Refer to figure.
RADIATION FOG. Caused by radiation of the earth’s heat at night, and the conductive cooling below dew point of the air in contact with the ground, in conditions of calm or very light wind and clear skies.

CONDITIONS NECESSARY FOR RADIATION FOG TO FORM

• LITTLE CLOUDS OR CLEAR SKY - increases the rate of terrestrial radiation and cooling of the ground.
• HIGH RELATIVE HUMIDITY - little cooling will be enough to cause saturation and
  condensation.
• CALM OR VERY LIGHT WIND – which will result in rapid cooling. 2 - 8 kt to mix the layers of air causing turbulence => promotes a thicker layer.
• SURFACE - only occurs over land, because ground surface is a better heat conductor than sea surface => we can, therefore, exclude the options which contain “sea”.
• PRESENCE OF PRESSURE SYSTEMS. High Pressure systems/ Anticyclones, Ridges and Cols are very favourable to radiation fog formation.
  • Due to subsidence, clear skies and calm winds are typical of a High-Pressure system. On the other hand, a Low-pressure area is characterised by the presence of clouds.