WX BRIEF

Updated essentials about thunderstorms that every pilot flying IFR needs to know

We often share the skies with these giant forces of nature, but complacency can be dangerous and caution is always advisable.

By Karsten Shein
Comm-Inst, Climate Scientist


Although pilot awareness and training, coupled with airport alert systems, have greatly reduced downburst accidents, these rapid pulses of downdraft air still present a danger to aircraft at low altitude and airspeed due to the sudden transition from a strong headwind to a strong tailwind.

He was a veteran pilot with 9000 hrs, and as the clouds enveloping the aircraft grew darker he knew he was going to encounter some rough weather. He'd been flying along inside a cumulus deck for a while when the storm scope began registering a cluster of lightning strikes 10–15 miles ahead, although none appeared directly in front of the aircraft. At the same time, though, the air was becoming increasingly turbulent.

The pilot had dealt with a lot of adverse flight conditions in his long career, and he knew from several preflight weather briefings that he'd be flying through an area of active convection. However, he'd remarked to an acquaintance that, while he might have to work around some weather, "it didn't look serious."

Now, to avoid what he thought might be the worst of it, he asked ATC for a turn away from the weather ahead and began to initiate it when he entered the main downdraft core of an extreme thunderstorm. Within 30 sec, he could only watch as the wing leading edges buckled and peeled away as the aircraft around him began breaking apart. His fate was sealed and the aircraft's wreckage was scattered across a quarter mile of Virginia countryside.

Despite the fact that most of us learn early on in our flying careers that thunderstorms are nothing to mess with, each year a number of pilots stumble into these powerful monsters and emerge as fatal accident statistics.

What's alarming is how many of those pilots are highly experienced professional pilots. In the aforementioned incident, the pilot was none other than former rocket plane test pilot Scott Crossfield, and although what Crossfield experienced in his final flight can only be speculated, he did fly into the core of an extreme storm cell and his Cessna 210A broke up in flight as a result.

In fact, Crossfield's accident was just one of several inflight thunderstorm penetration accidents occurring in a short span of time between 2005 and 2006, prompting NTSB to issue a safety alert (SA011) to remind pilots about the dangers posed by thunderstorms and describing the possible limitations of ATC-provided weather information.

A thunderstorm's many forces

With any adverse weather situation, knowledge is often the key to addressing it effectively. In this case, the more a pilot knows about how a thunderstorm develops, and the forces it contains, can mean the difference between a successful flight and one doomed for inclusion in the NTSB accident files.

First and foremost, pilots must recognize that even the most modest "light" thunderstorm (a level 1 in old intensity parlance) still contains more energy than was released by the first atomic bomb blasts. This alone is reason enough to avoid storms at all cost. Your average aircraft is not designed to withstand the application of even a small fraction of that energy.

Thunderstorms can contain so much energy because of water. A typical thunderstorm will draw in roughly 5 billion kilograms (approx 550 tons) of water vapor, and each kilogram of water absorbed a bit of energy (2260 kilojoules) when it evaporated. As it lifts the air that contains the water vapor, the rising air expands and cools.

Diagram of the updraft region of a thunderstorm. Strong upward motion suspends precipitation in the front section of the storm, but may also allow hail to form in this area. Updrafts can be strong enough to cause structural failure of an airframe.

Energy stored in the water vapor is withdrawn as the cooling air compensates for the heat loss, which at the same time causes the water molecule to condense back to liquid form, ultimately creating cloud droplets and precipitation. But that infusion of energy keeps the air warmer than the surrounding environment, letting it continue its rise.

As the air continues to rise and cool, it continues to rob energy from the water vapor accompanying it, saturating ever higher levels and filling them in with cloud droplets. Eventually, either enough moisture is removed from the rising air that it no longer has a sufficient source of energy to draw on, or it rises into a temperature inversion, such as in the lower stratosphere, and its temperature quickly evens out with the surroundings.

At that point it ceases to rise. If an updraft is checked by a temperature inversion, but moisture continues to ascend, the upper-level winds will spread the cloud downwind as an anvil.

Vertical windspeeds in a thunderstorm are a function of the difference in air density between the rising or sinking air and the surrounding environment. Density in turn is dependent on the temperature, so the greater the temperature difference, the faster the air is likely to be moving. With updrafts, meteorologists can estimate the maximum possible upward vertical velocity by doubling the convective available potential energy (CAPE) of the troposphere and taking the square root.

The result is a maximum updraft speed in meters per second. Using this equation, we can determine that a storm which forms in an atmosphere with a CAPE of 1000 joules per kilogram (J/kg)—a relatively low thunderstorm energy—will have a maximum potential updraft speed of around 44 mps (8660 fpm or 85 kts), while in a high-energy environment of perhaps 3500 or more J/kg CAPE, the maximum updraft winds could exceed 80 mps (15,750 fpm or 155 kts), which is considered an extreme updraft.

1


1 | 2| 3 | next