Sometimes, they can appear gray and threatening. Other times, on a beautiful summer day, they can loosen and brighten up the sky. Cloud formations come in many different shapes and sizes. Some are so rare that you can consider yourself very lucky to spot them.
Clouds always consist of water droplets or ice crystals. However, they can have very different shapes, densities and colours. Meteorologists differentiate between around a hundred different cloud formations. Some types of cloud occur frequently, such as cirrocumulus clouds, which even non-experts can recognise. Others are rare weather phenomena that have only recently been classified in the International Cloud Atlas. The Atlas is published by the World Meteorological Organisation (WMO) and is the reference for meteorologists wanting to identify different clouds. After more than 30 years, a new edition was published in 2017. It now contains volutus for the first time: a tube-shaped cloud that appears to roll slowly about a horizontal axis.