Commercial aviation is sensitive to meteorological conditions for reasons of economy and safety. For long-haul flights, the atmosphere is most heavily sampled at cruising altitudes, typically in the range of 30,000 to 40,000 feet. Critical factors in the flight performance include the accuracy of the predicted and actual wind at these heights, the effectiveness of the airline's use of the forecasts, and how ‘enhancements’ can improve the forecast winds.
It is well known that winds increase with height, till tropo-pause, then they start deceasing. On the other hand, as height increases, friction also decreases but air density reduces, which adversely affects the performance of the aircraft. Thus, for airlines it is pay-off between fuel efficiency & cruising speed. In India, the cruising altitude are generally between 33,000-37,000 ft. These are levels at which aircrafts will encounter Jet-streams. Jet-streams are core of a very strong, narrow, quasi-horizontal beams of winds. They are Westerly in winters in North India & Easterly in Southern Peninsula. Westerly jet is of the order of 150 knots (270 kmph), while Easterly Jet is weaker with average speed of 80 knots (150kmph).
These jet-steams can play havoc with duration of flights, depending on whether winds are opposite to the direction of movement of aircraft (headwinds) or in the direction of movement (tailwinds). To amplify, a Delhi-Guwahati flight in the month of November at 35,000 ft, will take about two hours, but the same flight, returning at same level, few hours later, can easily take three hours.
The other off-shoot of the jet-stream is that it causes ‘Clear Air Turbulence’(CAT), which is turbulence outside clouds & in clear sky. It can be so severe that it can toss aircraft few hundred feet in a second causing temporary loss of control. This can be unnerving for aircrew & passengers & in extreme cases, can result in injuries to passengers & cabin crew. The worst part is that CAT is patchy in nature, difficult to predict as it does not have radar signatures.
A critical choice made before any flight is the amount of fuel to carry. One factor (of many) influencing that choice is an estimate of the flight time in the light of predicted upper air winds. It is known that the quantity of fuel loaded is quite sensitive to the winds provided, which in turn will decide number of passengers and hence, cost of operations.
