7 facts about clouds

1. Clouds made by fire

One of the ways in which clouds are formed is by the heating of the earth's surface which causes air to rise which condenses water droplets to form clouds.

Heating of the earth's surface is usually done by the sun, but wildfires and volcanoes can also cause intense heating which leads to the rapid formation of clouds known as pyrocumulus.

If enough water vapour is available, a pyrocumulus can become a thunder cloud which is called a pyrocumulonimbus which can produce dramatic lightning displays.

2. Up in the clouds

Feared by paragliders and hand gliders, the base of cumulus clouds can be a dangerous place when thermal updrafts contribute to a phenomenon known as 'cloud suck'.

In Februrary 2007, paraglider Eva Wiśnierska-Cieślewicz was sucked up into a cumulonimbus cloud rapidly climbing at a speed of 45 mph to an altitude of 9,946 m (32,600 feet) - close to the altitude of an airliner. 

Due to hypoxia she lost consciousness and miraculously came around after around an hour to successfully land her paraglider. With temperatures around -50 °C at that altitude, when she miraculously reached the ground she was covered in ice with bruising all over her body from the impact of hailstones in the cloud.

3. Why clouds are white

The reason clouds are white and the sky is blue is all to do with the colour spectrum of light.  Light from the sun starts out white but becomes scattered by particles in the sky. Atmospheric particles in the sky scatter away blue light more than other colours which gives the sky its blue appearance.

But as light passes through a cloud it interacts with larger water droplets which scatter all colours of the spectrum of light almost equally.  This means that the sunlight continues to remain white making the clouds appear white against the background of a blue sky.

4. The cloudiest place on earth

The earth's atmosphere is a very cloudy place.  NASA's Earth Observatory estimates that at any given time, around 67% of Earth's surface is covered by cloud.  

Based on observatory data between 2002 and 2015, NASA's Aqua Satellite image (below) clearly shows three zones which are the cloudiest, over the mid-latitudes and over the equator.

These zones are created by the interaction of large cells of air which are part of the earth's global circulation patterns.

5. How much does a cloud weigh?

While there are many factors that will determine the exact amount of water vapour a cloud holds (e.g. temperature, altitude, pressure etc.) we can work with an average of about 5 g of water per cubic metre of cloud.

This means an average size cumulonimbus would weight about 400,000 kg, roughly the same weight as an Airbus A380.

Find out more in a discussion in our Mostly Weather podcast:

6. The highest cloud of all

The title of highest cloud goes to the rare but beautiful but noctilucent cloud.   They hover at around 60,000 m (200,000 feet) and are too faint to be seen in daylight.

They become visible during twilight when sunlight from below the horizon illuminates them to reveal their faint ghostly outlines.

7. Word clouds

When making observations at a weather station, cloud cover is measured in 'oktas' (eights of a sky). A reading of zero oktas would mean there is not a cloud in the sky, which is termed 'enubilous'.

Across cultures clouds can have very different meanings, in the UK they tend to have a relatively negative symbolism, for instance, a cloud on the horizon means something bad is going to happen.  However in Iran, clouds are considered as very lucky, so it would be a blessing to say 'your sky is always filled with clouds.'