Numerous aviation accidents have occurred in the vicinity of thunderstorms due to the density of clouds. It is often said that the turbulence can be extreme enough inside a cumulonimbus to tear an aircraft into pieces, and even strong enough to hold a skydiver. However, this kind of accident is relatively rare. Moreover, the turbulence under a thunderstorm can be non-existent and is usually no more than moderate. Most thunderstorm-related crashes occur due to a stall close to the ground when the pilot gets caught by surprise by a thunderstorm-induced wind shift. Moreover, aircraft damage caused by thunderstorms is rarely in the form of structural failure due to turbulence but is typically less severe and the consequence of secondary effects of thunderstorms (e.g., denting by hail or paint removal by high-speed flight in torrential rain).
Cumulonimbus clouds are known to be extremely dangerous to air traffic, and it is recommended to avoid them as much as possible. Cumulonimbus can be extremely insidious, and an inattentive pilot can end up in a very dangerous situation while flying in apparently very calm air.
While there is a gradation with respect to thunderstorm severity, there is little quantitative difference between a significant shower generated by a cumulus congestus and a small thunderstorm with a few thunderclaps associated with a small cumulonimbus. For this reason, a glider pilot could exploit the rising air under a thunderstorm without recognising the situation – thinking instead that the rising air was due to a more benign variety of cumulus. However, forecasting thunderstorm severity is an inexact science; in numerous occasions, pilots got trapped by underestimating the severity of a thunderstorm that suddenly strengthened.
General hazards to aircraft Even large airliners avoid crossing the path of a cumulonimbus. Two dangerous effects of cumulonimbus have been put forward to explain the crash of flight AF447 that sank into the sea on 31 May 2009 about northeast of Brazil. It encountered a mesoscale convective system in the Intertropical Convergence Zone (known by sailors as the "doldrums"), where cumulonimbus rise to more than in altitude. However, the aircraft did not disintegrate in flight. A different hypothesis was put forward and later confirmed: accumulation of ice on the aircraft's pitot tubes.
The inconsistency between the airspeeds measured by the different sensors is one of the causes of the accident according to the final report.
The US FAA recommends that aircraft (including gliders) stay at least 20 nautical miles away from a severe thunderstorm, while a glider pilot could be tempted to use the updraughts below and inside the cloud. There are two sorts of danger for this type of aircraft. One is related to the shear effects between updraughts and downdraughts inside the cloud – effects that can smash the glider. This shear creates a Kelvin-Helmholtz instability that can generate extremely violent sub-vortices. The second danger is more insidious: the strong updraughts below a supercell cumulonimbus can cover a large area and contain little or no turbulence as explained below. In this case, the glider can be sucked into the cloud, where the pilot can quickly lose visual reference to the ground, causing conditions to quickly become Instrument meteorological conditions (IMC), meaning that pilots are forced to fly by instruments alone, without visual reference of the ground or sky. In these conditions, the aircraft (if not equipped for IMC flight and flown by a pilot experienced in IMC flight) is likely to enter a graveyard spiral and eventually break up by exceeding the wing load limit. In this situation, the cause of the disintegration of the aircraft is not atmospheric turbulence but is the inability of the pilot to control the aircraft following the loss of visual reference to the ground. In the case of an instrument flight, cumulonimbus can catch a pilot by surprise when embedded in a more benign cloud mass. For example, nimbostratus can originate from the spreading of a cumulonimbus (nimbostratus cumulonimbogenitus), making the presence of active convective cells likely. Small private airplanes are generally not equipped with on-board weather radars; and during an IFR approach, they can be sent accidentally by air traffic control to non-obvious active cells.
Updraft characteristics
The updrafts under a cumulonimbus can be extremely laminar, extensive, and uniform, this is particularly true during the buildup of the thunderstorm. They can last more than one hour and correspond to a steady state of the cumulonimbus. The updraft under the cloud is mostly due to buoyancy, but there is also a large pressure difference between the base and the top of the cumulonimbus (larger than would be found in this height range outside the cloud) and local low-level mechanical lifting such as the lifting generated by a downburst. The two last phenomena can overcome a stable air zone close to the surface by lifting cooler air parcels to a level where they are eventually warmer than the surrounding air. This can happen if these mechanical phenomena lift the parcel above the lifted condensation level (LCL), above which height the parcel's temperature Tp(z) decreases less with height (due to the release of latent heat and at approximately 6.5 K/km) than the surrounding air temperature Ts(z) decreases with height in the case of a conditionally unstable lapse rate aloft. In other words, the parcel can be lifted to a height where , where the former is the cooling rate of the parcel and the latter is the ambient lapse rate. In these conditions, the rising parcel may eventually become warmer than the surrounding air; in other words, there may exist a level above which . This scenario's conditionally unstable lapse rate aloft is relatively common when thunderstorms exist. In effect, at low level, such air parcels are sucked into the cloud as if by a vacuum cleaner. Soaring pilots refer to this near-base sucking as "cloud suck", a phenomenon known to generally be more intense the taller the cumulus cloud – and to thus be at maximum intensity with a cumulonimbus. Since the dynamic updraft is wide, the updraft velocity varies little laterally and thus the turbulence is minimised. So, it is said:In fact, Ellrod and Marwitz's paper is more general. These authors state that in general, the buoyancy beneath the cumulonimbus cloud base is often negative. This explains why updrafts underneath the base of a cumulonimbus are often laminar. This phenomenon is well known by glider pilots. (see below). The phenomenon is enhanced under the weak echo region of a supercell thunderstorm that is extremely dangerous. At approximately these smooth updrafts become suddenly very turbulent.
In general, updrafts reach their maximum intensity at above the ground. At this altitude, a phase change occurs where water droplets become ice crystals and therefore release energy in the form of latent heat and thus the updraft strength increases. Supercell thunderstorms or derechos can have gigantic updrafts at this altitude, updrafts with speeds that can exceed . Such an updraft speed corresponds to the wind speed of a small hurricane. The speed can even exceed . The maximum number in the Beaufort scale is 12 ("hurricane force" wind) and is assigned to wind speeds of 64 knots or greater. If the Beaufort scale were extended, these updrafts would have a Beaufort number of 14 in the vertical direction. The turbulence is then extreme at this altitude.
Moreover, the diameters of the updraft columns vary between 2 km (air mass thunderstorm) and 10 km (supercell thunderstorm). The height of the cumulonimbus base is extremely variable. It varies from a few tens of meters above the ground to 4000 m above the ground. In the latter case, the updrafts can originate either from the ground (if the air is very dry – typical of deserts) or from aloft (when altocumulus castellanus degenerates into cumulonimbus). When the updraft originates from aloft, this is considered elevated convection.
Dangers pertaining to downbursts
Downbursts are dangerous for many reasons. First, downdraughts under cumulonimbus can be severe and extensive. A sailplane flying at 50 knots in a downdraught of 15 knots has an approximate glide ratio of 3, meaning that it covers only about three metres of ground for every metre it descends. Assuming that the glider is at cloud base height at , if it remains in the downdraught the entire time, it will only be able to glide before being forced to land – likely under difficult and dangerous conditions. Even if the glider lands safely, it could be destroyed later by a wind gust. So when a rain curtain shows a downburst, it is of paramount importance to not land in this area.
Downdraughts of 50 knots are possible and can generate wind gusts of 60 knots or more. Safely landing a light aircraft in these conditions can be virtually impossible. Moreover, close to the ground, a glider or airplane pilot can be caught by surprise by a sudden reversal of the wind direction and transition from an upwind to a downwind situation. If the airspeed becomes too low, the aircraft will stall and may crash into the ground due to the altitude lost recovering from the stall. As a consequence of famous instances of crashes of this nature in the United States, a network of wind profilers and Terminal Doppler Weather Radars was developed in the vicinity of airports to monitor this wind shear. Based on FAA rules, every pilot must inquire about the wind speed and direction before landing.
Compared to airliners, sailplanes fly at low airspeeds. The usual approach speed of a sailplane is around 50 knots, but let's assume that the pilot is extra "careful" and flies his approach at 65 knots. William Cotton claims that the wind shear can be as high as 50 knots. In such a case, if the shear direction is such that the airspeed is reduced by the shear amount, this pilot's airspeed will drop to 15 knots, which is well below his glider's stall speed (typically 35–40 knots). If this airspeed drop occurs during the transition from the base leg to the final approach, the aircraft may enter into a spin from which there isn't enough altitude to recover. The exact quotation is the following:
So when the pilot encounters benign cumulonimbus, it may be a better choice to stay aloft and use the updraughts under the cumulus in front of the thunderstorm along the flanking line (or even under the cumulonimbus itself in its laminar region) and wait for the thunderstorm to dissipate instead of attempting a landing in the presence of possible downbursts.
Flight inside cumulonimbus
Soaring
In some countries, sailplanes are permitted to fly inside clouds. For example, during the 1972 World Soaring Championship at Vršac, Yugoslavia, Helmut Reichmann attempted to use the violent updraughts associated with cumulonimbus. Initially, he found an updraught of +8 m/s. After half a circle, he was in a downdraught of −15 m/s. He had to land very shortly afterward. The thunderstorm was in its mature stage. In another example, Terry Delore got trapped in a severe thunderstorm. He entered a seemingly innocuous cumulus at . This cumulus evolved into a large cumulonimbus. At first, the flight inside the cloud was turbulence-free. Then his glider suddenly became uncontrollable. He was either inverted, in a nosedive, or in a chandelle. The airbrakes became stuck open due to hailstones blocking the orifices. When he landed, the airfield was still covered by hailstones. The wind gusts were between 30 and 40 knots. Everyone on the ground feared for the pilot's life. In the same book, the author narrates that an Italian instructor at Rieti had his students climb inside cumulonimbus so that they get accustomed to them.