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Weather Reference - Dew, Fog & Frost
Location: London (Default) Lat: 51.5N Sunrise: 06:07BST
  Lon: 0E Sunset: 19:55BST

Dew | Radiation Fog | Advection Fog | Upslope Fog | Fog Stratus | Frost | Disclaimer


Dew occurs when the temperature of the ground, or any other surface, drops low enough to cause condensation in the air immediately above it. This results in the formation of water droplets on the surface.

DewThe same process can result in fog, however it is possible to have dew without fog, but it is not possible to have fog without dew.

The ideal conditions for dew are a still, clear night, high humidity in the air next to the ground and low humidity in the air above.

You can see dew bows, approximately 42 degrees from your head, and they are identical to a rainbow, except that it is an arc not a semi-circle.

Another effect is known as a heiligenschein, which is a diffuse of white light that occurs round the shadow of your head

Radiation Fog

FogThis is the most common form of fog and is so named because it is produced by radiational cooling of the ground which happens at night, when the heat absorbed by the earth's surface during the day is radiated back into space.

The highest degree of radiational fog occurs on clear nights, when there are no clouds to radiate the heat back to earth.

Radiation fog varies in depth from only 3 feet to approximately 1,000 feet. It is always at ground level and results in reduced visibility.

The fog will normally disperse soon after sunrise, when the suns rays gradually warm the ground. Because a clear sky is need to result in radiation fog, a fine day normally follows.

Advection Fog

Advection is the transport of heat by a stream of gas or air.

Advection FogThe condensation is caused by moist air drifting into a cold environment (or cold air moving up into a moist environment) and is normally formed during the day, unlike Radiation fog.

Fogs at sea are always advection fog's as the warm air associated with a warm current drifts over a cold currant and condensation take's place.

The fog can be drawn inland by low pressure.

Advection fog can also occur when moist maritime air drifts over a cold inland area and usually happens at night when the land temperature drops as a result of radiation cooling.

Valley fog which is air that has cooled during the night drains into a valley from surrounding hillsides, condensation then takes place and the valley fills with fog this is also another form of advection fog.

Upslope Fog

Upslope fog occurs when moist air is blown up a hill or mountainside to a level where condensation occurs. The air currents that produce upslope fog are weak, almost non-existent.

The fog begins at the bottom of the mountain and covers a wide area. Upslope fog is common in all mountain ranges.

Fog Stratus

This fog develops at a higher level than other fogs and occurs because the sun's ray's first heat the ground near the edges of the fog, causing the perimeter to dissolve.

Some of the heat warms the ground underneath which results in the heat from the ground to evaporate the fog at low level. As the fog erodes from the edges toward the centre and from the underside, up resulting in a layer of fog a distance off the ground.

Because this fog form's in still conditions, there is seldom any wind at ground level and the stratus lifts more or less vertically as it erodes.

On some occasions light wind may occur soon after the bank of stratus had formed and gently blows the fog across the ground as it is dispensing.

Stratus fog has normally dispersed by mid to late morning.


FrostFrost tends to occur on clear, cloudless nights which allow heat to rapidly radiate from the ground, resulting in a significant drop in temperature for frost to form; the temperature must fall to below 0 degrees Celcius.

There are two types of frost:

  • Air frost 0 degrees, 1.25m above the ground
  • Ground frost 0 degrees at ground level

Frost is more likely to occur in low lying area's that are surrounded by hills.

True frost is known as Hoar frost and this occurs when a thin layer of moist air near the ground cools to below freezing and immediately forms ice crystals, without condensing as a liquid.


All information is presented in good faith. We cannot accept any liability for any incorrect or incomplete information. You are strongly advised to seek clarification on any information presented. If you find any errors or omissions, please contact us to let us know so that we can put it right.

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