Fog, mist and haze | Previous Top

Fog, mist and haze have in the past been used, rather loosely, to describe decreasing degrees of obscurity in the atmosphere. Modern practice reserves the description 'haze' for occasions when the obscurity is caused by solid particles such as dust or sea salt. Fog and mist are akin in that they are both composed of minute water drops and may thus be distinguished from haze. In practice the distinction is usually made by means of the dry-and wet-bulb readings. The following table gives the approximate criterion for the reporting of mist and haze at various temperatures. Intermediate values may be obtained by interpolation. If the depression of the wet-bulb is more than about that shown in the relevant column B, haze should be reported. If the depression is less, the obscurity should be reported as mist. In the case of the UK Met. Office, a relative humidity of 95 per cent is used as a guide to the dividing line between mist and haze

DEPRESSION OF THE WET BULB CORRESPONDING TO A RELATIVE HUMIDITY OF 95%

Column A | Columns B | Columns B
Dry-Bulb °C | Stevenson screen Depression °C | Aspirated psychrometer Depression °C
40 | 0.8 | 0.8
35 | 0.7 | 0.7
30 | 0.7 | 0.7
25 | 0.6 | 0.6
20 | 0.5 | 0.6
15 | 0.5 | 0.5
10 | 0.4 | 0.4
5 | 0.3 | 0.3
0 | 0.3 | 0.3

The further distinction between mist and fog is only one of degree and is arbitrarily assigned. When the visibility is reduced to less than 1 km or 0.54 n. mile the obscurity is described as fog; when greater than 1 km it is known as mist.