Weather

Pilots who rely on weather training materials based on FAA publications for thunderstorm knowledge could end up confused when they use National Weather Service reports and forecasts or tune into their favorite television meteorologist to find out about tomorrow’s flying weather.

The Pilot’s Handbook of Aeronautical Knowledge chapter on weather theory describes
two kinds of thunderstorms: “air mass” and “steady state.”

Meteorologists rarely talk about “steady state” thunderstorms. They will sometimes mention “air mass” thunderstorms, which television meteorologists often call “pop up” thunderstorms.

You’ll hear meteorologists talk about supercells, multicell clusters, and pulse thunderstorms. Supercells are thunderstorms that produce the most destructive and deadly tornadoes, among other kinds of dangerous weather.

DON’T DISMISS ‘AIR MASS’ THUNDERSTORMS. The Pilot’s Handbook of Aeronautical Knowledge chapter on weather theory says: “Some storms occur at random in unstable air last for only an hour or two and produce only moderate wind gusts and rainfall. These are known as air mass thunderstorms.”

That doesn’t sound too bad. Nevertheless, the first weather lesson all pilots should learn is that they should avoid all thunderstorms by at least 20 miles. Even a relatively small, short-lived thunderstorm could produce severe turbulence.

One reason to avoid even small thunderstorms is that one of those innocent-looking air mass thunderstorms could be what meteorologists call a “pulse thunderstorm.”

The word “pulse” tells you that the thunderstorm’s updraft develops quickly, and quickly dies out. A thunderstorm begins dying when rain begins to fall, which drags cool air down with it.
This cuts off the supply of warm, humid air feeding into the thunderstorm—its energy supply.

Pulse thunderstorms can quickly reach severe levels, with winds of 58 mph or faster or hail three-quarters of an inch in diameter. Hail is a sign of fierce turbulence in the thunderstorm that’s producing it.

WHAT MAKES PULSE STORMS DANGEROUS? Jeffrey Halverson, a professor in the Department of Geography and Environmental Systems at the University of Maryland, Baltimore County, describes pulse storms as usually “air mass storms on steroids.”

Halverson also writes for The Washington Post’s Capital Weather Gang blog. In July 2013, he wrote: “These cells stand out on radar because of their unusually tall and narrow appearance, perhaps topping 60,000 feet, and high intensity. The storm quickly begins to collapse and enters a brief, severe phase. The cascade of descending rain is so dense it creates a blinding cloudburst.

“If sufficient ice mass develops aloft, small hailstones (marble- to quarter-sized) accompany the torrent. Lightning may be nearly continuous. The downdraft may accelerate to high velocity. As it spreads along the surface, it creates a fan-shaped microburst with gusts to 60-80 mph. That’s enough to snap tree limbs and down electrical lines.”

A storm officially becomes “severe” when winds blow at 58 mph or faster, or hail that’s three-quarters of an inch or larger begins falling.

Pulse storms often become severe without the National Weather Service issuing a severe storm warning. There’s a chance, however, that a NWS Watch—which means severe storms are possible—would be in effect at the time.

PULSE STORMS CAN OUTRUN RADAR. Pulse storms can develop quickly—so quickly that weather radar, both ground-based NWS Nexrad Doppler radars and those aboard airplanes, often can’t keep up with them.

Halverson explains: “Because these cells grow so rapidly, and the Nexrad radar volume scan takes eight or so minutes to process, a lot can happen in terms of cell evolution in between scans. So during the growth and maturity phase, a ‘snapshot’ from the scan can underestimate the true height and vigor of these cells.”

In other words, a pulse thunderstorm storm could be worse than it looks on the NWS radar or an airplane’s radar.  

Halverson says pilots who have onboard weather radar or who download weather radar images on tablets should be especially wary of thunderstorms with a high and unusually narrow reflectivity core, which is usually shown in red on weather radar displays.

EFFECTS CAN REACH HIGH AND FAR. Pulse storms can affect even airline pilots flying higher than 30,000 feet and miles from the storm. Halverson explains that a pulse storm’s fast updraft shoots into the stratosphere’s stable air “like a fist” that creates “ripples of energy—a train of gravity waves at high altitudes.”

These waves can create extreme clear air turbulence miles from the thunderstorm like “ripples in a pond,” Halverson says.

Gravity waves are any waves caused by a force that pushes a liquid or a gas up. The force of gravity then pulls the fluid down. The up-and-down push and pull creates the waves. Ordinary ocean waves are the best-known example. The waves spread out from the source of the initial disturbance. Atmospheric gravity waves usually are invisible, although clouds sometimes show them.

DANGERS INCLUDE MICROBURSTS. Pulse thunderstorms often can produce microbursts: winds that blast down and spread out, sometimes faster than 150 mph when they hit the ground.

During the 1970s and 1980s, microbursts caused an average of one airliner crash each 18 months. Research scientists discovered what causes microbursts and how to spot their early signs. The scientists, pilot organizations, aviation companies, and others developed technologies to spot microbursts and warn pilots, and programs to train pilots how to avoid microbursts and what to do if caught by one.

The last air carrier microburst accident in the United States was the July 2, 1994, crash of a USAir DC-9 at Charlotte, North Carolina, which killed 37 people. A pulse thunderstorm caused it.