Apart from very small water droplets, visibility may be reduced by solid particles or precipitation.
Smoke: Smoke consists of solid particles produced by combustion. Conditions will be worse under STABLE (subsiding air) conditions. Smoke may cause widespread reductions, e.g. forest fires in Indonesia.
The reduction will depend upon:
• Rate of production
• Rate of dispersal by wind
• Distance from the smoke source
The particles provide ample hygroscopic nuclei for vapour to condense on to, thus increasing the severity of radiation fog.
Dust: Dust is a particle less than 0.08 mm in diameter. Because of its lightness, it may be carried high into the atmosphere. The surface wind speed is likely to exceed 15 kt and as the speed increases, so will the height to which the dust will rise.
Dust storms mainly occur in daylight due to the DV of wind, but simple dust is very small, it may stay in suspension and visibility not improve for a day or so. Examples are the Khamsin and Haboobs, which will be covered later.
Sand: Sand consists of particles between 0.08 and 0.3 mm in diameter. Wind speed will be 20 kt or more. The greater weight of sand particles means that they will only be carried a few
feet above the surface. Again, more a daylight event, due to the DV of wind.
Visibility: In dust storms or sandstorms visibility is likely to be reduced to less than 1000 m.
Precipitation: Reductions in visibility caused by precipitation have already been covered in Chapter 13 Cloud Formation and Precipitation, but to recap, they are:
Drizzle: 500 to 3000 m
Rain: Moderate: 3000 m to 10 km – Heavy: < 1000 m
Snow: Moderate 1000 m – Heavy: 50 to 200 m
Drifting: (<2 m above the surface) will reduce the above.
Blowing: (2 m above the surface) will GREATLY reduce the above.