Wicked whirlwinds can wreak havoc on airports and aircraft
By Karsten Shein
Comm-Inst Climate Scientist
Amidst the post-sunset gloom and heavy rain, not even the tower controllers saw it coming. A commuter pilot taxiing out to the furthest runway was the first to see it.
With his wings rocking in the sudden wind, he radioed in and watched helplessly as the gray funnel passed over the airport boundary fence, perhaps 500 ft in front of him, headed for an area of hangars.
ATC immediately ordered a ground stop and hastily evacuated the tower. Less than a minute later, the danger had passed, but, in that minute, the tornado produced a 300-ft-wide swath of twisted debris.
Thankfully, there were only minor injuries, and the tornado had spared the main terminal with its thousands of passengers awaiting their flights.
However, the general aviation hangars had taken a direct hit, and several dozen aircraft owners would be phoning their insurance companies in the morning. Inch for inch, tornadoes are easily the most violent weather phenomena on Earth, even though they are nothing more than rapidly moving air.
Despite this, tornadoes tend not to be something that pilots would normally consider a danger to their flight – unlike turbulence, icing, windshear and even the thunderstorms from which tornadoes are born. However, tornadoes do have a history of danger to aviation.
In October 1981, a Fokker F28 inadvertently flew into a tornado after departing RTM (Rotterdam, Netherlands). The ensuing crash claimed 17 lives. Investigators noted the pressure altimeter rose and fell dramatically, corresponding to the sharp pressure drop inside the tornado, and the aircraft encountered +6.8 to -3.2 G over the course of a few seconds, detaching the starboard wing.
A police officer reported seeing the disintegrating aircraft descend from the 1200-ft cloud base. More commonly, tornadoes strike airports and damage parked aircraft. They also mangle homes, business and communities, sometimes affecting the very people and businesses that own or use the aircraft you fly.
In addition, by virtue of being a pilot, your passengers and bosses will expect you to be the company’s font of weather knowledge. This is why understanding tornadoes is important for any pilot, whether or not they live and fly in “Tornado Alley.”
Whether named after the Spanish verb tornar (to turn) or the noun tornillo (a screw), a tornado is a rapidly rotating column of air that is in contact with both the ground and the base of a thunderstorm. Thus, while waterspouts are a type of tornado, dust devils, gustnadoes and firenadoes are technically not.
Tornadoes are invisible, as they are only wind, but frequently, the intense low pressure of the tornado condenses water into a funnel-shaped cloud that gives the tornado its characteristic visual imagery.
Once a tornado contacts the ground, it may also kick up dust, dirt and debris, producing a debris cloud at its base.
Funnel clouds may occur in the absence of a tornado, simply extending from beneath the base of a storm. However, the presence of a funnel cloud usually indicates that a tornado is embedded within, and that the funnel has simply not yet condensed all the way to the surface.
In these cases, a rotating debris cloud at the surface – with or without a funnel – will indicate a tornado in progress. Although tornadoes have been scientifically studied for over a century, and modern technology allows atmospheric scientists to look within their structure, the mechanisms behind the transition from a supercell to a tornadic supercell are still not fully known.
Meteorological research and observation suggest that tornadoes normally begin forming long before they become the wedge or rope linking cloud and ground. Most significant tornadoes form as a part of a large and rotating supercell thunderstorm.
Low-level windshear produced by the interaction of the downdraft gust front and the warm air flowing into the storm’s updrafts can initiate a rotation of the air. Within the storm cell, the rotating up- and down-drafts interact, producing an area near the rear of the storm known as a mesocyclone.
The mesocyclone stretches vertically throughout the lower 2/3 of the storm. Around the mesocyclone, downdrafts are drawing it toward the base of the cloud. The mesocyclone may eventually extend below the cloud base as a rotating wall cloud.
As the rear flanking downdraft of the storm cell descends and spins, it produces low pressure around the central axis that draws the air inward, concentrating it toward a small area at the surface and producing the common wedge or inverted cone shape characteristic of many tornadoes.
As the radius of rotating air decreases toward the ground, the rate of rotation (and wind speed) increases, not unlike a figure skater who brings her arms inward in order to increase her spin on the ice. If the spin increases enough, and if the descending air is not too cold, a tornado may result.
The same lowered central pressure of the tornado vortex is also drawing in the warmer, humid air beneath the storm as a rotating updraft. The rapid inflow forces the downdraft air to reverse direction vertically rather than to spread out at the surface as would be expected from a typical downburst.
Instead, the area around the “eye” of the tornado is known as the corner region, where inflowing winds from the surface boundary layer, which is just a few meters deep, transition from horizontal flow to vertical, entraining the downdraft wind into the updraft.
Above the boundary region, additional warm and humid air is drawn inward in a counterclockwise direction (clockwise in the southern hemisphere) to feed the updraft, but not quite as violently as nearer the surface.
The temperature and pressure drop of the inbound air produces the condensation that forms the funnel cloud to make the tornado visible.
In situations where the surface air is relatively dry, a funnel cloud may not form, and the tornado will remain difficult to see except for the debris cloud at the surface and possible rotation of the storm cloud above. The corner region, like the eyewall in a hurricane, is the most violent part of the vortex, and is where the debris cloud is found.
Using mobile Doppler radars, winds in the corner region (at about 100 ft [30 m] above the ground) have been clocked at over 300 mph (484 kph), and the updrafts can exceed 150 mph (240 kph).
Importantly for aviation, and yet another reason to keep well clear of thunderstorms, is that surface debris (and even fish and frogs) can and has been lofted tens of thousands of feet into the air, and can land well over 100 miles (160 km) from its source. Impact with even a small rock at several hundred kts could prove catastrophic.
When we think of tornadoes, we often associate them with the wide open farmlands of the central US, known colloquially as “Tornado Alley.” However, tornadoes have been observed on every continent except Antarctica.
In fact, although the most violent and destructive tornadoes occur in an area from Texas north to the Dakotas and east from Alabama north to Michigan, and the US is home to 4 times more tornadoes than the next most frequent place (Europe), many of the deadliest tornadoes in the history have occurred in Bangladesh.
Because even with a network of radars we are still not able to accurately estimate the speed of each individual tornado, we rely on a category rating first devised by meteorologist Ted Fujita in 1971, who based estimates of wind speed on the tornado’s path of destruction.
An F0 was associated with light damage and winds less than 73 mph (also the threshold between a tropical storm and hurricane). The scale increased up to an F5, associating “incredible damage” with winds over 261 mph. In 2007, more data connecting wind and damage replaced the Fujita F-scale with the Enhanced Fujita (EF) scale, that sets EF0 winds between 65–85 mph, and EF5 at above 200 mph.
Because of its ease of translation to the public, the EF scale has been widely adopted by meteorological services worldwide. Tornado statistics reveal their widely diverse characteristics. As one would expect, EF0 and EF1 occur most frequently, as very strong supercells are normally required to produce higher EF tornadoes.
Also, the most common place to encounter tornadoes is actually Florida, where near-daily weak to moderate storm cells produce frequent water/land spouts, which are classified as tornadoes. Waterspouts are also frequently encountered in the maritime storms of the intertropical convergence zone.
Just as most tornadoes are relatively weak, they are also mostly short lived, often lasting only a few minutes before dissipating; short traveled, moving less than a mile (1.6 km); slow moving or even stationary; and narrow of path, being less than 330 ft (100 m) wide.
However, tornadoes can reach extreme proportions. The tri-state tornado that crossed Missouri, Illinois, and Indiana in March 1925 reached a record forward speed of 73 mph (117 kph), cut a path of 219 miles (352 km), and lasted 3.5 hrs, leaving 695 dead.
While most tornadoes occur in the afternoon because they are dependent on strong storms that, in turn, need daytime heating to reach peak strength, sharp fronts and mesoscale convective systems that carry on through the night have produced devastating tornadoes at all hours.
Many nocturnal tornadoes rank among the deadliest, simply because they catch most people sleeping, and those who are awake may not be able to spot the tornado approaching in the dark. In addition, those organized convective systems sometimes produce a tornado outbreak involving dozens to hundreds of tornadoes.
Favorable conditions for tornado formation are well known to forecasters, but the actual prediction of a tornado remains elusive. We see this in the fact that well-trained tornado researchers attempting to intercept a tornado for study (storm chasing) simply choose a large area on the map where tornadic storms are likely, but they’re rarely able to anticipate when and where a storm will produce a tornado in time to intercept it.
Similarly, forecasters, such as those at the NOAA Storm Prediction Center in Norman OK, will issue a forecast for a likely area of supercell formation and tornadic activity bounded in space and time. But these are very general bounds, often covering thousands of square miles and several hours of time.
For aviation, these areas are translated into convective sigmets if the area covers at least 3000 sq mi (7800 sq km) and strong (level 4+) storms are expected to affect more than 40% of the area during the sigmet’s 2-hr duration. In a sigmet (or metar remarks), the word “tornado” will always be spelled out.
But even if tornadoes are not specifically mentioned, the storms within a sigmet area will have the potential to become tornadic. In addition, although not explicitly mentioning tornadoes, TAFs and area forecasts will also relay the probability of thunderstorms, the former within 5 miles of the airport.
If the airport is within a convective sigmet, or if conditions are conducive to strong storms, any forecast of thunderstorms in the vicinity should be viewed as a potential tornado threat.
Since tornadoes will normally affect only aircraft that are on the ground, as well as airports, houses, businesses, and communities, pilots, briefers, and controllers should also pay close attention to severe thunderstorm and/or tornado watches issued by weather authorities.
In some places, especially where tornadoes are rare, a severe thunderstorm watch will include the potential for tornadoes. In the US, a tornado watch is issued in place of a severe thunderstorm watch when severe storms capable of producing tornadoes are forecast or approaching the area.
A storm or tornado watch should be treated as an opportunity to prepare for the worst, like reviewing what you would do if faced with a tornado or extreme weather situation. Do you know where you would shelter? Do you have emergency supplies at your disposal?
Regarding your flight operations, can you advance your departure time to clear your aircraft from the area? Do you know where to find a tornado shelter or other reinforced interior room? Airports are particularly dangerous places because the large hangar buildings with massive cross-sections are often not built to survive extreme winds and flying debris, and the acres of wide-open space afford few places to shelter from an approaching storm or tornado.
If weather radar indicates a tornado signature, or if trained storm spotters have seen a tornado, then a tornado warning will be issued for that location and an area downwind of the storm movement. At that time, or if you actually see an approaching tornado – regardless of whether a warning has been issued – it is imperative to seek safety.
Contrary to some myths, tornadoes can strike anywhere, including cities, mountains, rivers, and even airports. Large hangars and aircraft on the ground are not good places to be in or around as a tornado approaches. If time permits, abandon them and seek safer shelter.
In tornado areas, most buildings, including hangars, will have a dedicated tornado shelter – usually a reinforced interior room on either the ground floor or in a basement.
Look for tornado shelter signs. In the absence of that, seek shelter in any interior room with a small footprint and, preferably reinforcement in the wall, such as the pipes surrounding a restroom.
Some protection may also be found in stairwells. Don’t waste time trying to open windows or secure loose items. Tornadoes move fast and their violent forces can tear up or knock down most anything, even if has been well secured.
Although there have been few recorded incidents of people killed in aircraft struck by a tornado, there is ample evidence from motor vehicles that they do not offer any substantial protection from a tornado, and you are safer if you can find shelter outside of the vehicle.
If you are at a gate or parked at the FBO, offload your crew and passengers back to the building as quickly as possible. In the open, the best shelter is a culvert or other low spot in the landscape. Often, taxiways and runways will have drainage ditches on one or both sides.
Similarly, sheltering in the lee of the small utility blockhouses scattered about the airport may also provide some protection from blowing debris. The key is to either put a substantial obstacle between you and any flying debris, or make yourself as small a target for debris as possible.
Remember, the strongest winds are just above ground level, and it is the debris carried by those winds that poses the greatest danger. Importantly, do not try to outrun a tornado, even in a taxiing aircraft. Tornadoes can travel fast, and they may move erratically.
Large storms may even spawn multiple tornadoes, meaning that you may be running from one and head straight into the path of a second vortex that may be less visible. However, from the air, we are often in a position to see the weather unfold.
If you are avoiding storms and notice that they appear to be supercells or even see a funnel forming beneath them, send in a pirep or simply alert ATC so they can relay the information. In a tornado situation, giving people as much advance warning as possible is everything.
Karsten Shein is cofounder and science director at ExplorEiS. He was formerly an assistant professor at Shippensburg University and a climatologist with NOAA. Shein holds a commercial license with instrument rating.