What is a thunderstorm?
Generally thunderstorms require three conditions to form: moisture, instability and lift. They are produced by cumulonimbus clouds which are created when warm, humid air is lifted. As the air is lifted it cools and the moisture in the air condenses to form a cloud. If the air continues to rise the cloud will grow taller and water droplets in the cloud grow in size.
Further lifting above the freezing layer can result in ice crystals and hail forming in the cloud. A cumulonimbus cloud can develop from these conditions as powerful upward air flow, atmospheric instability and condensing water vapour create towering clouds of between 6,500ft and 60,000ft in height -this is when thunderstorm activity is most likely (1).
Thunder and lightning occur when a charge separation in the cumulonimbus cloud results in a discharge of electrical activity from the cloud. Once ice crystals have formed in the cloud there is potential for a thunderstorm. Turbulence in the cloud causes tiny splinters of ice to break away from the hail and become electrically charged. These fragments carry an electric charge of opposite polarity to the hail and the lighter ice particles are lifted upwards. These particles grow in size as they rise above the freezing layer and pick up more water droplets continuing the cycle and increasing the charge separation. This results in the upper section of the cloud becoming positively charged whilst the lower section is negatively charged.
The separation of charged particles creates an electrical charge difference which when large enough discharges to the surroundings. This can either be between clouds (cloud to cloud) or between the cloud and Earth’s surface (cloud to ground) and is observed as lightning. As the electrical energy is discharged, large amounts of heat are released causing the surrounding air to expand rapidly. Thunder is heard as pressure waves reverberate through the cloud following the discharge of electrical energy.
Impacts to airlines and aircraft…
Cumulonimbus clouds and thunderstorms present a serious risk to aviation. Aircraft are susceptible to hail damage, lightning strikes, strong updrafts and downdrafts, aircraft icing and powerful winds. Aircraft which attempt to fly through cumulonimbus clouds expose themselves to severe danger. If a plane were to fly from an updraft into a downdraft it may lose control and structural damage can result due to stress on the plane. As cumulonimbus clouds also hold large amounts of ice, above the freezing level icing can occur and may be severe enough to freeze engines. In addition, the hazards presented by cumulonimbus clouds and thunderstorms are not limited to planes in the sky. There are also problems aircraft on the ground need to be aware of with the main danger arising from sudden changes in wind direction as the storm arrives.
Aviation accidents related to adverse weather conditions form a substantial proportion of total aviation accidents and guidelines suggest pilots should avoid thunderstorms by 10 miles whilst keeping up to 20 miles clear from severe storms due to the dangers present. Accidents related to thunderstorms are most commonly due to pilots flying into a storm that either they didn’t know existed (i.e. an embedded storm) or underestimated the storm severity (2).
Impacts to air traffic controllers…
Thunderstorms and cumulonimbus clouds result in an increased workload for air traffic controllers as they divert aircraft around hazardous zones and can result in delays and/or cancellations for air travellers. Since summer 2012 the Met Office has provided a forecast of the risk of thunderstorms and deep convection within the LTMA (London Terminal Manoeuvring Area), the section of airspace over the London region, to the UK Air Navigation Service Provider (ANSP) in order to aid their management of any disruptive systems in the area. Advance warning of thunderstorm activity and cumulonimbus clouds in a particular area enables the UK ANSP to reduce disruption caused by altering air traffic flows prior to the event.
Deep convection often results in reduced operational capacity at airports as strong winds, heavy rain/ hail and lightning bears down. As a result departure and/or arrival regulations are put in place to limit the number of planes using the airport. This helps to allow the airport to reduce congestion as the tower won’t start as many planes in order to avoid gridlock.
Thunderstorms not only affect the airports directly by reducing their operational capacity but can take out stacks for the airports resulting in longer stacking queues. In some instances, air traffic controllers will be able to slow planes down on route to help avoid congestion in the airspace. Deep convection will also affect the number of planes changing sectors in the airspace as diverts are put in place. This increases the number of handovers of planes between air traffic controllers and ultimately raises their workload.
Katie Brown – Aviation Scientist