METEOROLOGY FOR DGCA CPL/ATPL EXAM

METEOROLOGY FOR DGCA CPL/ATPL EXAM LESSON 6 #FOG #MIST #HAZE #VISIBILITY #RVR
1. Visibility is one of the most critical meteorological factor which every pilot encounters during flying. Good visibility always gives a pleasant feeling and poor visibility makes the pilots uncomfortable. History of civil aviation is filled with large number of accidents like CFIT, Disorientation, unable to spot runway for landing due to poor visibility. Visibility would govern whether flight conditions are VMC or IMC. It will also dictate the Aerodrome minima or suitability of an aerodrome for landing. So, knowledge of current visibility, its trend and your visibility minima’s is very important in day to day flying.
3. Visibility. Visibility is defined as the maximum distance at which a dark object can be seen by an observer with normal eyesight. Visibility is a measure of atmospheric clarity or obscurity.
Visibility in atmosphere reduces because of the presence of one of the three things; first, water droplets in the form of fog, cloud & rain; second solid particles like sand, dust & smoke; third is presence of water in the form of ice.
Poor visibility is associated with stable atmospheric conditions like anticyclone, COL, inversion and light winds.
4. Fog. It is called fog when visibility is less than one kilometer and the obscuring agent is water droplet and relative humidity is near 100%.
5. Mist. it is called mist if visibility is one kilometer or more; obscuring agent is water droplet & relative humidity is more than 95%. Upper limit of reporting mist is visibility 5 kilometer.
6. Haze. It is called haze if obscuring agent is extremely small solid particles like sand, dust or smoke (not water droplets) when visibility is 5 Km or less.

TYPES OF FOG
1. Radiation Fog. You must have seen thick fog in winters in early morning hours. This is radiation fog and it is caused by radiation of earth’s heat at night and the conducive cooling of the air in contact with the ground to below dew point temp.
conditions necessary to form radiation fog are three
clear skies (helps in cooling of earth at night)
high relative humidity (slight cooling achieves saturation)
light wind of 2 to 8 Kts (water droplets are kept afloat in air otherwise the condensed water will settle down on ground as dew).
Radiation fog generally occurs in night and early morning in autumn and winter. The latest time at which radiation fog can form is about 30 minutes after sunrise. Strong wind as well as insolation (heat from sun) lift the radiation fog and improves visibility. Radiation fog will always form over land and never over sea since sea surface temp does not change much due to cooling effect in night (high specific heat capacity of water)
2. Advection Fog. In video lesson 3 on Temp, we have already studied that advection means horizontal movement of air. So, advection fog is formed due to horizontal movement of air. It forms when warm moist air moves over cold surface and achieves saturation or condensation. This can either be over land or over sea.
conditions necessary to form advection fog are three
cold surface with temp below dew point of moving air
high relative humidity (slight cooling achieves saturation)
wind up to 15 Kts (to move the warm moist air over cold surface).
Advection fog generally occurs in winter and early spring over land; late spring and early summer overseas. Strong wind (more than 15 Kts) as well as change of air mass will lift the advection fog and improves visibility.
3. Steaming Fog. Steaming fog also called Arctic smoke. It is formed only over sea in polar regions. It is caused when cold air from land mass moves over warm sea. This results in evaporation, saturation, condensation and formation of fog.
4. Frontal Fog. Frontal fog occurs when warm front replaces cold front (occlusion/) The fog belt can be 200 nm wide and moves with the front.
5. Orographic Fog. It is also called Hill Fog. You must have seen this type of fog in hill stations when the hill tops are covered with strati form clouds
Presence of dust and sand particles also reduce visibility drastically. Dust can be picked up at wind speed more than15 kts and sand at speed more than 20 kts. Visibility with dust storm may drop below 1 km.
Visibility is measured by an instrument called Transmissometer or Scopograph.
RVR (Runway Visual Range): RVR has been designed since the visibility value is most important at the place and in the direction of takeoff and landing. RVR is the max distance that a pilot 15 feet above the runway in touch down area can see in the direction of takeoff/landing. RVR is reported when visibility drops below 1500 mtr. It is called IRVR, when RVR readings are instrumented. IRVR is reported at three places. Touch down zone, mid point and runway end.
Some facts about visibility: By day, visibility is good looking away from sun, by night vis is good looking to the side of moon. Vis is poorer in drizzle than in rain.