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When air cools, the water in it condenses around dust or other particles in the air, called condensation nuclei. These small droplets then become visible as clouds. As more and more droplets join together they become too heavy and fall from the cloud as rain.

Clouds form from water or ice that has evaporated from Earth’s surface, or from plants that give off water and oxygen as a product of photosynthesis. When it evaporates—that is, rises from Earth’s surface into the atmosphere—water is in the form of a gas, water vapor. Water vapor turns into clouds when it cools and condenses—that is, turns back into liquid water or ice. In order to condense, the water vapor must have a solid to glom onto. This solid “seed” may be a speck of dust or pollen, or a drop of water or crystal of ice. Dew is water vapor that has condensed back onto Earth’s surface—on grass or a car’s windshield, for example. Rain is when water falls from clouds in droplets that are bigger than 0.5 mm. Droplets of water that are about 0.2mm to 0.45mm big are called drizzle. Rain is a kind of precipitation. Precipitation is any kind of water that falls from clouds in the sky, like rain, hail, sleet and snow. It is measured by a rain gauge. Rain is part of the water cycle.

Clouds will often absorb smoke to create rain, commonly referred to as "nature's laundry" due to this process.

Some places have frequent rain. This makes rainforests. Some have little rain. This makes deserts. Rain probably exists in the atmosphere of Venus, where it is sulfuric acid. Titan may also have rain, where it is methane.

Convectional rain[change | change source]

When the Sun heats the Earth's surface, the ground heats the air above it. Convection makes the air rise and cool. When it cools to the dew point, clouds form and rain follows.This usually occurs on flat land. This type of rainfall often causes summer showers and thunderstorms.

Relief rain[change | change source]

Relief rain usually occurs along coastal areas where a line of hills runs along the coast. When wet onshore wind from the sea meets a mountain, hill or any other sort of barrier, it is forced to rise along the slope and cools. When the air temperature falls to its dew point, water vapour condenses to form clouds. When the clouds can no longer hold the water droplets, relief rain begins to fall on the windward slope of the mountain. On the leeward slope, air sinks, it is warmed and further dried by compression. Therefore, the leeward slope is known as rain shadow. Moist winds blow in from the sea and are forced to rise over the land. The air cools and the water vapour condenses, forming rain drops. The rainiest places in the world are places that have relief rainfall.

Frontal rain/Cyclonic rain[change | change source]

diagram of a cold front causing rain

Frontal rain happens when cooler air and warmer, humid air meet in a weather front. The less dense warm air rises and condenses forming clouds. These clouds grow and eventually create rain. In some places on the northern temperate zone the cold air front tends to come from the north west and the warm air front comes from the south west.

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