Atmospheric obscurity in the lower levels (closer to ground) is mainly due to suspended water particles in air saturated with pollutant particles leading to the formation of smog. Poor visibility ranging from zero to few hundred meters becomes very common in North India, hampering air operations. This phenomenon sometimes takes place at a very short notice thereby taking airport authorities by surprise.
Visibility, which is vital for clearing air operations, is reported manually as well as with instruments. Most of the important airports have instruments to record and report visibility in the range of zero to 2000 mtrs. In manual observations, the visibility reported is generally the lowest in all the directions. Visibility along the runway is commonly termed as runway visual range or RVR, which matters the most for air operations. Take off and landing of airplanes usually takes place in the direction of wind, prevailing at that point of time.
Advanced training of the aircrew has made them compliant of operating in marginal weather conditions without any compromise with the safety of both man and machine. Nowadays, most of the airlines train their pilots to be categorized under CAT-I to CAT-III which virtually permits them to undertake air operations in very poor visibility conditions. Advanced technology and equipment available on board and at airfields have enhanced the operation of airplanes in adverse weather conditions. However, forecast of poor visibility conditions with adequate notice still remains a challenge for meteorologists.
Dense fog- visibility of 200mtrs or less
Very dense fog- visibility less than 50mtrs
Smog- fog or haze intensified by smoke and atmospheric pollutants