Take a moment in your day to look up at the skies, and you’re likely to be confronted with a wide variety of continually shifting cloud formations. Gazing at the clouds can be calming, exhilarating, and awe-inspiring. Have you ever wondered what a cloud actually is, or thought about the different types of clouds you can see?
The sky is full of a gas called water vapour, which we usually can’t see. Higher up in the Earth’s atmosphere, where the air is cooler, this water vapour turns to tiny water droplets; a visible mass of these water droplets forms a cloud. A cloud usually seems white, because the dense mass of water droplets reflects sunlight, which our eyes interpret as white. When the air gets cooler still and it’s about to rain, the water droplets cluster together into raindrops with more space between them, and less sunlight is reflected, making the cloud seem darker in colour. Because these raindrops are heavier, gravity causes them to fall to Earth. If the air is really cold, the raindrops may become sleet, hail or snow.
In 1802, the British chemist and amateur meteorologist Luke Howard invented a system for naming clouds which is still in use today. Howard divided clouds into three main types: stratus, cumulus and cirrus. These names are Latin words which indicate their shape: stratus means ‘flattened’ or ‘spread out’, cumulus means ‘heap’, and cirrus means ‘tuft of hair’.
Stratus clouds are low-lying, horizontal and stratified (layered). They can look like white or grey blankets. The appearance of stratus clouds often means the weather is turning cold and dull.
Cumulus clouds are large clouds which stretch vertically, and form low down or in the middle of the Earth’s atmosphere. They can signal fair weather, but if they build up they can cause showers.
Cirrus clouds form high up, and are wispy and curly, resembling feathers. They’re sometimes known as ‘mares’ tails’. They’re usually a sign of fair weather, but can also indicate wind and/or a change in the weather.
It gets a bit more complicated beyond these definitions, however: there are also intermediate cloud classifications such as ‘cirrocumulus’, ‘altostratus’, and ‘cumulonimbus’. The prefixes and suffixes in these cloud names describe the height of the cloud above the Earth. The prefix ‘nimbo’ and suffix ‘nimbus’ refer to low-level clouds lying less than 2,000 metres above the Earth. The prefix ‘alto’ refers to mid-level clouds that lie between 2,000 and 6,000 metres above the Earth. Perhaps you’ve heard of ‘mackerel sky’; this expression describes cirrocumulus or altocumulus clouds which have a rippling pattern resembling fish scales. Finally, the prefix ‘cirro’ refers to high-level clouds that lie more than 6,000 metres above the Earth.
So it’s time to get cloud-spotting: not only are clouds beautiful and fascinating, they can also help you to predict the weather! Sunrise and sunset are often the best times for cloud-gazing, but clouds can be enjoyed at any time of day. Don’t forget to take photographs to record the beauty and drama of the cloudscapes you see. For more information, check out the National Geographic and Met Office websites.