Tropical storm Merbok lifted the curtain on Hong Kong’s typhoon season on Monday, disrupting hundreds of flights and sending travel plans into chaos. During inclement weather, some flights do manage to take off or land at Hong Kong International Airport while others do not. What factors determine whether flights are delayed, cancelled or can go ahead? According to some airlines and the Hong Kong Airport Authority, wind speed and direction are crucial during conditions such as thunderstorms, lightning and fog. When faced with adverse weather conditions that make air travel unsafe, the Hong Kong International Airport operates an emergency centre for coordinating all necessary contingency measures including flight rescheduling and crowd management. As of 5am on Tuesday, almost 400 flights were disrupted, with 44 flights cancelled and 350 flights delayed as Merbok approached and finally hit the city on Monday night. Because the airport handles 68 flight movements per hour and 1,160 flights a day, storms like this pose a great challenge for staff, who have to handle flight disruptions at a total of 182 parking stands. What is the procedure for flight movements in bad weather? When the Hong Kong Observatory issues weather warnings for the days ahead, such as rainstorm, lightning or tropical storm alerts, the emergency centre is activated so contingency arrangements can be made. The Airport Authority and all the airline companies then work closely together on flight rescheduling to facilitate aircraft rotation at the airport based on forecasts by the Observatory and the Civil Aviation Department. What are the factors affecting flight cancellations or delays? A Hong Kong Airlines spokeswoman said many factors were taken into account when making decisions on flight rescheduling. The most significant factors are wind direction and speed as all planes have certain wind limitations for taking off and landing, she said. Ground equipment at the airport, such as jet bridges and conveyor belts, is also subject to wind speed limits. For example, due to Merbok, strong southwesterly winds gusting up to 35 knots struck the airport at about midnight, causing serious flight disruptions. Nearly 400 Hong Kong flights disrupted by Tropical Storm Merbok Heavy rain and cloudy skies can significantly reduce visibility, which will determine whether a plane can take off. “Land lightning limits outdoor activities. Refuelling procedures are not allowed during this period of time,” the spokeswoman said. Flight operations can continue after a thorough risk assessment is performed and the following conditions are fulfilled: wind and visibility conditions fall within permitted limits; no lightning in the vicinity of the airport that prevents refuelling or restricts the movement of people on the ramp; and all precautions have been taken, such as carrying extra fuel for possible diversions. What about Cathay Pacific Airways? A Cathay Pacific spokeswoman said the issuing of a typhoon signal was only one of the factors affecting their flight operations as there were other weather-related factors to consider. “Safety is always the top priority at Cathay Pacific and Cathay Dragon. We assess the overall weather situation, including wind velocity, direction, visibility and precipitation to determine the safe operation of our flights,” she said.