Clouds

Morning Glory Cloud - Mick Petroff, 2009 The roll cloud ‘Morning Glory’. © Mick Petroff, 2009.

By Royal Society Dorothy Hodgkin Fellow Dr Charles Williams

What are clouds?

Clouds are visible masses of water droplets (either liquid or frozen) suspended in the atmosphere above the Earth's surface. Alone, these water droplets would be invisible, being approximately 0.01 mm in diameter. When surrounded by millions of other droplets, however, they become visible as clouds, appearing in a variety of shapes and colours.

Why do they occur?

Typically, clouds occur as warm, moist air moves up through the lower atmosphere. In this type of convective cloud, sunlight warms the surface and causes moisture to evaporate and rise. As it rises, it cools, condensing back into water droplets which then cluster together. This is the most common type of cloud, and can be between several hundred metres to several kilometres across. Alternatively, stratiform clouds are much larger and are formed by much larger areas of more slowly rising air.

Why do they look so different?

Clouds appear different due to several factors, such as their vertical height, density of the water drops and the surrounding environment. Clouds are classified according to their base height: for example a ‘low cloud’ like nimbostratus would be below 2,000m.

What do you think is the most interesting cloud display?

One of the most interesting phenomena occurs over the Gulf of Carpentaria in Northern Australia, and is called the ‘Morning Glory’. It is a roll cloud - a long horizontal tube-shaped cloud – that occurs at a vertical height of 1-2km, is up to 1000 km long and can move at speeds of up to 60 km/h. It is accompanied by sudden gusts of strong wind at the surface, and is popular among glider pilots who aim to 'ride the wave'.