1 e Visibility

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ISIBILIT


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VI IBILITY

In meteorology, visibility is a measure ofthe distance at which an object or light can beclearly discerned. It is reported within surfaceweather observations eitherin meters or statute miles, depending upon the

country. isibility a!ects all forms of tra"c#roads, sailing and aviation. $eteorologicalvisibility refers to transparency of air# in dar%,meteorological visibility is still the same as in

daylight for the same air.In e&tremely clean air in 'rctic or mountainousareas, the visibility can be up to 7( %ilometresto 1(( %ilometres . )owever, visibility is often

reduced somewhat by air pollution and


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VI IBILITY

isibility can be reduced by li*uid or solidparticlesin the air as in the following cases#+a$ist or fog+b-recipitation

+cpray+dmo%e+e ust, volcanic ash, etc.isibility can therefore vary in di!erent

directions.0ater vapour is a transparent gas and does notreduce visibility.


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VI IBILITY he main criteria for fog formation is water2saturated air. 0hen air is saturated, the watervapor turns into li*uid water droplets in a processcalled condensation. he two ways air canbecome saturated are cooling the air to itsdewpoint temperature or evaporating moisture

into the air and increasing its water vaporcontent.he 3ve most important factors for fog formationare

1 long nights during colder months45 clear s%ies and light winds4moist air4 a low2level temperature inversion and

8 su"cient supply of condensation nuclei.


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VI IBILITY he term fog is used when visibility reducesto less than 1%m and the relative humiditye&ceeds =8>.

0hereas mist is reported when visibilitye&ceeds 1%m.

?bscuration by dryparticles is de3ned as haze , which

must not be mi&ed up with the popular,but somewhat vague term @ha:y s%ies@describing high thin clouds +often cirrusobscuring the sun.

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VI IBILITY Types ! !" #$a%iati& ' "'(&% !" 2 in mountain areasand cold pools often seen as valley fog. Aadiation

fog might be rather shallow over damp and wetground after arainy night.uring the night, land gives o! its heat

very *uic%ly. ?n clear nights, theradiation of heat from the land surfaceinto space is *uic%er as it isunobstructed by clouds. he air in contact withthe ground thus gets cooled and if cooled belowits dew point, a large *uantity of dew isdeposited. If, however, a light bree:e is blowing,

turbulence causes the cold from the land surface

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VI IBILITY Types ! !" #$a%iati& ' "'(&% !" 2he visibility at eye level above this ground fog

may be good but, in the fog, it may be only acouple of hundred metres orless. If the wind is a bit stronger, radiation fogmay e&tend upto a height of about 18( metres or

so above the ground. trong winds cause toomuch turbulence, resulting in low clouds +stratustype and no fog.Aadiation fog, which can form over land only,may drift on to rivers, harbours, la%es and othercoastal regions. ;or e&ample# fog on the hamesAiver, over traits, the andheads of the

)ooghly, etc. Aadiation fog reaches its ma&imum

VI IBILITY


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VI IBILITY Types ! !" #)%ve*ti& !" 2 is also called sea !", whenwarm air


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VI IBILITY Types ! !" #)%ve*ti& !" 2 IC he south coast of the J in winter,

whenever 0 winds blow. hesewinds come from lower latitudes andblow over the sea and are hencewarm and moist, compared to the cold land

surface. he possible time of occurrence of advection

fog can sometimes be predicted by plotting thetemperature of the sea surface and the dew pointtemperature of the air as two separate curvesagainst ship@s time

'dvection fog is often associated with the

passage of cold and warm fronts and thus is also

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VI IBILITY Types ! !" #Sea sm+e ' 'ive' sm+e.$ainly caused by evaporation.

his type of fog is rather shallowand lies generally below 1(m inheight from the surface and islocally restricted to places with

large temperature di!erenceswith the air colder than thewater. 0hen very cold, dry air passes over arelatively warm sea surface, the water vapour,evaporating from the sea surface, is *uic%lycondensed into water2droplets and it appears as ifvertical strea%s of smo%e are rising from the sea

surface. It is commonly seen in the 'rctic ?cean.

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VI IBILITY Types ! !" #Sm" is radiation fog mi&ed with industrialsmo%e.

Sm+e -" .Sm"It is a thic%, blac%, oppressive blan%et, which

not only wets all e&posed surfaces but also ma%esthem blac% due to carbon particles in the smo%e.It is found in most big cities.

VI IBILITY


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VI IBILITY Types ! !" #/ill !" ' '"'aphi* !"# 0hen awind comes against a mountain

range and begins to climb over it,it progressively cools adiabatically.'fter dew point is reached, anyfurther cooling causes the e&cessmoisture to condense into waterdroplets forming hill fog or orographic fog

1 e Visibility

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