Frost comes from the atmosphere when the temperature falls below freezing, and invisible water vapour in the air turns into white ice crystals, without first becoming a liquid. It usually occurs when the skies are clear, when there is no wind and when a mass of cold air descends on the land.
This often happens during the night in the spring and autumn of areas with temperate climates. In the morning the fields and roofs are white with what would be dew if the temperature had been above freezing point. It is the most common type of frost and is often called hoar frost.
Sometimes only the leaves of plants are fringed with white rime. This is formed when very small droplets of the moisture from fog have frozen on coming into contact with a cold object.
There is also black frost. This occurs when water vapour turns first into liquid and then freezes into a thin layer of ice instead of white crystals. As it is invisible, it is particularly dangerous when it forms on roads.
The beautiful patterns, looking like trees, ferns or feathers, which are sometimes seen on windows, are made when the water vapour in a cold room condenses.
Picture credit: google
