Visibility is the transparency of the atmosphere and is defined as the max. distance at which an object can be clearly seen and distinguished in normal daylight.
Visibility can be reduced by liquid or solid particles in the air as
By Mist or fog
Dust, volcanic ash.
Visibility can therefore vary in different directions.
MIST / FOG:
Mist is said to exist when visibility is reduced by water particles that have condensed on dust, minute particles of salt, etc. but are so small that they remain suspended in the air.
If mist becomes dense & reduce visibility to 1 km. or less it is called fog, Mist can occur when RH is as low as 80%. Mist is always experienced before and after fog.
If visibility is reduced by solid particles such as dust, sand ,volcanic ash, in suspension in the air, haze is said to exist.
Spray is the name given to small droplets of water driven by the winds from the top of the waves. Sprays affect visibility when the wind force is 9 or more.
Also called land fog because it forms only over land, not over sea. During the night land gives off it’s heat very fast. On clear nights, the radiation of heat from the land surface into space is quicker as it is unobstructed by clouds. The air in contact with the ground thus gets cooled and if cooled below its dew point, a large quantity of dew is deposited.
If, however, a light breeze is blowing, turbulence causes the cold air from the land surface to be communicated to the air a couple of metres above the ground – shallow fog called ground fog results.
The visibility above at eye level above this ground fog may be good, but in fog it may be very poor.If the wind is bit stronger the radiation fog may extend upto 150 mtr from the surface.
Radiation fog which originates over land may drift on to rivers, lakes,harbours.
For example; Fog on the Thames river, Dover strait, the sand heads of the Hooghly.
Radiation fog reaches its maximum about half an hour after sunrise, because air temperature is at its lowest at that time. It generally dissipates after the sun has shone for a few hours and the land surface has warmed up.
SEA FOG (Advection fog):
It is called sea fog because it is mostly found over sea.
It is formed when a moist wind blows over a relatively cold surface of sea or land. When the moist air is cooled below its dew point , the excess water vapor condenses into small droplets of wateron dust or minutes particles of salt, resulting in seafog.
Wind causes sea fog to form and also to spread. If the wind is quite strong , turbulence cause fog to form to considerable depth. However very strong winds carry the moisture too high resulting in low clouds.
Examples of Sea fog Area;
1.On the grand bank of Newfoundland , where the warm moist Westerlies ,blowing over the warm Gulf stream , cross over cold Labrador current.
2. Off the east coast of Japan where the warm, moist westerlies, blowing over the warm KU-RO-SHIO current cross over the cold OyaShio.
3. The south coast of UK in winter, whenever SW winds blow. These winds come from lower latitudes and blow over the sea and are hence warm and moist, compared to the cold land surface.
Advection fog forms when warm, moist air passes over a cool surface. This process is called advection, a scientificname describing the movement of fluid. Inthe atmosphere, the fluid is wind. When the moist, warm air makescontact with the coolersurface air, water vapor condenses to create fog.
Smog is radiation fog mixed with industrial smoke.
Arctic sea smoke:
When very cold, dry air passes over a relatively warm sea surface, the water vapor evaporating from the sea surface is quickly condensed into water droplets and it appears as if vertical strips of smoke are rising from the sea surface.