Rain. It's water from the sky. That's pretty amazing in itself really isn't it? But there's an awful lot to know about the stuff. Read on.

What is Rain?

Rain¹, a type of weather, is droplets of water (chemically H₂O) that form in the atmosphere from water vapour. When the vapour condenses it presents as clouds that, when weather conditions are right, deposit the water back to the Earth as raindrops. Rain makes things wet, and like any variety of water, is vital for the existence and survival of animal and plant life on the planet. Some other forms of precipitation include sleet, hail and snow.

Here Comes the Rain Again

Itsy Bitsy Spider² climbed up the water spout.
Down came the rain and washed poor Itsy out.
Out came the sun and dried up all the rain.
And Itsy Bitsy Spider climbed up the spout again.
- Children's Rhyme

Rain plays a major part in the process commonly referred to as the 'water cycle'. This doesn't mean anything to do with a kind of bike you can ride in wet places, it's the way that nature keeps the 'blue planet' well, blue.

In the process, the sun heats large expanses of water, which causes some of the water to evaporate into vapour. This water vapour rises into the atmosphere with warm air currents, but as the warm air rises it cools and the water vapour then changes into tiny droplets. As the droplets grow in size they create clouds and when the clouds are big and heavy with water droplets, 'burst' so to speak, and the water droplets fall back to the ground as rain (although if the air is cool enough, the droplets may form hail or snow).

The rain then soaks into the ground where it is either taken up by plants and returned to the air, or filters through the soil back into aquifers³ where it is often pumped from for use in agriculture or extravagant water features. Failing that, it simply falls back into water courses that flow into rivers, that then meet larger bodies of water again, like the sea. Then the cycle begins all over again. However, not all rain reaches the Earth's surface; some evaporates while falling through dry air. When no rain reaches the ground through evaporation, it is called virga - a phenomenon often seen in especially hot and dry desert areas.

The Science of Precipitation

The scientific explanation of how rain is made and falls is called the Bergeron-Findeisen Process. It goes something like this;

The equilibrium of vapour pressure over water is greater than that over ice and therefore in a cold cloud, the water will be out of vapour pressure equilibrium and hence will evaporate to reach equilibrium. The ice in the cloud will then condense this vapour and grow into a larger ice crystal. Eventually this ice crystal will grow large enough to fall and during descent become a raindrop.

Confused? It just means that when water vapour gets cold high up it forms little ice crystals in clouds. When they are big and heavy enough they fall and heat up a bit, becoming water droplets again. And you get wet if you're under them. Simple really.

Types of Rain

Rain
Feel it on my finger tips,
hear it on my window pane
- Madonna, entertainer

There are supposedly three main types of rain - relief (orographic), convectional and frontal (cyclonic). Frontal rain occurs when warm air meets cold air, which is denser, and forces the warm air to rise. As the warm air rises it cools, causing a build-up of condensation - the result is big clouds full of rain. Relief rain is common in hilly and mountain areas near the sea and is the result of warm, moist air pushing up and over a tall peak. As the warm air rises up over the apex, it starts to cool and begins to form clouds. Convectional rain is the most common, as it comes from the heating of the Earth's surface, with air expanding and reducing in density. The warm air rises and as it increases in altitude gets colder, forming clouds that will either become rain, or even hail or snow. But rain comes in many other different shapes and sizes. And names. Here are some of the more recognisable, and a brief description and definition to aid identification in the field (or while you're on a bus, or running under awnings).

• Spitting - This is the rain that makes you think 'Oh, better get the washing in!' Or the rain that is precursor to a downpour (see below). Or the rain that never is - you feel a few specks, prepare for the inevitable and...nothing.
• Drizzling - A fine step up from spitting, drizzle is a constant trickle of random raindrops from above. Not enough to get you wet, unless of course it lasts for days. And days. And days. Which it often does.
• Tiddling/Light - Some would say light rain is perfect rain. It is enough to be called proper rain, and enough to give the flowers a much needed drink, top up the reservoir, make sure the local wildlife can put muddy footprints through the fields and generally make the grass grow. There are, however, some light rains that for some obscure reason, no matter how much waterproofing you do, will ensure you still get wet socks and a cold after going out in it.
• Showering - A constant rain, but can be pleasing. Showers often produce rainbows in the spring or summer, what with the sun being out after the rain clouds have passed. But it also prevents outdoor activities (aside from those that involve chasing some spherical object about and injuring people).
• Teeming/Moderate - A constant rain. It just doesn't really seem to stop. Even when you think it might.
• Bucketing - Imagine standing under a bucket of water. Then it up-ends itself all over you. Magnify that by ten fold, add some wind, mud and the fact the person you've been waiting for is ten minutes late. Not pleasant.
• Drumming - Just that, the rain makes a constant drumming on roofs. Can be quite soothing, or downright aggravating.
• Squalling - More common to seaside areas, squalling rain is incredibly wet. Mostly because it is aided by either wind and/or waves crashing against the sheets of it that fall from above.
• Blattering/Heavy - A strange driving rain that seems to have a supernatural ability to get you wet, even when you just look at it from the safety of your front door. It hammers against windows, and waves of water seem to pour off roofs, down gutters and against bus shelters.
• Downpour - Put all your clothes on. Run a bath. Get in. This is the equivalent result of being caught in a downpour.

All of the above can also be classified again into:

• Cold - Self explanatory really. The rain is cold, because it's downright freezing both up in the air and on the ground. Not far away from being hail or snow. Horrible when it drips down the back of your neck and causes you to shudder and cringe.
• Warm - And the other side of the coin. Warm rain can be most unpleasant, as it feels like someone has chosen to urinate over you in some manner (unless of course you enjoy that kind of thing). However, warm rain can also be wonderful, especially either on a cold day, or when it is extra hot and the rain is like having a shower - great to go and play in without fear of catching a cold.
• Clean - The kind of rain you'll tilt your head back into and gleefully allow to tickle your tongue.
• Dirty - Rain you wouldn't wash your uncle's Y-fronts in.
• Impenetrable - Even spitting rain can become impenetrable. Usually because you can't be bothered going outside into it. But some rain will feel like it is trying to batter you down and ensure that you will never ever decide to quickly nip down the shops for some milk because you really, really need it - even though it looks like the neighbours are building a raft from their fence.

Many more categories of rain have been identified by fictional character Rob McKenna - lorry driver and Rain God in the Douglas Adams' novel So Long, And Thanks For All The Fish, part of the Hitchhiker's Guide To the Galaxy trilogy in five parts, but besides these relatively normal types of rain, you can get some a little more bizarre.

Raining Cats & Dogs

While it's not very common for it to actually rain cats and dogs, numerous reports (some of which may be urban myth or folklore) from throughout human history of strange rainfall have occurred. From raining blood, to fish and other sea-life or even sand, from chickens and frogs to voles and moles, various strange rains have fallen in towns and cities the world over. Some of these may have coined phrases like:

It's raining pitchforks!

Woah, it's a real toad-strangler!

or the remarkably obvious,

Nice weather for ducks.

Many different things have been blamed for these freak weather conditions such as god(s), witches, aliens, or the Apocalypse - but it is more likely that a combination of wind, rain, storms and hurricanes over lakes, rivers and the ocean have been the cause⁴.

Hard Rain

Rain can be dangerous. Just like custard can be dangerous. A little won't hurt, but too much could be life-threatening. In order to protect yourself from rain simple measures can be put into place. If you want to go walking in the rain, wear a raincoat/jacket/mackintosh, wellington boots and take an umbrella. You can combine all three as suggested, or just one. It depends on your preferences, and the availability of the items.

If you're outside and it rains, but you have none of the protective clothing, you still have a few options. You can go inside. Houses and many other buildings are well protected from rain. They have roofs, some especially designed with gutters and other measures to ensure rain doesn't get inside them. If you do go into a house, make sure you take off your shoes. Most home-owners don't appreciate muddy footprints traipsed through their domicile.

If you're nowhere near a house or any other structurally-sound building⁵, you can take shelter under a tree. However, there are many risks to take into consideration. Trees are usually tall⁶, thus are prime targets for lightning. And rain usually accompanies storms, which produce lightning. If you are under a tree that gets struck by lightning you will either:

1. Be fine. The tree will take the brunt of it all.
2. Be burnt to a crisp. The air around lightning strikes can reach temperatures three times hotter than the surface of the sun.
3. Have a tree fall on you, which could either be lethal or uncomfortable.

Puddles

Doctor Foster went to Gloucester
In a shower of rain.
He stepped in a puddle right up to his middle
And never went there again.
- Children's Rhyme

Puddles, not to be mistaken for paddling pools (although many small children often do) are the remnants of rain. That is, they are small dips or holes in the surface of the Earth where water has pooled. Puddles have the uncanny ability of being able to make you even wetter when it is raining, for a number of reasons:

1. You don't know their depth, so you could step into one thinking it is shallow and end up in a sinkhole up to your armpits.
2. An animal, usually a dog, rolls in a puddle, then proceeds to shake itself dry all over you.
3. It is muddy and you slip and fall in one.
4. You enjoy splashing in puddles like a child, but forget that you have a hole in your shoe until you get halfway home and wonder what the funny squelching noise is coming from your left foot.
5. While you're walking by a road, a car casually drives past, its weight and speed causing the entire contents of a nearby puddle to surge out and all over you.

Flooding

As stated, puddles tend to form if you get a lot of rain. However, if you get even greater quantities, such as in a monsoon, then the danger is greater still. The result can be a lot worse than getting a bit wet. Floods have long been a terrifying side-effect of rain, ever since one Noah was told to build an Ark⁷.

So, the moral? Don't take risks when it comes to rain, or any sort of extreme weather.

Stormy Weather

Pitter patter raindrops,
I'm wet through.
So are you.
- Children's Rhyme

Rain comes from clouds, right? Yes indeed. Clouds are just big fluffy floating white things up in the sky made up of condensed water vapour⁸. So how do you know which clouds have rain in them, and which ones don't? Rainclouds are easy to spot though as they are usually greyish in colour. Well, easy to spot if the sky is usually blue. If the sky where you are is for the most part, grey, you might have difficulty seeing a raincloud in it. There are two very distinct types of rain cloud though, these being:

• Cumulo-nimbus - Does in fact mean 'heaped rain cloud'; these types of cloud are usually towering columns in the sky, big and really look like they need to loosen their belts. Often seen just as you've managed to get the lawnmower working - looming over the horizon, ready to spill their guts.
• Nimbo-stratus - Classic storm clouds, these hang low to the ground and are long and flat in shape, and very dark. Get inside, close the windows and wait for the downpour when you see these up above.

By observing the clouds, you can often tell when it is about to rain. Another general rule of thumb is if you can't see the sun, and it's not night-time, it's probably going to rain. There's also a certain smell in the air, and it can get very cold. Some people (often really old ones) swear that sometimes their bones will ache before a storm, and others say there's a change in the atmosphere in that it gets decidedly more electric. That doesn't mean you can plug your kettle into a cloud and make a cuppa, it just means that the air feels more charged. But, if you're still not sure, take a look at what way the wind is blowing by checking out the tops of tall buildings. Many of these will have either flags or weather vanes that will point out the direction of wind. If the wind is blowing your way, and the clouds are rushing towards you, best prepare for rain. There will also be a slight drop in pressure, and an eerie silence that precludes a storm. Also keep an eye on humidity. Humidity is actually the measure of water vapour in the air, and it is generally more humid just before it is about to rain. You can check humidity levels with two thermometers and a pair of socks - one of which should be wet. How?

Take your pair of socks and get one wet. Take off your wet sock and tie it around the end of one of the thermometers. The theory is that if the air is very dry, the water will evaporate quickly and the thermometer in the wet sock will show a lower reading than the one not wearing a sock. If the air is humid, however, the thermometers will have roughly the same measurement.

Advantages of this are that you will tell if it is about to rain, and make preparations. Disadvantages are you will have a wet sock. It is up to you to weigh up the options.

Measuring Rainfall

Rain can be measured using, funnily enough, a rain gauge. You can make a simple rain gauge with the following;

• A plastic drink bottle
• Scissors
• Felt-tip pen/marker
• A ruler

Method

1. Wash out the plastic bottle.
2. With the scissors, cut the top off the bottle, so it fits upside down in the bottom of the bottle - making a funnel shape.
3. With the ruler and felt-tip pen/marker, mark on the side of the bottle measurements - 5mm (millimetres), 10mm, 15mm, 20mm and so on.
4. Place your rain gauge outside (as it doesn't rain indoors all that much), either in a shallow hole or on level ground away from buildings or trees.
5. Keep a record of rainfall by recording measurements every 24 hours.

A rain gauge is particularly helpful for gardeners and farmers as crops rely on rain to assist in growing - but the making of a rain gauge and keeping of a rainfall diary is also a good school science project for younger children.

Where Does It Rain?

The rain in Spain falls mainly on the plain.
- Linguistic Rhyme

If you are in the United Kingdom, the general response to this question would be 'everywhere'. However, this is not entirely true. Compared to some parts of the world, the British Isles are fairly rain-free⁹, and the wettest place on Earth is actually Mawsynram, India, which has an annual average rainfall of 467.4 inches (11,873mm)! So if you find you don't really like rain, best not visit there. The area around the equator is also exceedingly wet all year round because of the heat and the amount of water vapour being produced through evaporation, and this is why many rainforests can be found in countries along the equator. Still, many other parts of the world near the sea, or surrounding large lakes, often have a lot of rain too. Areas of high rainfall also include mountainous regions, since warm air rises taking water vapour with it and high peaks and hills often have clouds around them. Wind will blow these clouds about, and it is the changes in warm air currents meeting colder air currents from the polar regions of the Earth that help move cloud formations about, thus affecting where gets rain, and where will miss out. This is why some places in the world have deserts, while others have lush green forests.

The landscape of the planet has been largely created by rain. It is a major player in the process of erosion - forming caves, canyons, gorges, valleys and many other natural wonders. Into the 21st Century, Global Warming will also influence how the world changes, in particular the amount of rainfall. With a warmer planet, rainstorms will become more frequent and more intense due to the fact that the land warms up and cools down faster than the sea so that, as global warming increases, the contrast in temperature between the land and sea will also increase. Warmer seas cause more water to evaporate which, in turn, comes down as heavier rainfall, and more flooding. As the ground has less chance to absorb this excess water there will be an increase in erosion, possibly altering the landscape. With changes to the way the Earth looks, such as lower hilltops or expanses of desert, rainfall patterns will also vary. Not enough to stop it from raining on your birthday though.

Looks Like Rain

Raindrops keep falling on my head...
- Burt Bacharach

Many people think that raindrops are tear-shaped, or at least this is how they are often depicted in pictures and cartoons. This is, however, complete bunkum¹⁰. Only drops of water dripping from some sources are tear-shaped at the moment of formation, like those coming from melting icicles or in certain cave formations. The shapes of raindrops were studied by Philip Lenard in 1898, who discovered that rain is in fact more spherical in shape, like a beachball. Larger raindrops actually flatten out and change into a more 'bun'-shaped variety, while even larger raindrops look like parachutes, with a curved upper part but a concave bottom.

Acid or Alkaline Rain?

Rain has a pH slightly under 6. In some areas, dust in the atmosphere contains enough calcium carbonate to counter the natural acidity of rain, causing the pH to be neutral or even alkaline. Rain with a pH below 5.6 is considered acid rain, a worrying variety. Acid rain is formed when pollutant gases present in smoke from sources such as factories, fossil fuel power stations and the internal combustion engine, containing for the most part chemicals like sulphur dioxide (SO₂), oxides of nitrogen (NO_x) and - more rarely - carbon dioxide (CO₂) dissolve in water vapour in the atmosphere and eventually precipitate out as rain from acid rainclouds. You can't tell the difference between acid rainclouds and normal rainclouds, although one might assume that acid ones are technicolour hues that dazzle and spin. This is a silly assumption. When acid rain falls it can be devastating, much like showers of vinegar, and can kill off plant life and leave forests - such as the Black Forest, Germany - in ruin.

Rain and Travel

Rain, rain. Go away.
Come again another day.
- Old rhyme

Human transportation has the ability to take people from place to place, regardless of the weather conditions. Cars provide you with a nice little enclosed place, with window-wipers that swish and swosh rain away so that the way ahead can be seen. However, rain can cause problems for any kind of travelling:

• Walking - Walking in rain can be fun. Outer water-resistant garments are recommended, like a raincoat and Wellington boots, though, if you would rather remain dry. An umbrella will also assist in keeping the rain off. Pedestrians must be careful near traffic though, as drivers of other vehicles have less control of their transportation in wet weather. It is also advisable to wear bright colours or reflectors if walking out in the rain, just so as you can be seen better.
• Car - As a car drives along the road, its tyres rub against the surface of the road creating friction - which in turn assists the tyres grip to the road giving better control. When it rains though, the water on the road makes the surface smoother, the tyres lose grip and aquaplane, meaning the tyres will slide across the smooth surface, causing the car to skid or spin out of control. Rain also diminishes visibility, and can leak in through windows making things a bit damp.
• Bus - The same as with a car, except a bus is bigger.
• Bicycle/Motorcycle - The same as with car and bus, but the rider of a bicycle or motorcycle is more open to the elements. You can get wet and muddy if it rains.
• Train - Rain only really affects trains by causing problems on their tracks. If you commute by train in wet weather, delays in your journey are often blamed on a build-up of wet leaves on the track. If there's an awful lot of rain there may be flooding which, as trains are commonly not able to float, could cause stoppages.
• Aircraft - Aircraft can fly above rainclouds, so can remain unaffected by rain. Unless of course they are landing or taking off. Then aircraft can be afflicted by the same problems that occur to cars and other land-based vehicles, such as decreased visibility and aquaplaning.
• Ship/Boat/Hovercraft - Ships and other watergoing vessels don't really have a problem with rain. It is the addition of wind and other weather that can cause problems such as soaking sails, filling up with water, and sinking - which isn't something you really want a boat to do.
• Submarine - As they are generally found underwater, rain doesn't especially affect the traveller on a submarine. If the vehicle surfaces and it's raining, you may get wet if you leave the craft whilst it is on the surface of the water.

Rainmaker

Why does it always rain on me?
- Travis, band

Not as easy as putting a silly hat on and dancing around in a circle outside, gesticulating wildly and beseeching gods or goddesses, or both, making rain is something best left to nature. However, 'cloud-seeding' (a relatively new idea made practical in around 1947 by Bernard Vonnegut, brother of author Kurt Vonnegut), is a scientific method incorporating the Bergeron-Findeisen Process that has had some success in artificially creating rain.

To 'cloud-seed', aircraft fly into the lower atmosphere and drop a mixture of chemicals into some clouds. These are usually silver halides - two strongly-charged particles which form a dipole (a molecule with a positive charge at one end and a negative charge at the other). Because of the strong charges, the dipolar molecules attract a lot of water, which is in itself a dipole. Thus, moisture collects around the crystals formed from the chemicals, and then as the mix passes through the air it forms into larger particles - which in turn make water droplets. The problem with the process though is that you do actually need some clouds to begin with otherwise you just end up dropping loads of silver iodide - the most common chemical used - onto the ground from a great height. Therefore, if you want rain, it's probably just as productive to stage a 'rain dance'¹¹.

However, there do seem to be some other tried and true methods of invoking rain such as firing up a barbecue, sunbathing, planning a field trip or any visit to an outdoor location, holidays, camping, remarking what a nice day it is, hanging out your washing or doing some gardening - and of course, singing a certain song at a music festival.

Rain and Religion

I wish to God it would stop raining!
- English woman waiting for a bus

Many cultures around the world have beliefs and mythologies surrounding rain. Arguably the most well-known in the western world is the story of Noah and his Ark. Here, Noah collects two of every creature and boards a giant wooden ship that he builds on the command of his god, as the people of the world are to be punished and cleansed with forty days and forty nights of rain. The end result is an olive branch, a rainbow and the continued survival of the species, including cows and chickens and kangaroos.

The native Australian Aranda people believed that Atain-tjina, a creator being, would hunt young men and then hurl them into the sea. They would be eaten by a giant water-snake, and then after two days the snake would release the young men, and they would fly up into the sky as clouds. Turning themselves upside down, their drooping hair would become rain, after which Atain-tjina would take the men and together they would travel away, the rain going with them.

In ancient Phoenician mythology Ba'al was, among other things, god of thunder and rain. In India, the nature gods include Parjanya - the Rain, and the Chinese Taoists have the Master of Rain, Yu-tzu, who sprinkles water from a pot he holds with the end of his shining sword. The Japanese have Susanoo, the ever-changing god of Thunder, Storm and Rain, while the Incans gave thanks to Mama-Cocha, the goddess of rain and water.

There are many other gods and goddesses related to rain in some way, including the Norse Thor, the Roman nymphs and the Slavic sprites - but all show just how important rain is to human society.

Love it or Loathe it

I'm only happy when it rains...
pour some misery down on me
- Garbage, band

Many people find the smell of rain, both during and immediately after a rainfall, exceedingly pleasing. The source of this scent is in fact a chemical known as petrichor - a natural oil produced by plants, then absorbed by rocks and soil, and later released into the air during rainfall. There is also something quite refreshing about walking out in a light misty rain, it has the feeling of cleansing everything and washing away your worries. However, rain can also induce feelings of sadness and woe. There is even a condition known as SAD (Seasonal Affective Disorder) that can bring on bouts of depression amongst some people when it is dark, cold, grey and raining outside.

If in Doubt

You could check the latest weather forecast, or just recall some of the many old sayings about rain:

• If cows are seen to be lying down, rain is imminent (this, supposedly, is because when it rains the ground gets wet. So cows, like any other sensible creature, don't like to lie down in the wet. Hence, they lie down before it rains).
• A reddish sun has water in his eye; before long you won't be dry.
• Birds flying low, expect rain and a blow.
• The louder the frog, the more the rain.
• A wind from the south has rain in its mouth.
• Mares' tails and mackerel scales make tall ships take in their sails (Mares' tails and mackerel scales are in reference to high wispy cloud formations, that in time build into rain-bearing clouds).
• Before it rains, ants are very busy, gnats bite, crickets are lively, spiders leave their nests; and flies gather in houses.
• Red sky at night - shepherds' delight. Red sky in the morning - shepherds' warning.
• The daisy shuts its eye before rain.
• Don't believe the weather man¹².

After the Rain

For the rain, it raineth every day
- 'Twelfth Night', William Shakespeare

Rain doesn't last forever. Sometimes it feels like it, granted. However, there are precautions to take after rain. Surfaces are often wet and slippery, so take care. Trees and other areas are full of water. Just because it has stopped raining doesn't mean that you will not get wet. Most of all, if you're experiencing rain for the first time - don't worry. It will come back. Especially if you don't want it to.