Rain

Rain falling

Rain are droplets of water falling from clouds in the sky that are bigger than 0.5 mm. Droplets of water that are about 0.2mm to 0.5mm 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.

Convectional rain

Convectional rain happens in places of the world that are hot and wet. Sometimes, it also takes place in tropical deserts and inland areas during summer, when temperatures are hot. During the day, the sun makes the ground very hot. Air near the ground surface is heated by conduction. The heated air expands, becoming less dense and rises in a strong upwards air current. When the temperature of the rising air falls to the dew point, water vapor shrinks into thick clouds and forms convection rain. Depending on the temperature it may fall as sleet or snow.

Relief rain

Relief rain usually happens along coastal areas where a line of hills runs along to 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 vapor 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. Relief rain is also a very dense and cold mixture of precipitation.

Relief or orographic rain is formed when air is forced to cool when it rises over relief features in the landscape such as hills or mountains. As it rises it cools, condenses and forms rain. The highest annual rainfall totals occur in mountain areas. There is often a rain shadow effect whereby the leeward (downwind) slope receives a relatively small amount of rain.

Frontal rain/Cyclonic rain

Frontal rain happens when a cold front meets a warm 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.

Rainstorm

A Rainstorm is a sudden heavy fall of rain. It may cause floods on land which is not much above the level of the sea for example, flat coastal land and river basins. Too much water can make rivers overflow and cause floods. Also, landslides may happen. This is bad news as people may drown. For example, over 6 million people died just last year from rainstorms.

See also

• Water cycle

Other websites

• BBC article on rain-making.