If you’ve ever tried to make out faces or animals in the contours of the clouds, you might have wondered why they come in such a variety of shapes. Moisture is always present in the air as water vapour, but when it condenses into liquid droplets or solid ice particles, these particles scatter light, making them visible as clouds.
The shape of a cloud is determined by air temperature, density and movement. Differences in temperature and density prevent water-laden air from mixing with the surrounding air, giving clouds a distinct shape and creating crisp edges. Air movement pulls clouds into different formations.
Although no two clouds are exactly the same, they can be grouped into categories. Cumulus clouds are puffy and cotton-like, forming low in the atmosphere. When water vapour condenses into liquid water, it releases some heat, and if atmospheric conditions are unstable, this heat is enough to give cumulus clouds buoyancy, causing them to rise and build into cumulonimbus clouds. These large, dark clouds topped with billowing turrets are formed by rapid updraughts of air and are often accompanied by thunderstorms.
In contrast, stratus clouds form as wide layers when a large region of air rises.
Cirrus are delicate, wispy clouds formed of ice crystals at high altitudes. As they gradually sink, they pass through air currents that push and pull them in different directions, forming long, feathery wisps of cloud.
- Why are there two layers of clouds?
- How do large clouds stay in the air if they can weigh tonnes?
- Why are some clouds flat underneath?
- How do I see noctilucent clouds (NLCs)?
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Dr Claire Asher is a science journalist and has a PhD in Genetics, Ecology, and Evolution (GEE) at the University of Leeds. She also works part time as Manager of the UK Robotics and Autonomous Systems (RAS) Network, based at Imperial College London. Asher is also the author of Brave Green World: How Science Can Save Our Planet.