Why thunderstorms cause big flight delays and how pilots deal with them

Jun 25, 2022

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This week it was reported that thousands of flights have been cancelled across the USA due to severe thunderstorms. Whilst frustrating for passengers, it’s also frustrating for the airline crews who also have families and social events to get home to themselves.

However, when it comes to Mother Nature, thunderstorms are not to be messed with. Along with lightning, there are a number of other factors that pilots must take into consideration before deciding to fly near them.

The Problems With Thunderstorms

Severe Turbulence

Thunderstorms form due to a combination of moisture and rising air. As this rapidly rising air in the thunderstorm cloud runs out of energy, sometimes as high as 50-60,000ft above the ground, what goes up must come down.

As a result, the interior of a storm cell is a maelstrom of rapidly changing winds and these changes of wind direction and strength can be so strong that the effects of an aircraft can be brutal.

Flying through a thunderstorm can cause severe turbulence (image Getty Images)

Whilst these will not cause an aircraft to crash, they are certainly strong enough to cause serious discomfort to those inside. Severe turbulence can cause items as heavy as drinks carts to become airborne, with the potential to cause serious injury to those on board the aircraft.

Strong Winds

Along with the rising columns of air inside the cloud goes the moisture picked up from the surface. As this runs out of vertical energy, it cools in the colder temperatures aloft. Then this moisture falls to earth as rain or hail, creating the brief but intense showers you experience on the ground.

As it falls, it creates downdrafts of air that also head down to earth. When these downdrafts hit the surface, they have nowhere else to go but outwards. Think of pouring a glass of water onto the floor. As the water hits the ground, it splashes outwards. Keep pouring the water and the splashing isn’t uniform. Sometimes it goes farther in one direction, other times not so far.

A profile view of a microburst event. Image: Wikipedia


It’s the same with the wind. As it hits the surface, it spreads out at different rates. It’s this difference in speed that creates the gusts. As the thunderstorm cloud moves across the ground, these gust fronts push out ahead of the cloud, giving a good indication that a thunderstorm is imminent.

For pilots, this change in wind speed and direction can have a major effect on the aircraft known as wind shear, a well-known and understood threat to aircraft.

Heavy Rain

Stopping a 200-ton airliner safely on the runway takes accurate calculation and effort by the pilots and like when driving a car, an aircraft stops more efficiently when the runway surface is dry. The more flooded it becomes, the less effective the brakes become.

When too much water builds up on the surface of the runway, the aircraft’s tires can lose contact with the surface in a phenomenon known as hydroplaning.

When this happens, the effectiveness of the brakes is massively reduced as the aircraft in effect is just surfing along the top of the runway.

With this in mind, the runways at most large airports are built with grooves cut into the surface and are angled off to the sides so that the centre of the runway is actually higher than the edges. This helps excess water drain away from the paved surface, reducing the chances of hydroplaning.

How do these elements affect flights?

As your pilots, our prime concern is for your safety and comfort. As a result, avoiding the effects of thunderstorms is high on our agenda. The best way to do this is to avoid them altogether.

During the day, some storms are easy to spot. However, at night, or when the storms are shrouded by other clouds, they are not so easy to identify. For this, we have our weather radar.

Weather detection on an aircraft is based on a transmitted energy beam hitting water droplets in a cloud and then bouncing back to the aircraft. As water droplets vary in size and density, the more dense the water droplet, the greater the radar return.

Thunderstorms show up on the screens in the flight deck (Photo by Charlie Page/The Points Guy)

As a result, particularly dense water droplets such as hailstones will have a greater return than less dense droplets such as fog. These returns are then depicted in the flight deck on the navigation display. Green areas are those of low returns, whilst red indicates areas of high returns.

Most weather radar systems on newer aircraft also feature a turbulence detection function. This uses the Doppler effect to detect the movement of the water droplets and areas of turbulence are depicted on the screen in magenta.

Storms enroute

When we have identified a thunderstorm on our route, we will then try to determine the best way to avoid it. Normally we will endeavour to fly upwind of the storm as the downwind area tends to be the bumpiest. However, when there is a line of storms dotted across our path, this may not always be possible.

When we have decided on our preferred route, we have to request this from ATC who will in turn check that our new route will keep us clear of other aircraft.

The problem comes when there are extensive thunderstorms in a relatively small geographic area and multiple aircraft are trying to deviate around them.

Pilots use the weather radar to navigate around thunderstorms. (Photo Charlie Page/The Points Guy)

When ATC all of a sudden have 50% of the airspace to fit in 100% of the aircraft, there is an obvious problem. To stop this from happening, ATC restricts the number of aircraft in their sector to ensure that they are all safely separated at all times.

Obviously, once an aircraft is in the air, there is no way to make it wait. So, ATC predicts how the forecast weather will affect the flow of traffic during the hours ahead and issue delays to aircraft before they leave the gate.

This is why only a handful of flights from a certain airport may be delayed due to weather whilst all the others are able to leave on time. The bad weather causing your delay may not be outside your window, but it could be hundreds of miles away along your route.

Storms near an airport

Storms en route tend to be relatively simple to deal with from the pilot’s perspective, even if they do cause headaches for ATC. However, storms near or over an airport pose much more of a threat and logistical problems for those of us flying the aircraft and our colleagues working on the ground.

On The Ground

For those people working hard on the ground to load and unload bags from the aircraft holds, cater the aircraft for its next flight, pump fuel into the wing tanks and empty the aircraft toilet waste tanks, lightning from thunderstorms is a real threat to their safety.

When lightning is detected nearby, most airports mandate that the ramp area is closed. This means that all the time-critical jobs that need completing before a flight can depart are put on hold until the storm has passed.

Not only does this delay the outbound flight, but it can also cause knock-on delays for inbound aircraft waiting for that gate.

Lightning can be a threat to ground staff (Photo by Ethan Miller/Getty Images)

There is currently no centralised guidance for airports in the United States to determine when and for how long the ramp should be closed in these situations. Some airport decisions makers use data as rudimentary as hearing thunder and waiting a certain time period until the lighting was last observed before opening up again.

However, many bigger airports have sophisticated equipment installed around the airfield that can detect lightning and accurately pinpoint where it occurred and how much of a threat it is to the airport.

On Departure

Take-off is one of the most safety-critical stages of flight so the effects of any adverse weather such as thunderstorms must be taken into account.

In these situations, it’s the threat of wind shear that can cause the biggest delays to flights.

An aircraft is at its most vulnerable when it is flying at slow speeds, particularly in the moments just after it gets airborne. If it were to encounter wind shear, a sudden and severe change in the direction or speed of the wind, there is a risk that the wings would no longer be able to generate enough lift to keep it flying.

If pilots are not happy the location of a storm in relation to their departure route, they will delay the take off (Photo credit should read MANAN VATSYAYANA/AFP/GettyImages)

As a result, when there are thunderstorms around the airfield on departure, if we feel that they could be a threat to us in those early stages of flight, we will simply tell ATC that we are not happy to take off at that time.

This may even mean lining up on the runway and taking a good look at the weather radar in the direction that we are departing. If there is a storm cell sitting over the departure end of the runway, or in our initial routing, we will inform ATC.

Depending on other aircraft waiting to depart, this may mean sitting in place until such time that we are happy to go, or, taxiing off the runway to allow another aircraft with a different routing to get airborne. This is something that I have done many times in my career.

On Approach

As the aircraft nears the ground, any changes to its flight path can quickly become a serious threat to its safety. Once again, wind shear is a major threat.

As we near the airport, we will keep a constant eye on the weather radar and listen to other aircraft on the radio to find out their experience on the approach. Before starting the approach we will always either be sure that we can make the approach without the risk of being affected by the storm or, if we do encounter it, ensure that we have a plan for a safe ‘escape’ route.

Windshear has been a known threat to aircraft for several decades, so much so that aircraft now use the weather radar system to detect it. Should a hazardous situation be detected ahead, the systems alert the pilots with an audio and visual warning, either instructing them to avoid the area of wind shear or to perform a go-around.

More often than not, if there is any doubt about making an approach, pilots will take up a holding pattern well clear of the weather and wait until it has passed and it’s safe to make an approach. If this takes longer than expected, they may have to divert to another airport where a safe landing is assured.

Another threat comes from heavy rainfall.

Before each landing, we calculate how much runway we will need to stop at our expected landing weight in the expected weather conditions.  The is affected by the temperature, wind strength and direction and also how slippery the runway is.

If we know how slippery the runway is, we can factor this into our landing distance calculation.

The problem comes when a thunderstorm has dumped a massive amount of water on the runway in a short amount of time and the actual surface conditions are worse than reported by ATC.

In these situations, there is a very real risk of not being able to stop in time and going off the end of the runway.

As a result, if there is any doubt as to the surface condition of the runway, we will either take up a holding pattern until its state can be verified or divert to another airport.

Bottom Line

When it comes to safety, pilots and ATC do not take any risks. If a flight needs to be delayed because of the weather conditions, your pilots will not hesitate to do so. We understand that it’s frustrating for our passengers but we are there to get them safely to their destination, even if it is several hours late.

As the pilot’s saying goes, “It’s much better being on the ground wishing you were up there than being up there and wishing you were on the ground”

Featured photo by Getty Images.

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