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How Thunder, Fog and Other Weather Conditions Affect Flying

Weather is one of the most important factors to consider when planning a flight. Pilot Tapio Siivola and Karri Hannula, Vice President at Air Navigation Services Finland, discuss the most challenging weather conditions.

Thunder, fog, blizzard and wind. Difficult weather conditions challenge both pilots and air traffic controllers. Pilot Tapio Siivola and Karri Hannula, Vice President at Air Navigation Services (ANS) Finland, discuss how weather conditions affect their work.  

1. Thunder – a summer nuisance 

“An aircraft will start descending for landing 10–15 kilometres from the runway, and if there is a thunder cloud on the line of approach, the runway in question may be unserviceable. The same problem exists with takeoff. You can’t fly through or underneath a thunderstorm cell because of the risk of convection currents and hail. Sometimes you need to go to great lengths to fly around them. It’s easy to see thunder clouds during the day, but at night, you can only detect them with the weather radar,” says Siivola.  

“The conditions change quickly during a thunderstorm. Wind direction may turn 360 degrees within half an hour. Consequently, thunder restricts the number of planes departing from and arriving at the airport. A thunder cell close to the airport with lots of lightning may prevent the fuelling, loading or unloading of planes. If lightning strikes an airplane, people in the plane are safe, but someone standing outside it may get hurt. The worst-case scenario is a thunder cloud bringing the airport to a complete standstill,” explains Hannula. 

2. Fog may prevent landing  

“If the weather is foggy, the distances between approaching planes must be kept longer, and planes may have to circle the airport before landing. If you can’t detect the runway from 30 metres when landing, you may have to pull back up and try again or head to another airport. Wind will clear out fog, so in a foggy weather it’s also very calm. In such conditions, there is no rush, and schedules will have to give way. The objective is a safe landing,” Siivola continues. 

“In dense fog we apply low-visibility methods, which means increasing the distances between planes in the air and on the ground, decreasing the speed of vehicles and pausing work on the apron. If you have 36 planes landing per hour on each runway in normal conditions, then foggy conditions reduce their number to 24 planes per hour,” says Hannula. 

3. A blizzard makes runways slippery 

According to Siivola, snowfall is a challenge especially if there’s also a strong wind. “We always try to land upwind, because crosswind makes landing difficult. Crosswind conditions can be good enough in summer, but during a blizzard, the slippery runway sets limits to landing, and we may not be able to land at all. We always determine a backup airport with a good weather forecast prior to takeoff,” he says. 

4. Gusts complicate flying 

“If the wind is strong but steady, it doesn’t really bother flying. On the other hand, a gusty wind that changes direction at a fast pace is a real challenge. When the plane and the pilot are both shaking, it is difficult to see the meters and you really need to bend over backwards to manage the plane. Moreover, landing in a strong wind is no easy task. Rapid changes in speed during landing may force you to suddenly pull the plane back up and try again a bit later – or head to a backup airport. The wind can be very strong in Norway and the British Isles, for example,” Siivola says. 

“For example at Helsinki Airport, strong winds may render the main runway unusable. Such conditions will force us to resort to other runways, which limits the number of planes landing and taking off. This causes delays in air traffic,” Hannula shares. 

Iuliia Tore