In this topic you will learn about clouds and how they produce rain or snow.

Clouds are found only in the troposphere, the lowest layer of the atmosphere. Clouds are made of tiny droplets of water or ice. There are three basic cloud shapes. Cumulus clouds are puffy and have a flat bottom. Stratus clouds form in blanket-like layers. Cirrus clouds are high-altitude clouds made out of ice crystals. Cirrus clouds look wispy or feathery. A cloud at ground level is called fog.

Clouds are made of tiny water droplets or ice crystals so light that they are suspended in air. When these tiny particles clump around particles of dust, called condensation nuclei, they become large enough to fall. Any form of water particles that falls through the atmosphere and reaches the ground is called precipitation.

Liquid precipitation is rain. Snow, sleet, and hail are examples of solid precipitation. The type of precipitation depends upon the air and ground temperatures. Rain will fall if both the air and ground are warm. If the air is warm and the ground is cold, sleet will form. Snow forms when both the air and the ground are cold.

Cumulus clouds produce heavy rain or snow showers that usually last only a short period of time. Precipitation from stratus clouds is usually longer-lasting with small snowflakes or small drops of rain.

Hail is a pellet of ice and snow that forms in very tall clouds with strong upward winds. In these clouds, ice crystals are hurled upward many times. As the crystals fall, they become coated with water, which freezes when the crystals rise again. Then the crystals are hurled upward again. Each up and down cycle adds another layer to the hailstone.

Some of the precipitation that reaches Earth's surface evaporates right away. Some of it flows into streams, rivers, lakes, or oceans. Some of it seeps into the ground. Much of this water eventually moves to the oceans. Water at the surface—including the ocean's surface—evaporates into the atmosphere.

The pattern of condensation, precipitation, seepage, and runoff is called the water cycle. The water cycle is the continuous movement of water between Earth's surface and the air, as it changes from liquid to gas to liquid.

Rain is vital for life on Earth. It helps crops grow, which means food for us. It also increases our water supply. Yet it can also be destructive. For example, hail can damage or destroy crops, cars, and buildings.