Fog

The meteorological conditions required for the formation and dispersal of:

(a) radiation fog;

When the surface cools, the air immediately above it cools. Should the air have a high RH (therefore high dew point) it will only take a little cooling to create condensation.

If there is no wind, dew will form on the ground as water vapour within the air condenses, but with a light wind mixing will take place and the layer of cooled air becomes thicker.

Fog will form so long as the air is stable. After day break the sun will induce mixing in the upper layers initially deepening the fog layer before burning it off.

• Forms at night over land as surface temperature cools due to the loss of terrestrial radiation.
• Requires moist air to form with 2-8 knots of wind.
• Most likely in winter during an anti-cyclone.
• Settles in valleys and low lying areas.
• Thickest just after sunrise.
• Can persist if a layer of high level cloud cuts off solar radiation, i.e. surface warming.