Definitions

Severity of fog

The official definition of fog is a visibility of less than 1,000 m. This limit is appropriate for aviation purposes, but for the general public and motorists an upper limit of 200 m is more realistic. Severe disruption to transport occurs when the visibility falls below 50 m. Useful labels for these three categories are aviation fog, thick fog and dense fog. The reduction in visibility is due to tiny water droplets suspended in the air. The thickest fog tends to occur in industrial areas where there are many pollution particles on which water droplets can grow. The Met Office...

Severity of frost

Frost occurs when the temperature of the air in contact with the ground, or at thermometer-screen level, is below the freezing-point of water (‘ground frost’ or ‘air frost’, respectively). The term is also used of the icy deposits which may form on the ground and on objects in such temperature conditions (GLAZE, HOAR-FROST). Since the sensation of cold depends not only on air temperature but also on the accompanying wind speed, the fourfold classification of frosts used in forecasts of this condition in the United Kingdom is varied with wind speed. Thus, frost is classified as ‘slight‘, ‘moderate‘, ‘severe‘, or...