Rain is one type of liquid precipitation and is the result of water vapour condensing and precipitating.
This type of liquid precipitation can be described and seen as drops of liquid water falling from the sky.
To become rain, the water condensed in the clouds must be heavy enough to fall to Earth. For this to occur, the tiny droplets need to grow into drops by acquiring more water and becoming larger.
Some droplets collide with others in order to become larger whilst others will grow as water condenses out the air into the droplet. When these drops become too heavy to stay in the cloud, we get rain.
The size of raindrops is highly variable, from as small as 0.5mm in diameter to 6mm.
Rain is classified according to how it is generated such as by an approaching front, by convective cloud or by a cyclone.
There are three main types of rainfall:
Frontal rain occurs when two air masses meet. When a warm air mass meets a cold air mass, they don't mix as they have different densities. Instead, the warm less dense air is pushed up over the cold dense air creating the 'front'.
As a result, the warm less dense air cools, and the water vapour condenses into water and falls as raindrops.
This type of rain can happen anywhere in the UK.
Orographic rain is produced as a result of clouds formed from the topography - or shape - of the land. Where there is high ground moist air is forced upwards producing cloud and potentially, precipitation.
Mountainous areas close to prevailing westerly winds are most likely to experience this type of rainfall. The geography of the UK means that this type of rainfall is most common in the north and west of the UK where warm moist air from the Atlantic cools as it is forced upwards over high altitudes.
Convective rain is produced by convective cloud. Convective cloud is formed in vertical motions that result from instability of the atmosphere. One way that the atmosphere can become unstable is by heating from the sun. The ground warms up, causing moisture in the ground to evaporate and rise, and the hot ground also heats the air above it. As the water vapour rises, it cools and condenses into clouds and eventually rain.
When you heat the air from below like this, much like in a boiling kettle, you tend to get "bubbles" of rising air, known as updraughts. These are much smaller than the large-scale lifting of air that occurs at fronts and over mountain ranges. This tends to give us smaller areas of rain, with clear spells in between, commonly referred to in the UK as "sunshine and showers".
This type of rainfall is most common in the south and east of the UK, where it is typically warmer. This area is also prone to very heavy showers and thunderstorms, this is because the warmer air can hold more water.
Sometimes, you can get all three types of rain at once, and this can lead to severe flooding.