BBC HomeExplore the BBC
This page was last updated in July 2006We've left it here for reference.More information

25 October 2010
Accessibility help
Text only
BBC Weather Center BBC Weather Centre

BBC Homepage
Skip this navigation panel

Contact Us

Like this page?
Send it to a friend!


WeatherWise - Fact Files - Weather Systems

A humidity dial Dew and Frost

Even the driest air contains some water vapour. The condensation process is the same whether we talk about dew or cloud. However, if the air is cooled sufficiently, at night for instance, some of the water vapour will condense on surfaces as dew. The temperature at which air, at a level of constant pressure, can no longer hold all the water it contains is called the dew point. However, when a sample of air is lifted up through cooling air to a level at which is condenses, we use the term condensation level instead of dew point.

Frost and ice can be a hazard to drivers Frost is a Hazard

When there is no cloud cover at night, the Earth loses heat rapidly and cools the layer of air just above the ground. Temperatures fall to the dew point and water (dew) condenses on surfaces. As the day warms up, the dew will evaporate into the air, but further cooling may lower the temperature to freezing point and produce a white frost - often called a ground or hoar frost. Water droplets freeze into ice crystals, which are deposited on vegetation and other exposed surfaces. Frost is a major hazard to farmers across the world. Temperatures below 0°C (32°F) can damage and even kill young plants. Frost pockets occur when there are natural dips and valleys. Cold air drains into these low-lying places, where frosts occur more frequently than higher up hillsides or slopes. A type of thick frost called rime can form in cold foggy conditions. The water droplets of fog freeze on impact when they strike an object. It can form a coating of thick, white frost on bushes, trees and even the fronts of cyclists! When rain falls through very cold air, or onto cold objects, it can freeze instantly into a see-through coating of clear ice known as glaze. Rime and glaze can be very hazardous. Ice is heavy, so objects coated with glaze collapse. Power lines sag and snap. Branches break off trees and bushes. And a thick coating of rime caused a TV mast to collapse a couple of decades ago.

Also in this section:
The Basics series:
Fact File series:

Back to Top
The BBC is not responsible for the content of external websites.


About the BBC | Help | Terms of Use | Privacy & Cookies Policy