Atmospheric Obscurity

For a PPL, CPL & ATPL aspirants who wants to become a good pilot, during flying prior to departure or enroute, earth atmosphere plays a vital role for safety of the aircraft. To ensure this, pilot has to be well versant with earth atmosphere & other topics related to aviation meteorology. Study Kwik eLearning module of Aviation Meteorology helps the PPL, CPL & ATPL aspirants to cover one of the important topic of  Atmospheric Obscurity.

ATMOSPHERIC OBSCURITY

Visibility

Visibility in pure air should be infinite. However, air is never pure and is laden with dust, smoke, industrial pollutants, aerosols etc. Presence of these suspended obscurities reduces the visibility.

Horizontal Visibility

Horizontal Visibility is the maximum distance along the horizontal up to which prominent objects can be seen with the naked eye and are recognized as such under natural light. Visibility may be different in different directions. The visibility reported by Met Office is the lowest Horizontal Visibility. During day Visibility is estimated by seeing objects at pre-measured distances, within and around the airfield. During night existing lights in and around the airfield at known distances are used. These are called Visibility Land Marks.

Runway Visual Range (RVR)

It is the distance up to which the pilot can see from the cockpit along the runway. RVR is reported when the visibility is less than 1500m. RVR is reported in multiples of 25M when between 50M & 150M, in multiples of 50M when between 150 & 800M, in multiples of 100M when between 800 & 1200M, and in multiples of 200M when more than 1200M.

Poor Visibility Phenomena

1. Haze: Atmospheric obscurity due to very small particles of smoke, dust, water etc., wherein the visibility reduces to 5000M or less (but more than 2000M) is termed as haze. When the Relative Humidity is 75% or more, this is termed as Moist Haze(or simply Haze) whilst if the RH is less than 75% it is termed as Dust Haze or smoke Haze. It gives a milky appearance to the sky.

2. Dust Haze: Atmospheric obscurity due to dust particles in suspension with RH less than 75% reducing visibility to 500oM or less is termed as Dust haze. It is caused by the strong winds in the desert or arid areas arising due to a steep pressure gradient kicking up dust in the stable lower layers of the atmosphere.

3. Smoke Haze: Atmospheric Obscurity due to suspended smoke from industrial or domestic sources reducing visibility to 5000M or less is termed as Smoke haze. Smoke Haze occurs when wind is calm and there is strong ground inversion.

4. Mist: Atmospheric obscurity due to suspended droplets wherein visibility reduces to 1000 M to less than 2000 M, with RH being 75% or more.

5. Fog: Atmospheric obscurity due minute water droplets suspended in the air reducing visibility to less than 1000M.In Fog RH is nearly 100%. Fog may be termed as thick, moderate or light depending upon visibility.

6. Smog: When smoke haze & fog co-exist, it is called Smog. In smoke visibility is very poor.

7. Dust Storm: The visibility in dust storm is 5000M or less. If the dust storm is followed by rain visibility improves fast.

8. Precipitation: The visibility in precipitation depends on the size of the drop and intensity of precipitation. In moderate Rain vis may reduce to 1-3 km, in heavy Rain or showers to less than 1000M, in drizzle to 3 km and in snowfall it may reduce to less than 1000M.

Horizontal, Vertical & Slant Visibility

1. Visibility reported by Met Office is minimum horizontal vis.

2. Vertical visibility may be much better during shallow fog as it is the distance up to which one can see objects along the vertical.

3. Slant visibility is the visibility at an angle and can be defined as the distance an object can be seen along the path that is at an an angle to the horizontal( usually glide path or landing path).In shallow fog this visibility is more than horizontal vis but less than vertical vis.

Instruments for Measuring Visibility

1. Scopograph or transmissometer are used for measuring vis along the different runways on an aerodrome. RVR equipment are installed along various runways to measure RVR.

2. Close to take off & landing areas.

3. The equipment consists of a light source and a photo-electric receiver, which generates electric current according to the obscurities present between the transmitter and receiver. The current so generated gives a measure of RVR. The display units are installed in Met Office and ATC Tower.

Fog

1. Fog is composed of tiny water drops or ice crystals. It is nothing but a stratus cloud sitting smack on the ground. Fog occurs due to condensation of water vapour present in the layers of air close to the ground. Condensation can occur either by cooling of the air close to the ground or by feeding moisture in to the air to saturate it. Fog can be various types:

2. Radiation Fog: Radiation Fog forms due to nocturnal cooling of the ground and the adjacent layers. The conditions favorable for formation of Radiation Fog are:
a. Clear skies
b. High relative Humidity e.g. after Rain
c. Light freshening wind of 3 to 8 kt

3. Radiation fog is most frequent at about sunrise and dissipates within 2-3 hours after sunrise due to heating. If the Fog is thick it may take longer to lift. Generally fog lifts in to low stratus clouds before dissipating. Radiation Fog in India occurs mostly during winters after the passage of active western disturbances. It is almost a daily feature in winter season in the Valley of Brahmaputra River where adequate moisture is present and katabatic flow provides the required degree of turbulence and extra cooling. The most susceptible areas for radiation fog are NW India, UP, Bihar, Bengal and Assam (particularly south of Brahmaputra River).

4. Vertically radiation Fog extends to only a few hundred feet whilst sometimes it can extend up to about 1500 ft.

5. Advection Fog: Advection Fog forms when warm moist air from elsewhere is transported over the cold surface (ground or water) where the surface temperature is less than the dew point of the advected air. Advection Fog forms both over land & sea. Coastal fogs over Bengal and Orissa during winters are due to a combined effect of advection from sea and radiation cooling. The Brahmaputra valley, the coastal areas and hills of Indian Peninsula experience advection Fog.

6. Frontal Fog: Frontal Fog forms near the front due to saturation of air in the area of continuous rain ahead of the front. It also forms due to lowering of the cloud to the surface with the passage of a front.

7. Steaming Fog: This type of Fog is common in higher latitudes when cold air mass flows over a warm sea surface. The water vapour from warm sea when comes in contact with the cold air aloft condenses to form steaming fog. It is common over frozen land, ice covered Polar Regions and over Icelandic and Norwegian regions. Over sea it is often referred to as sea smoke. It looks like steam coming out of the surface.

Fog Dispersal:

1. Thermal Dispersion: When air is heated to temperatures above dew point, fog or cloud will dissipate.

2. Dispersal using Hygroscopic Nuclei: When hygroscopic particles are injected into fog water drops form. This lowers the relative humidity of the air and fog dissipates.