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 ot 2 mm. How does 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 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 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. The kind of precipitation that reaches the ground depends mainly on temperatures in the lower parts of clouds and near Earth's surface. Rain is precipitation that falls in the form of liquid water. Raindrops are at least 0.5 mm in diameter. Precipitation less than 0.5 mm is called drizzle. Do you think the precipitation shown in the photograph on the front of this card is rain or drizzle? Scientists use an instrument called a rain gauge to measure rainfall. A rain gauge has an opening at the top. Rain falls into this opening and is funneled down into a measuring tube. The measuring tube is marked with a scale measuring to the nearest 0.035 cm. Sometimes a layer of cold air occurs near the ground. If this air is cold enough it can freeze rain. Rain that freezes is called sleet. If the layer of cold air is thin, the rain may not freeze until it hits a cold object on Earth's surface, such as a tree, telephone line, or sidewalk. Rain that freezes when it strikes an object on Earth's surface is called freezing rain. Snow is precipitation in the form of spheres, or balls, of ice. Hail is usually associated with thunderstorms. Hail begins as a tiny ice crystal in a dark, towering cumulonimbus cloud. Updrafts and downdrafts are characteristic of cumulonimbus clouds. Swirling winds carry hailstones up and down through different ragions of the cloud. Every time a hailstone goes through the lower part of the cloud it picks up a fresh coating of moisture. This moisture freezes as the gailstone is carried to the upper part of the cloud. The hailstone grows larger as it travels up and down. This gives hail a layered look, similar to an onion. Eventually the hailstone becomes heavy enough to fall to Earth's surface. Hail can be as large as 10 cm in diameter. As you might guess, hail can cause a lot of damage to crops and property. In fact, all types of precipitation can cause damage. Heavy rains cause powerful floods. Heavy snowfalls can blanket a city and shut down services. Freezing rain and sleet make travel very dangerous