Flight Training - August 2014 - 51

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 quartersized) accompany the torrent. Lightning
may be nearly continuous. The downdraft
may accelerate to high velocity. As it
spreads along the surface, it creates a fanshaped 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 groundbased 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.
Jack Williams is an instrument-rated private pilot.
His latest weather book is The AMS Weather Book:
The Ultimate Guide to America's Weather.

RESOURCES
* To learn about the terms today's meteorologists use to describe thunderstorms, see "Weather:
Big Push," April 2012 Flight Training (http://flighttraining.aopa.org/magazine/2012/April/
weather.html).
* For more on microbursts, see "The Weather Never Sleeps: Menacing Microbusts," July 2007
Flight Training (www.aopa.org/members/ftmag/article.cfm?article=5992).
* Jeffrey Halverson's Capital Weather Gang blog on pulse thunderstorms (www.washington
post.com/blogs/capital-weather-gang/wp/2013/07/22/the-pulse-of-summer-when-pop-upthunderstorms-turn-severe).

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http://flighttraining.aopa.org/magazine/2012/April/weather.html http://flighttraining.aopa.org/magazine/2012/April/weather.html http://www.aopa.org/members/ftmag/article.cfm?article=5992 http://www.washingtonpost.com/blogs/capital-weather-gang/wp/2013/07/22/the-pulse-of-summer-when-pop-up-thunderstorms-turn-severe http://www.washingtonpost.com/blogs/capital-weather-gang/wp/2013/07/22/the-pulse-of-summer-when-pop-up-thunderstorms-turn-severe http://www.washingtonpost.com/blogs/capital-weather-gang/wp/2013/07/22/the-pulse-of-summer-when-pop-up-thunderstorms-turn-severe

Flight Training - August 2014

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