WX BRIEF

Thunderstorms

From isolated cells to squall lines, thunderstorms remain a consistent threat to pilots flying in unstable airmasses.


Lifting aids

Massive downburst near PHX (Phoenix Sky Harbor Intl, AZ). Thunderstorm downdrafts can exceed 100 mph and have contributed to a number of major aviation accidents. It is best to avoid flying within 2 nm of the base of any thunderstorm.

Fortunately, the conditions that would permit a storm to form spontaneously by punching through an overlying cap are relatively infrequent. Most storms require a little formative assistance from some type of lifting mechanism. A common variety in the middle latitudes is a front. Fronts are the boundary between 2 air masses – normally of different temperature and density. As the more dense/colder air undercuts the warmer air mass, it forces the warm air to rise. If the air is lifted to a point where it can continue to rise on its own, it can produce a convective updraft.

Another way warm air can be forced to rise is when it is pushed against rising terrain. Orographic uplift is a frequent producer of mountain thunderstorms. Such storms can be extremely dangerous because, in addition to the normal hazards of a thunderstorm, these storms often park themselves in one place for several hours. The clouds and precipitation they subsequently produce may obscure mountainous terrain and precipitate flash floods through the mountain valleys that house the region's airports.

A hybrid of both frontal and orographic uplift is a sea breeze. The sun heats the coastal land and the overlying air, which becomes buoyant and rises. This draws humid air in from the water that becomes entrained in the updraft to produce towering cumuli and in some cases thunderstorms.

Most of these storms move offshore at night when the sea breeze reverses itself. Stronger storms can be produced when the sea breeze front from one side of a narrow peninsula converges with the sea breeze front from the other side to create a convergence zone of enhanced uplift.

Globally, thunderstorms are most common to places where 2 warm and humid airmasses converge. On smaller scales, these are often large peninsulas where the diurnal sea breeze causes heated air to meet over the land and rise. Florida is a perfect example. On a planetary scale, this is the inter-tropical convergence zone (ITCZ), where warm and humid subtropical surface air flows toward the equator from both the northern and southern hemispheres, producing an equatorial band of towering cumulonimbi.

Importantly for aviation, low-level convergence of air at a much smaller scale is responsible for many aircraft wrecking storms. As 2 adjacent convective cells produce downdrafts, the outflow may converge in the space between the storms, forcing the surface air there to rise into a new storm cell.

Thunderstorm life cycle

This global map of thunderstorm activity is based on lightning detection in 2015. The greatest incidence of thunderstorms is along the equator, in particular over Africa. In the US, Florida has the most thunderstorms, followed by the Great Plains.

At first, a thunderstorm is nothing more than an updraft that is condensing water vapor into a cumulus cloud. At this stage, there is no precipitation, which is formed as cloud droplets collide and coalesce into larger droplets. Because of this, developing storms do not show up on radar - but the updrafts at this stage can still be dangerous to aircraft.

Also, we tend to think of a thunderstorm as an updraft contained entirely within a single cloud. In truth, the conditions that favor the formation of storm are regional. Air is ascending throughout the region. But in most cases, the cumulus defines the path of least resistance for the rising air and becomes a conduit for the airflow as the rising air lowers the surface pressure and draws in air from surrounding areas.

Eventually, the rising air cools and dries out to a point that it becomes denser than the air around it. At this point the storm is considered mature and produces a downdraft. Rain droplets are normally present and at this stage, the storm will appear on radar. In most cases, the storm forms in an environment with little wind shear, meaning the downdraft descends through the updraft air, disorganizing it and collapsing the convective cell within about a half hour of its formation. Such storms are called air mass storms.

Occasionally, however, a storm may rise into a region of moderate windshear, which tilts the updraft to a degree that the eventual downdraft descends adjacent to it. When this happens, the downdraft can reach the ground unimpeded, and actually help force more air to rise into new updrafts, creating a continuous cycle storm and sustaining the convective cell for hours.

The most severe storms are created when the wind not only shears in speed with altitude, but also in direction. This can produce a rotation within the storm complex itself, allowing multiple up and downdrafts to exist simultaneously and independent of one another. At its core, such a supercell storm will contain a mesocyclone, or region of rapidly rotating air from which tornadoes may be spawned.

Thunderstorm dangers

The reason that storms are so dangerous to aviation is that they contain so many adverse and extreme conditions within their envelope. One of the most dangerous aspects of a thunderstorm is, of course, the extreme amount of energy being relocated by up and downdrafts. While there are central updraft and downdraft cores to all storms, some may have multiple cores.

In a typical storm, updrafts will flow on the order of around 2500 ft per minute, while in strong, non-supercell storms that rate may increase to around 5000 fpm. Updrafts in supercell storms have been measured at around 12,000 fpm. At that rate, a storm could grow from a top of 25,000 ft to 50,000 ft in just 2 minutes.

Because of the density of air in the downdraft, these currents are normally even stronger than the updrafts, ranging from 5000 fpm in weak storms to well over 15,000 fpm. The strongest downburst winds measured at an airport were 158 mph (roughly 14,000 fpm). Compare these rates with the rate of climb of even the most powerful business aircraft, and it is easy to see how much power there is in even a run-of-the-mill storm and how fast things can change.

These vertical currents also often create turbulent eddies that shear off from the main flow, generating severe turbulence which can apply enough force to buckle a main spar or literally rip an engine from its mounts.

These eddies can and often do extend well outside of the thunderstorm cloud, which is part of the reason the FAA's Airman's Information Manual notes that "severe turbulence can be expected up to 20 miles from severe thunderstorms," and that pilots should avoid any area that has a thunderstorm coverage exceeding 50%.

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