Why Does it Rain?


5.4.13_Ljubljana_15_(8623814958)The United Kingdom is well known for its heavy rainfall, but what causes it to rain in the first place?

Rain occurs when warm, moist air cools down, creating condensation, or vapour. This water vapour originates from rivers, lakes and the sea, and rises into the air to form clouds of water or ice crystals. As the clouds rise higher, the air surrounding them gets colder, until eventually the cloud becomes too heavy, and the water vapour falls back to the ground as rain or even snow.

Although rain always forms in the same manner, it can be categorised into three different types – orographic rain, frontal rain, and convective rain.

Orographic rain is only produced over mountainous areas close to prevailing westerly winds, when moist air from high ground is forced upwards, producing clouds.
The geography of the UK means that this type of rainfall is most common in the north and west, and is one of the main causes of rain in the wettest part of the UK, the western Highlands in Scotland, where they receive three metres of rainfall every year.

Frontal rain occurs when a mass of warm air meets a mass of cold air. As the two masses have different densities they can’t mix together, so the less dense, warmer air is pushed up and over the cold, dense air. This creates a weather front which condenses the water vapour into raindrops. This type of rain is not subject to geographical factors, and can happen anywhere across the UK.

The final type of rain, convective rain, is most common in the warmer climate of the south and east of the UK. Convective rain is produced by clouds that are formed in vertical motions caused by the instability of the atmosphere. This instability is caused by heat from the sun warming the ground, and moisture from that ground evapourating and rising. At the same time as this, the hot ground also heats the air above it. This additional water vapour rises as well, cooling and condensing into clouds before turning to rain.

The flooding that has been so severe in the UK in recent years was caused by all three of these types of rain occurring at the same time.

Hopefully we shall not see too much rain in the next couple of months, but from past experience, that may not be too likely…

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Dr Kathryn Bates is a graduate of archaeology and history. She has excavated across the world as an archaeologist, and tutored medieval history at Leicester University. She joined the administrative team at Oxford Open Learning twelve years ago. Alongside her distance learning work, Dr Bates is a bestselling novelist, and an itinerant creative writing tutor for primary school children.

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