Extreme and violent vertical movements of air, know as updrafts, often
lead to thunderstorms. This uplifting usually creates dramatic cumulus
and cumulonimbus cloud formations that rise vertically, sometimes as
high as 70,000 to 75,000 feet. The uplift of air can be caused by heating
of the air from the ground (common in the Midwest), weather associated
with a cold front, or temperature differences caused by the meeting
of land and sea (common in the Gulf and southern Atlantic states).
Because temperature differences are the most common cause of
thunderstorms, thunderstorms usually occur in the afternoon, when
temperature differences between land and air or sea are the most
extreme. Since temperature differences between land and sea are not
as dramatic in the North Atlantic or West Coast (Pacific Ocean)
regions, thunderstorms in these locations are not as frequent.
A thunderstorm forms as rising warm air cools and condenses. The
higher the cloud stacks, the greater the level of cooling, and soon precipitation
begins. As the falling rain and ice crystals cool the air within
the cloud, the temperature difference between the cloud and the surrounding
air begins to equalize, leading to the creation of downdrafts
and heavy precipitation. The rush of cold air fans out ahead of the
storm, sometimes as much as 2 to 3 miles, and is a good indicator of
an impending thunderstorm.
As the downdrafts increase in intensity, the cloud becomes nothing
more than downward moving air. Cooling of air ceases, and since no
air is rising and condensing, the rain tapers off.
Lightning is caused by the attraction of unlike electrical charges
within the storm cloud or the earth’s surface. The friction caused by
rapidly moving air particles, churning from the violent updrafts and
downdrafts, leads to a building up of strong electrical charges. As the
electrical pressure builds, charges between parts of the cloud or from
the cloud to the earth are released, taking the form of lightning.
Up to thirty million volts can be discharged by one single lightning
bolt, and it is this power, or explosive heating of the air, that causes the
compressions of thunder.
It is possible to roughly judge the distance of an approaching
storm by observing the lightning’s flash followed by the resounding
boom of thunder. For this method count slowly, “one-one thousand, twoone
thousand, three-one thousand,” and so forth. This will approximate
one second elapsed for each thousand counted. Once the lightning
has flashed, begin to count. When you hear the thunder, stop counting.
Every five seconds of elapsed time indicates 1 mile of distance. In other
words, if the count reaches, “seven-one thousand,” it is safe to assume
that the approaching thunderstorm is approximately 11⁄2 miles distant.
Thunderstorms are dangerous companions when you are traveling on
a mountain peak. If a thunderstorm approaches when you and your hiking
party are on an exposed peak or ridge, take the following precautions:
Do not lie down or sit. Rather crouch on the balls of your feet with
your feet close together. The idea is to minimize the available surface
area through which possible ground currents from nearby
lightning strikes may move.
Do not huddle next to a single tree. That is rather like hugging a
lightning rod and expecting to be safe. Choose a cluster of trees
instead and place yourself in the middle (figure 5), preferably in an
Spread the group out, at least 25 to 30 feet apart. If lightning does
strike, the idea is to minimize the potential damage and injury. With
the group spread out, chances are only one person will be injured,
if, at all, and this leaves the rest of the party to provide lifesaving
assistance once the storm moves on.
Avoid depressions or caves. Such areas usually have moisture
present, which makes them more susceptible to conducting electricity.
If you are caught on a lake in a thunderstorm, assume a crouching
position toward the middle of the boat. Try to minimize your contact
with wet objects.
From the book Basic Illustrated Weather Forecasting, by Michael Hodgson. Copyright © 2008 by Michael Hodgson. Used by permission of FalconGuides, a division of Globe Pequot Press. Visit Falcon.com.