Arrival of winters over north, east and northeast India announces problems, both for operators and air travellers. Poor visibility ranging from zero to few hundreds of meters becomes very common which hampers air operations and troubles the commuters. This phenomenon sometimes comes with a very short notice thereby taking everyone with a surprise.
Air operations are largely dependent on prevailing visibility conditions. Range of visibility becomes the deciding factor for take- off and landing of airplanes in poor visibility. Mostly the air operations become restricted when the visibility drops to 500 mtr or less. Visibility range of 200mtr or less, which amounts to dense fog, substantially curtails the air traffic. Very dense fog with visibility less than 50 mtr is practically considered zero visibility and virtually stops all air operations.
Visibility which is so vital for clearing air operations is reported both, manually and with instruments. Many of the important airports have instruments to record and report visibility when it is in the range of zero to 2000mtr. Beyond this range, the operations are mostly unrestricted on account of visibility.
In manual observations, the visibility reported is generally the lowest amongst all the directions. However what matters for air traffic is the prevailing visibility, down the runway or runway in use. Take off and landing of planes is generally done into the wind, prevailing at that point of time. Visibility reported along the runway is commonly termed as RVR, which is runway visual range. RVR, in poor visibility conditions is invariably different from the general visibility reported for the airfield. What matters for air operations is RVR and this could be higher than the general reported visibility by as much as 500mtr or sometimes even more.
Available technology and equipment, on ground at airfields and on board the planes has enhanced the operating minima in adverse weather conditions. 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 machines. Most of the airlines have enabled their pilots, through training, to be categorized and compliant for CAT-I to CAT-III which virtually permits them to undertake air operations in very poor visibility conditions.
Poor visibility affects air operation for three months, from December to February. Episodes become more frequent and intense between 15 Dec and 31 Jan, specially on days when ground inversion becomes prominent. Indo- Gangetic plains become more vulnerable to dense fog and the hilly areas mostly remain clear of this impediment.
The forecast of such poor visibility conditions and that too, with adequate notice remains a challenge for the Meteorologist, even today. Truant nature and tricky characteristics of fog continue to hound the weathermen to put in more efforts to understand and forecast the phenomenon, effectively and confidently.
