Thunderstorms are, of course, well known for their thunder, as well as lightning, hail, tornadoes, and as a pretext for The Wizard of Oz. At ground level, lightning poses a threat to buildings, people, and other objects; hence, they were once referred to as "electrical storms." Much of that risk has been reduced over the years by Benjamin Franklin's invention of the lightning rod, which redirects electrical energy safely away from structures. While lightning strikes do pose a risk to aircraft, modern airliners are designed to withstand contact with both lightning and static electricity in general. In many cases, lightning strikes to an aircraft’s surface goes completely unnoticed by the occupants, aside from a momentary flash and possibly the sound of thunder.
Air-Mass Thunderstorm Over Southeastern US
Hazards Beyond Lightning
If lightning does not pose a significant risk to aircraft, why do pilots avoid it? There are two main reasons. First, even though lightning is not likely to disable an airliner, there is no reason to take that chance. Second, and more importantly, thunderstorms contain a host of other weather phenomena, including hail and convective motion, which is the air-lifting process caused when heat is released by condensing moisture.
For aircraft, the most hazardous part of thunderstorms is convective motion, commonly referred to as convective activity. Convective activity happens when moist air is forced upward by an outside force, such as, a front, a hot area of the Earth's surface, or wind flowing over a mountain. As the air moves upward, it is exposed to colder air at the higher altitude. This, in turn, cools the lifting air and causes the water within to condense. As the water condenses, it releases its energy, which reheats the up-going air. That air is then propelled higher by the temperature difference between its newfound heat and the surrounding air. This creates an almost perpetual process, which continues skyward literally for miles.
While convective activity certainly occurs inside thunderstorms, it can also happen in partially formed storms, along weather fronts, and in other places where a complete thunderstorm has not developed. The air being lifted by the convective process is what creates the clouds, rain, and hail produced in and around storms and other convective areas.
Moreover, a lack of lightning does not mean a lack of concern. Other convective storm features can occur entirely without lightning or thunder. Hail, in particular, poses a threat to aircraft. While falling hail can damage cars, a jet airliner flying through hail at 400-500 miles per hour can be rendered unairworthy in seconds, with cracked windows, dented wings, and significant engine damage.
Avoiding Downdrafts and Microbursts
Still, the biggest concern for pilots near convective activity is not lightning, rain, or hail, but the convective activity itself. The force of the up-going air can be extremely powerful with currents moving up and down faster than the planes themselves regularly climb or descend. These forces are not only strong, but rapidly changing. An aircraft that encounters convective activity can quickly be tossed hundreds or even thousands of feet higher or lower. The force can be enough to severely damage and even destroy an aircraft. Many planes have been lost to thunderstorm encounters in which the plane broke apart in the turbulence.
Not only can strong air motion break an airplane apart, but it can also cause a plane to lose altitude rapidly. This effect is referred to as a downdraft. Downdrafts can occur at a range of heights but are most common near the ground, where they pose the greatest threat to aircraft. Downdrafts are sometimes associated with microbursts, a short-lived – usually less than 15 minutes – event in which air moves downward from a convective source, striking the ground and spreading outward. Downdrafts and microbursts, in particular, have the strength to push a plane to the ground.
This was the unfortunate result on August 2, 1985, when a Delta Air Lines L-1011 came down short of the runway as Dallas-Fort Worth International Airport. The resulting crash left only 27 survivors out of the 165 people onboard. The cause was determined to be an undetected downdraft near the airport. In the years since that accident, airports have begun installing windshear detection devices, which can warn of such events.
For all of the above reasons, pilots, airlines, and air traffic controllers work hard to keep aircraft clear of thunderstorms and other convective activity. There are three places where an aircraft may encounter convective activity: at or near the departure airport, en route, or at or near the airport of intended arrival.
Encountering the Threat of Convective Activity
Takeoff and landing phases of flight incur the most significant risk from convective motion for two reasons. First, aircraft in the middle of their journey can simply go around most areas of undesirable weather, whereas airports are at fixed locations, limiting pilots' abilities to maneuver around thunderstorms. Second, planes taking off and landing are much closer to ground level, making them more vulnerable to microbursts and downdrafts. As a result, planes often delay takeoff or landing when convective activity approaches an airport. In some cases, the inclement weather lasts long enough to force planes to land at an alternate airport. This process is known as diverting.
While thunderstorms are most disruptive near airports, they also affect aircraft between airports. Before departure, pilots, airline dispatchers, and air traffic controllers work together to build a flight path that avoids large areas of unsafe weather. (Airline dispatchers are the pilots’ counterpart on the ground. They create flight plans, manage weight and balance, and provide in-flight support to flight crews.) Once airborne, pilots work directly with air traffic control to maneuver around small areas of weather.
Ground Stops and Flight Delays
Of course, for the business travelers who are now late to a meeting, the wonders of the Earth's atmosphere may not be the most pressing issue. Instead, they are interested in the practical questions of the day: “Why is my flight delayed and when will we get there?" As noted earlier, there are three points where the plane may be held up. First, the flight may not be able to depart due to conditions associated with a storm. Last year, an Aerolitoral Embraer 190 (operating for AeroMexico) attempted such a takeoff from the Durango, Mexico airport. The plane came to rest more than a thousand feet beyond the end of the runway before bursting into flames. Fortunately, everyone survived, and no major injuries were reported.
Second, a plane may be delayed because its planned route of flight has convective activity present or that is about to become present. Air traffic control will not allow a flight to depart knowing that it will encounter hazardous conditions. In these situations, the controllers will work to find the plane a new (safer) route. However, in high-traffic areas like the northeastern US, there may not be enough airways (routes in the sky) available to accommodate all of the planes that need to be rerouted due to weather. The result is often lengthy ground delays.
Finally, there is the arrival airport to consider. When convective activity is present or suspected, airliners often delay landing and may even divert to another airport to wait out the weather. The waiting aircraft are directed by air traffic control to specific locations to prevent collisions with other aircraft. In many cases, the planes are placed in holding patterns, which allow the aircraft to circle a specific area while waiting.
If aircraft are holding in-flight or are expected to begin holding, air traffic control will often issue a ground stop order. Ground stops force planes inbound to a particular airport to remain at their departure airport if they are not already airborne, to prevent having too many planes in the air simultaneously. Once the ground stop order is lifted, aircraft are again allowed to depart in the direction of the weather affected airport. In other cases, airplanes are not stopped entirely but are forced into a flow control program, which delays and sequences airplanes at their departures airports to prevent too many planes from arriving at the same time.
Area of Air-Mass Thunderstorms As Seen By The Pilots Of A Commercial Flight
Predicting Convective Activity
Is there any way to predict convective activity? Often, yes, but the type of prediction depends on the cause of the convective weather. Weather fronts sometimes referred to as air-mass fronts are relatively predictable, even days in advance. Convective frontal weather is particularly common in the spring and fall in the continental US. Because frontal weather can be predicted with some level of accuracy, flights that are likely to be affected are delayed or canceled in advance, allowing an opportunity for passengers to be re-accommodated on other flights.
The hardest type of convective activity to predict is air-mass thunderstorms. These storms are the result of uneven heating of the Earth’s surface by the sun. They are particularly common in Florida and the Southern States; however, they can form anywhere over land as long as sufficient moisture exists. Desert regions are less prone to this type of convective activity because of the lack of humidity ordinarily present in the air.
While it is possible to know where air-mass storms are likely to occur, exact predictions simply are not feasible. Air-mass storms form rapidly, change continuously, and are not detectable by radar in the early stages of development. Pilots often navigate around this threat visually, looking out the cockpit windows. This type of storm is most common in the summer, typically developing in the afternoon and dissipating later in the day, as the sun sets.
There is a third type of convective activity, which is caused by terrain features such as mountains and mountain ranges. This type of activity is referred to as orographic and is caused by orographic lifting: air moving upward because of the land it encounters. Thunderstorms caused by orographic lifting tend to be localized and appear in recurring locations. Pilots and air traffic controllers who work in that area will be familiar with the places where this occurs and will have plans in place. This type of convective activity usually has little impact on commercial air traffic.
Timing the Weather
One factor that most airline passengers overlook is the time of day. While frontal and orographic storms occur at any time of day or night, air-mass (summer) thunderstorms occur predominately in the afternoon. In parts of Florida, summer thunderstorms are a daily event, occurring at almost the same time every day for weeks at a time. Many airline pilots intentionally request to fly early morning flights in the summer, to avoid the hassles associated with handling thunderstorms and the inevitable flight delays and cancellations.
Flying the Point Home
Modern airliners provide a level of safety greater than ever before and even offer safer travel than many forms of ground transportation. However, thunderstorms and convective activity, in general, are still forces to be reckoned with and must be treated with great respect. Delays and cancellations are inconvenient, but as the old pilot saying goes: “It is better to be on the ground wishing you were in the air than in the air wishing you were on the ground."
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