Rain

Without clouds, it would not rain. Clouds are made up of water droplets formed when warm, moist air rises high into the sky and cools. That water vapour in it condenses and forms what will become raindrops. These droplets move around in the cloud and bang into each other. As they do this, they increase in size until they are heavy enough to fall from the cloud.

However, not all rain arrives in such force or so quickly, and different clouds can cause considerable variation in rainfall. Layered clouds such as the thinner and lighter nimbostratus usually result in slower, steadier rainfall that can last for hours, and even days, on end.

If the raindrops are very fine, they fall as drizzle – and this usually happens with low stratus clouds. Drizzle is defined as water droplets under 0.5 mm in diameter (width).

Rain can also vary from place to place. Short, heavy showers are common in warm places, such as the tropics, because the higher temperatures there make air rise rapidly to create big rainclouds. Some of the most torrential rain of all is associated with the monsoon season in Asia. In this area, countries such as India are soaked by heavy rains for six months of the year, but are dry and parched for the rest of the time. In contrast, temperate areas such as Europe receive rainfall throughout the year. Similarly, areas around the Equator are wet all year round, whereas desert regions such as the Sahara in Africa may have no rain for several years in a row. Polar regions also get very little rainfall because much of the water in these areas is locked up as ice. The air is also very cold here and cold air holds very little moisture compared to warm air.

Raining cats and dogs

We have all heard the expression "raining cats and dogs", but do you know where it originates from? Although we don’t know for sure, many people believe that this expression is based on the ancient Chinese spirits for rain and wind, which were often illustrated by a cat and a dog.

However, the strong upward movements of air (updraughts) that bring heavy rain have been known to bring other things apart from just water. Creatures such as maggots, fish and even frogs have been lifted up into the air, so maybe the saying does have some basis in truth. Let’s hope it does not happen to any of our pets though!

Like many weather-related sayings and expressions, it is hard to know where they originated from, although many do seem to be based on meteorological facts. To find out more about such phrases, take a look at the weather lore section.

The equivalent French expression for "it’s raining cats and dogs" is "il tombe des cordes" (it’s raining ropes). So who knows where that phrase originated!

Cows and rain

Apart from cats and dogs, another animal is often associated with rainfall – cows. This time though they are connected with predicting rainfall, as some people say that cows in a field all lie down when rain is on its way.