NEW DELHI: Last month, over 75 flights were diverted when thunderstorms with speeds of 50-80 kmph hit Delhi airport. While thunderstorms pass in a few minutes, they cause a cascading effect, leading to flight delays throughout the day.Around IGI airport, 45 thunderstorms normally occur in a year, and of these, 29 strike from May to Aug. Thunderstorms are often accompanied by lightning, wind gusts, squalls and hail, impacting flights during different stages of operations, including takeoff, cruising and landing. When a thunderstorm hits an airport, wind direction changes frequently. A flight cannot take off or land if the wind direction is not uniform.In May 2018, about 70 Delhi-bound flights were diverted — the highest impact of thunderstorms in summer in the past few years. However, in the last month, six major thunderstorms happened in the city, impacting flight operations.On May 2, an intense thunderstorm, accompanied by a squall of 74kmph, diverted three Delhi-bound flights to other cities. Over 500 flights were delayed due to a ripple effect on the network of airlines.When another such thunderstorm occurred around IGI airport on May 17, with winds touching 74kmph, no flight was diverted, but six flights performed a “go-around” due to wind pressure while over 300 flights were delayed. In aviation terminology, a “go-around” describes a safety procedure where pilots abandon their landing attempt and the aircraft ascends back to a safe altitude to either attempt another landing or divert to another airport.Significant impact was seen on May 25 when a thunderstorm diverted 49 flights to other cities.“Of all the weather-related hazards, thunderstorms have the highest impact on aviation across the world. During a thunderstorm, an aircraft experiences turbulence as air currents are unstable, leading to updrafts and downdrafts,” said R K Jenamani, senior scientist, IMD.Updraft is a strong column of rising air that can suddenly lift the aircraft while downdraft is a rapid column of sinking air, which often happens after the updraft weakens, causing an aircraft to lose altitude.A study, ‘Characteristics of Thunderstorms and Squalls over IGI Airport’, which analysed data from 1995 to 2005, found the maximum thunderstorms in June followed by July, and maximum squalls in May followed by June.“More than 80% of thunderstorms in each season are of duration less than three hours, with the remaining mostly 3-6 hours. For pre-monsoon months, the most favourable timing of thunderstorms and squalls is 1200-1500 UTC, while for monsoon, it starts earlier,” said the study.Another study, ‘Bad Weather and Aircraft Accidents – Global Vis-à-vis Indian Scenario’, found 21% of aircraft accidents in India during 1992-2008 were weather-related. Across the globe, it was 26-32%.The study revealed the highest number of these accidents was due to wind-related factors, like gusts, updrafts, downdrafts and low visibility, each responsible for 16% of total weather-related accidents. About 12% of weather-related accidents were due to cumulonimbus and thunderstorms. In the US, 48% was due to bad wind conditions, as found for India, followed by 20.5% due to low visibility and ceiling.
