An air mass, or ordinary, thunderstorm evolves through three stages
over a period of about an hour. Updrafts and warm in- cloud temperatures
of the cumulus stage give way to updrafts and downdrafts, precipitation,
lightning, and thunder in the mature stage. The dissipating stage is
characterized by downdrafts and light precipitation. Regions of intense
electric charge develop in a mature thunderstorm through processes not
fully understood. Hypotheses include convection currents that carry
charge from the ground and the upper atmosphere into different regions
of the cloud, and collisions of precipitation particles within the cloud.
Lightning is a sudden flow of electricity between two charge centers,
discharging them. Thunder is the sound made by the violently heated,
expanding air along the lightning's channel. Multicelled, frontal or
squall line, and supercell thunderstorms all show significant differences
from the basic, air mass thunderstorm model. Interactions among neighboring
cells, coupled with the influence of large-scale convergence and divergence
patterns, may stimulate dense regions of thunderstorm development called
mesoscale convective complexes (MCC) or mesoscale convective systems
(MCS). Severe thunderstorms tend to form where shear and/or rotation,
dry air aloft, and frontal or orographic lifting supplement the basic
requirements of all thunderstorms: warm, moist, and unstable air. Tornadoes
form most frequently from supercell storms, in which a rotating air
column called a mesocyclone exists. Under some circumstances, the mesocyclone
is stretched vertically upward and downward, intensifying as it does
so, and extending its influence below the cloud base, where the tornado
forms. Tornado winds may reach 300 mph in the most violent cases, causing
total destruction of objects in their path. The southern United States
is the most tornado-prone region in the world. Thunderstorm and tornado forecasting are showing substantial improvement,
thanks largely to the National Weather Service Doppler Radar network
using their Weather Surveillance Radar-88 Doppler (WSR-88D)that
became operational in the 1990’s. In some cases, potentially life-threatening
storms can be identified more than 20 minutes in advance. However, since
the worst parts of these storms can have life-spans as short as a few
minutes, warnings with ample lead time cannot always be given. The most
dangerous storms are sometimes those that occur late at night, when
most people are asleep. Most summer thunderstorms do not cause damaging winds, hail or tornadoes.
Just look around: if destructive storms were really common, would there
be many older buildings, trees or other vegetation in your community?
It is estimated that even within a typical area covered by a severe
thunderstorm or tornado watch, less than 3% of the area is actually
affected by damaging conditions. The short movie here shows the development and movement of clouds as the main part
of a thunderstorm approaches the observation point. Notice how fast-moving
low clouds are coming toward the camera while higher clouds are moving
from left to right. What does this tell you about whether warming or
cooling is occurring in the layer between the two cloud decks? If you
answered that cooling is occurring because the wind backs with height,
you are correct. As you watch the low clouds advance, you will notice new ones forming
near their trailing edge. Why? There is a ridge in the picture, and
the air from the other side of the ridge climbs over the ridge. As it
climbs, it cools and eventually becomes saturated. At that point, the
clouds form. But why don’t the clouds evaporate as the air descends
from the ridge crest? You cannot really answer that question from the
movie alone. However, it turns out that the slope on the other side
of the ridge is much longer than on the side you see. In other words,
the air climbs a considerable distance before the clouds form, whereas
the descent from the ridge you see is less than 300 feet. Before the thunderstorm arrived, it was rather tranquil at the location
from which the video was shot. Only a slow crescendo of distant thunder
and the darkening western sky hinted at the storm’s approach. Even though
the early signs may be subtle, that time is the best for moving to a
safe location. In addition to the danger of lightning and the prospect
of getting soaked, the appearance of fast moving low clouds racing toward
you from the base of a thunderstorm tells you that strong gusty winds
are on the way. By the time you see these clouds, those winds are just
minutes away. Many people are fooled by the dark low clouds from an approaching thunderstorm.
It usually looks brighter right behind the advancing cloud line (which
can form a solid roll cloud or wall cloud). It is tempting to think
that once that dark line of clouds passes, the storm has "blown
over"; i.e; that the worst is over. Quite often, the brighter area
has a very uniform or milky look. You cannot see individual clouds in
that area the way you easily see the churning cloud elements as the
roll cloud approaches. Why? Most of the time, the uniform appearance
is a sign of heavy rain. In fact, there is so much rain that it literally
hides the individual clouds you could otherwise discern. So, instead
of the worst being over, the main part of the thunderstorm is probably
still on the way. Each storm is different, but the whole range of thunderstorm
products may be found within the cloud area that at first looks relatively
benign. There may be window rattling, tree trunk twisting winds, gutter
gushing rivulets of flash-flooding rain, plenty of lightning, and hammering,
shrub-stripping, crop-flattening hail..maybe even a tornado! However,
all of that is hidden. That is why it is so very important to move to
the safest place possible before the storm hits. Much of this chapter
will deal with safety related issues. Literally, we hope it helps you
avoid injury or death. To end this section on a lighter note, the movie’s
second half runs in reverse: the clouds retract and dissipate as they
back up over the ridge. If only we could press the reverse button when
the real thing hits! Thunderstorms and Tornado Simulation (14110.0K)
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