Copyright © 2007 Dorling Kindersley
The moisture gathered in clouds eventually falls to the ground as liquid rain or drizzle, or as solid, frozen SNOW or HAIL. Any kind of falling moisture is called precipitation. Rain forms when tiny droplets of water floating in clouds collide to form bigger drops. If the drops get large and heavy enough, the air can no longer support them, and they fall as rain. Rain also forms when falling snowflakes melt high in the air.
The bright arch of a rainbow appears in the sky when the Sun shines through falling raindrops. Inside each raindrop, the sunlight is refracted (bent and split) into the seven colours that make it up – red, orange, yellow, green, blue, indigo, and violet.
Moisture rises from the Earth’s surface and falls back in a never-ending cycle driven by the Sun’s energy. As the Sun heats the surface of lakes, oceans, and icefields, moisture evaporates (turns into water vapour), rises into the air, and gathers in clouds. At cooler temperatures, the water vapour condenses (turns into a liquid) and falls back to Earth as rain, snow, or hail.
Hailstones are balls of ice that grow from ice crystals. They form inside storm clouds that tower up to 10 km (6 miles) above the ground. The temperature at the base of a storm cloud is warmer than at the top. This causes powerful vertical air currents. Water droplets in the cloud freeze and are whirled up and down. A fresh layer of ice forms around a hailstone each time it is tossed up to the frozen cloud top. Eventually, it gets so heavy it falls to the ground.
Snow forms in clouds high in the atmosphere, in temperatures of -20 to -40°C (-4 to -40°F), as water vapour condenses to form ice crystals. The crystals collide and combine to form larger snowflakes, until they get too heavy to float, and drift to the ground. Sleet is a mixture of snow and rain, or partly melted snow.
A snow crystal is a single crystal of ice. A snowflake can be one snow crystal, or several stuck together. All snow crystals have a hexagonal (six-sided) structure. This is formed because the tiny water molecules inside ice line up in a regular hexagonal shape called a lattice.