Thunderstorms
Understanding convection and the dangers it creates is key to managing risk.
By Karsten Shein
Comm-Inst
Climate Scientist
Although the tower had advised them to accept a hold, the runway was clearly visible and the 2 pilots had done the calculations in their mind, deciding they could punch through the gap between 2 active thunderstorms that framed their approach.
With fingers crossed that they could reach the FBO and offload their passengers before the storms reached the airport, they followed the glidepath between walls of gray-white cloud lit internally by frequent flashes of lightning.
As they descended through 900 ft, a severe jolt forced the wings to rotate perpendicular to the ground and the nose to drop. Almost immediately, the GPWS blared its warning while the pilot flying applied full power and wrestled with the stick to arrest the unplanned descent.
While they regained control and leveled the wings, they couldn’t reverse the rapid descent, clipping the runway approach lights, landing hard, and sliding to a smoking, gearless stop a quarter of the way down the runway.
Notwithstanding non-instrument-rated pilots flying into IMC, statistics for Part 91, 121, and 135 accidents show that, while thunderstorms are contributing factors, the conditions these storms produce, including extreme turbulence, strong and gusty winds, and windshear, are the most common conditions contributing to aviation accidents.
Unfortunately, despite the obvious danger these massive and energetic convective cells pose to aircraft, more than a few experienced pilots continue to lose their lives to them each year.
Understanding thunderstorms
While the physics behind them can be complex, thunderstorms are simple phenomena. They form when hot and humid air is lifted from the surface into an environment where it is able to continue to rise on its own.
Its growth and lifespan are supported by a continuous supply of that same warm and humid air that may not even need the same initial lifting mechanism to rise, as the already rising air creates an updraft to serve that purpose.
Towering cumuli and cumulonimbus cover the evening sky. Pilots are advised to maintain at least 20 miles clearance around any active thunderstorm and avoid flying between cells with a gap of less than 40 miles.
The air cools as it rises. As long as it remains warmer than that environment, it will continue to rise, aided by the condensation of water vapor that releases heat energy into the air. Continuous condensation of the water vapor in the rising air produces the towering cumulus. However, if the air aloft is dry, those condensed droplets can be re-evaporated quickly, stopping the formation of the storm cell.
Eventually, the rising air will have released enough energy through cooling and condensation that it becomes denser than its surroundings. This air forms a downdraft, signaling the mature phase of the storm.
If there is no wind aloft, the downdraft falls through the updraft, disorganizing it and causing the storm to die quickly. Weak windshear aloft, however, will tilt the updraft, resulting in the downdraft descending away from it and allowing the storm to form an internal circulation cell that may persist for an hour or more.
The forces within even a weak thunderstorm can easily create a loss of control and far surpass tested design loads, destroying aircraft in midair. This is why pilots are taught to steer clear at all costs. Furthermore, these forces are not limited to the inside of a thunderstorm, but can extend to the surface below, several thousand feet above, and many miles around the periphery of the storm.
Fortunately, very few pilots ever find themselves inside a thunderstorm. Those who have, have mostly entered inadvertently. This can happen for different reasons, but often this situation occurs when a pilot loses situational awareness and blunders into a storm they didn’t know was there, or flies into airspace occupied by a developing storm, which matures around the aircraft.
The latter occurs because, during the developing phase, there is limited precipitation that might show on radar, and no lightning to identify it as a thunderstorm. If in IMC in an area of embedded storms, these developing storms can be difficult to avoid. Most often, however, they form near other storms, as the outflow of an existing storm can serve as the lifting mechanism for a new one.
While simply avoiding IMC flying in any area with active embedded storm activity is the best option, staying at least 20 miles away from any active cell visible on radar is essential.
If an area of storm activity is large and active enough (storms covering at least 40% of an area of at least 3000 sq mi, or a line of storms at least 60 mi long), the weather service will issue a convective sigmet to warn pilots away from the area.
However, convective sigmets are only issued if, in the next 2 hours, meteorologists expect the storms to produce severe turbulence, severe icing, winds exceeding 50 kts, and/or hail exceeding ¾ inch.
Smaller regions or areas where perhaps only moderate storm activity is expected may not receive a sigmet, although a thorough weather briefing will note such areas.
Overflying storms is not recommended unless an aircraft can clear it by several thousand feet, as mature strong storms may produce turbulence far above the cloud tops and even throw hail upward.
Achieving this clearance often depends on the time of year, and even the latitude. Storms will normally top out when they encounter a strong or deep temperature inversion aloft.
Frequently, this inversion occurs at the top of the troposphere, which rises during the summer and lowers in winter, and which decreases in altitude with increasing latitude away from the equator.
Storms in the lower latitudes year-round and those in the middle latitudes during summer may have tops above 50,000 ft, while those at higher latitudes or during colder months in the middle latitudes may top at around 20,000 to 30,000 ft.
Depending on winds aloft, these inversions may force the cloud to flatten and propagate downwind, developing an anvil appearance. Because the area beneath the anvil is frequently a zone of severe turbulence and hail, pilots should avoid flying in this area.
The storm’s outflow
Similarly, hail and turbulence can occur 20 mi or more from the storm itself, which is why pilots should give even the weakest storms at least a 20-mi clearance when flying near them.
If flying between 2 storms (a “gap”), that clearance should be doubled. In addition, flying radar gaps between storms in IMC should be avoided, as the outflow from the storms often converges in the gap, generating a new storm that will grow to occupy it.
That same outflow presents a significant danger to pilots on departure or approach. The outflow can spread out several miles from the storm and travel at speeds exceeding 50 kts, producing even higher gusts.
Many accidents have occurred due to these gust fronts producing low-level windshear (LLWS) that suddenly decrease airspeed and causes unintended descents. The effect can be even more dangerous if an aircraft flies directly beneath the storm and encounters a downburst – an enhanced pulse of downdraft flow that, if not recognized immediately, may be impossible to recover from.
Avoiding flying beneath any storms and performing takeoffs and landings is advised when storms are within around 2 miles of the touchdown/takeoff zone. Towered airports will often hold aircraft in such situations or when LLWS is detected.
In the rare instances that pilots finds themselves unintentionally inside or about to penetrate a thunderstorm, the recommended course of action is to reduce speed to turbulent air penetration speed and turn around 180º in hopes of returning to clear air.
A caveat to that is that it takes only an instant for a developing storm to mature, and what may have been clear air behind you may have become an active cell by the time you turn. Because airborne radars are forward-looking, you can’t really tell that until you make the turn, and ground-based radar feeds can be up to 10 minutes old.
Newer radar algorithms have reduced attenuation effects and may show you what lies beyond the immediate reflectivity, and whether or not continuing forward is a better option. Neither case, unfortunately, is guaranteed to get you out of danger.
The best option is to not tempt things by flying into such an area in the first place – and, if you do encounter a thunderstorm or dangerous convective conditions outside of the storm, please report it with a pirep.
Karsten Shein is cofounder of 2DegreesC.org. He was director of the Midwestern Regional Climate Center at the University of Illinois, and a NOAA and NASA climatologist. Shein holds a comm-inst pilot license.