Condensation
- What is Condensation?

Cloud
- How do Clouds form?
- How do we measure clouds?

Fog
- What is Fog?
- Fog Danger

Dew
- What is Dew?
- Dew Trap

Frost
- Frost

Condensation

There are two major types of condensation. One is present on the surface of objects, such as dew and frost. The other one is suspending in air, such as clouds and fog. Clouds are made up of millions of tiny water droplets, formed when air is cooled. They are formed in several ways and are named according to their shape, height and size. Clouds help to forecast the type of weather, which may follow. They are often associated with precipitation ( rain, snow, sleet or hail), but not all clouds lead precipitation.

How do clouds form?

Clouds form when warm air rises and cools down enough for some of the water vapour in it to condense into tiny water droplets or ice crystals. Billions of these make up a cloud. Water vapour can also condense on to smoke or dust specks in the air.

Making a cloud

Measuring cloud cover

The number of clouds covering the sky is measured in oktas. The number of oktas indicates how much the sky is covered by clouds. Oktas are measured on a scale of 0 to 8 ( 8 oktas means that the sky is completely covered). For example, a weather forecaster may describe the sky as having four oktas of cloud, which means that half the sky is obscured by clouds.

Fog

Fog is a really low cloud, which forms when air near the ground cools. Sea fog forms when warm air from the land flows over cold seas. In the Arctic, fog can rise up from the sea like steam rising from hot water. It is called sea smoke.

Fog Danger

Fog reduces visibility and causes accidents on land at sea. In 1962 two trains crashed in thick fog in London. 90 people were killed and many more injured.

Dew

Dew

Dew is the small water droplets which form on cold surfaces during the night. As air cools at night there is a point when it cannot hold any more water vapour and condensation begins. This is called the dew point and dew forms on objects on the ground, such as on blades of grass. It evaporates in the morning when the air warms up.

Dew trap

Farmers in Lanzarotte, Canary Islands, collect dew to water their crops. The dew traps look like moon craters, 3 m wide and 2 m deep. A layer of volcanic ash inside makes a good surface for condensation. Vines planted in the craters can live on the dew if it does not rain.

Frost

At night the ground cools and, in turn, cools the air above it If the temperature falls below freezing point, water vapour sublimes into ice crystals which are called frost. Hoar frost often forms around keyholes and delicate fern frost on windows.

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