 The average raindrop is a hundred times bigger、 with a diameter of 2 mm。 How does ma a microscopic cloud droplet grow into a raindrop that is so much bigger? In middle latitudes、 precipitation often begins to form high in the tops of clouds where temperatures are below freezing。 Water droplets in the tops of clouds are supercooled。 This means that they do not freeze at 0 ° C。 Instead、 they may not freeze until temperatures reach- 40 ° C。 Ice crystals are also present in the cold 3 tops of clouds。 When the air is saturated and can hold no more water vapor、 the vapor in the air begins to condense。 It turns be to ice and accumulates on the ice crystals。 The ice crystals grow in size until they become too heavy to remain aloft。 As they fall through the cloud、 the ice crystals come into contact with more water droplets。 The water droplets freeze onto the ice crystals。 The ice crystals grow into delicate snowflakes。 Nearly all precipitation begins as snow in the high parts of clouds.