Rain

Rain is one of the most common and essential weather phenomena. It sustains ecosystems, replenishes water supplies, and influences our daily lives. But what exactly causes rain, and why does it vary so much in form and intensity? Let’s explore the science behind rain, its different types, and some intriguing facts.

What is rain?

At its simplest, rain is described as drops of liquid water falling from the sky.

Clouds form when water vapour (a gas) cools and condenses to form water droplets (a liquid) but these droplets are so tiny they stay in the sky. To become rain, the water droplets must grow 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 video and text below explain how rain forms, and the different types of rain that we get in the UK.

Transcript: Why does it rain?

What are the different types of rain?

Rain is classified according to how it is generated, with three main types of rainfall:

Frontal rain

This is when cold air meets warm air in a ‘weather front’. Warm air rises, so when it bumps into the cold air, it rises above it. However, like when you’re climbing a hill, the higher you go up in the atmosphere, the colder it is. So the warm air that has risen up in the atmosphere now begins to cool down. As a result, the water vapour (a gas) condenses into water (a liquid), forms a cloud, and eventually falls as raindrops.

This type of rain can happen anywhere in the UK, and usually brings grey overcast skies and persistent rain through a large portion of the day.

Orographic rain

When the wind pushes air towards a hill, it is forced upwards, as it cannot go inside the hill. When it reaches the top of the hill it cools down (as the air higher up in the atmosphere is colder), and if it has enough moisture, the water vapour (gas) will condense into water (a liquid) form a cloud and eventually fall as raindrops.

This is why we tend to see more rain on the hills than anywhere else.

Convective rain

This is when the Sun shines on the ground and heats a shallow layer of air close to it. This air is now warmer than the surrounding air, so it rises up in the atmosphere (warm air rises). The higher you go up in the atmosphere, the colder it is. So the warm air that has risen up in the atmosphere now begins to cool down. As a result, the water vapour (a gas) condenses into water (a liquid), forms a cloud and eventually falls as raindrops.

This type of rain is also known as showers and usually produces smaller areas of rain, which is why on some days you may get a downpour, whilst a neighbouring village may have stayed dry. This is the type of situation where you will have rain on and off through the day, with sunshine in between and it seems like the weather cannot make its mind up!

The reason that showers are much smaller in size, and much more unpredictable is because the ground surface varies greatly across the UK. This means the air close to the ground is heated at different rates, which is why the showers can pop up anywhere, and be blown around in different directions by the wind. If, for example, there is tarmac on the ground (a darker surface) this will heat the air more quickly than a lighter surface (such as grass) or one made of water (such as a lake).

Drizzle

Drizzle is a type of liquid precipitation consisting of very small droplets of water falling from low level stratus clouds. In the UK we're very familiar with drizzle. Our prevailing winds from the Atlantic pick up lots of moisture, and therefore clouds, as they travel over the sea.

To be classed as drizzle, droplets must be less than 0.5mm in diameter. These are larger than the droplets in the clouds, but smaller than raindrops. 

For the drizzle to form, the cloud must be fairly low to the ground (usually below 1500ft from the surface), and so it is more likely to occur in hilly areas.

Interesting facts about rain

Raindrops

Raindrops are usually represented in the shape of a teardrop, however, in reality they are not. When the form high up in the atmosphere they from a spherical shape as the water molecules bind together held by surface tension. As they begin to fall their shape changes as they hit other raindrops, while air resistance causes the bottom of the drop to flatten and curve resembling the shape of a jellybean. The size of raindrops is highly variable, from as small as 0.5mm in diameter to 6mm.

Phantom rain

Did you know that under certain conditions, rain can fall from the sky without ever reaching the ground. It happens when rain falling from a cloud evaporates or sublimes as it approaches the earth's surface. This creates what is known as Virga clouds, a tail or wisp extending from a cloud in a downwards direction and are generally seen to extend from Cirrocumulus, Altocumulus, Altostratus, Nimbostratus, Stratocumulus, Cumulus or Cumulonimbus clouds.

Rain has a smell

Many people believe they can smell when it is going to rain, and there is some evidence to support this. When a higher humidity is experienced as a precursor to rain, the pores of rocks and soil become trapped with moisture forcing some of the oils to be released into the air.

But the strongest smell is released when rainfall arrives. Raindrops landing on dusty or clay soils trap tiny air bubbles on the surface which then shoot upward - as in a glass of Champagne - and burst out of the drop throwing aerosols of scent into the air where they are then distributed by wind.

This is what is responsible for the familiar smell of rain and is called 'Petrichor'. The word comes from the Greek words 'petra', meaning stone, and 'ichor', which in Greek mythology refers to the golden fluid that flows in the veins of the immortals.

The phrase was coined by two researchers at the Australian CSIRO science agency in a 1964 article for the journal Nature. In their research, rocks that had been exposed to warm, dry conditions were steam distilled to reveal a yellow-coloured oil that had become trapped in the rocks and soil, a substance they discovered was responsible for the smell.

The source of this oil is a combination of oils secreted by plants during dry weather (which signals to halt root growth and seed germination) and chemicals released by soil-dwelling bacteria.

The wettest place in the world

The title of wettest place in the world goes to Mawsynram in the Maghalaya State of India.  It receives an astounding average of 11,971 mm rainfall each year. For comparison, the UK's average annual rainfall is 1154 mm.

The reason for its torrential downpours is its proximity to the Bay of Bengal and its situation 1400 m high in the Garo Hills, to the south of the Himalayas. The south-westerly monsoons pile in with moist winds which are caught against the mountains creating huge amounts of rainfall on the village.