In this article we find out more about what banner clouds are and how they form.
A banner cloud is one of a number of different cloud types that are grouped as orographic clouds. This defines them as being caused by the shaped of the land, with orographic (from the Greek óρος meaning 'hill') referring to the relief of land and mountains.
When the wind blows against a hill or mountain it is forced to lift. Air cools as it rises and the water vapour within it condenses to form a cloud. As the name implies, these clouds form in a layer and then remain also stationary as the wind continuously flows from one direction. The prevailing wind determines the particular microclimates found around a hill or mountain.
Areas to the leeward side of the hill can often experience shelter, with more sunshine, less rainfall and higher temperatures than areas to the windward side. Banner clouds form on the sheltered leeward side of the mountain.
The Matterhorn banner cloud
Particular mountains, such as the Matterhorn in the Alps in Switzerland, have become famous for the frequent appearance of a banner cloud as pictured below.