Download Chapter 8 Thunderstorms and Tornados

Chapter 8 Thunderstorms and Tornados
8.1 Thunderstorms
A thunderstorm, figure 8.1, is a local storm produced by a cumulonimbus
cloud. By definition it is always accompanied by lightning and thunder, and is
usually found with strong gusts of wind, heavy rain, and sometime hail.
Thunderstorms occur mainly in the tropics but occur further north in the
summertime.
Figure 8.1 A thunderstorm.
The basic requirements for the formation of a thunderstorm are:
(1) unstable air
(2) some type of lifting mechanism
(3) a very high moisture content of the air.
(1) Unstable Air
As we discussed in chapter 5 on the stability of the atmosphere, when unstable air
is lifted to the point where it is warmer than the surrounding air, it then moves
upward freely. This rising air drags even more air in from the ground below.
(2) Lifting Mechanism
Even if the air will be unstable when it moves upward into the atmosphere, there
must be some way to lift the air up into the atmosphere where it can become
unstable. That is, there must be some kind of lifting mechanism to initiate the
instability of the air. The lifting mechanisms are the same four lifting mechanisms
we discussed in Chapter 5 for the formation of clouds, namely :
(a) Convection Normal heating of the ground by short-wave radiation from
the Sun causes the ground to warm up. The ground radiates long-wave
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Chapter 8: Thunderstorms and Tornados
radiation that is absorbed by water vapor and carbon dioxide in the air. The
air at the surface of the earth is thus warmed. The warm air expands and
becomes lighter. The lighter air now rises by convection, and expands and
cools adiabatically.
(b) Convergence The convergence of wind currents or air masses causes a
lifting of the air. As we saw in section 6.6, air spirals into a low pressure area
at the surface of the earth, and produces a vertical motion upward of the air.
Convergence also occurs over a region like Florida. Air blows inward
from the Gulf of Mexico and the Atlantic Ocean. The air converges over
Florida. The only place for it to go is upward.
(c) Frontal Lifting Since a front is a boundary between two different air
masses, when these two air masses collide the warmer air mass, being
lighter, will move up over the colder air mass. Hence, the front will cause
lifting of the air.
(d) Orographic Lifting Orographic lifting occurs when air pushes up
against a mountain barrier. There is no place for the air to go but upward.
Hence the air is forced to rise.
(3) A very high moisture content of the air The greater the amount of
water vapor in the air the greater is the amount of latent heat energy that is
available to increase the instability of the air. That is, as the moist air rises it cools
adiabatically to the dew point temperature, becomes saturated, and condensation
occurs. When condensation occurs the latent heat of vaporization is given off by the
water vapor in the air, and this heat energy will cause the air to warm up. This
warmer air will then rise further upward into the atmosphere pulling more air
below up with it. As the air below rises, it goes through the same process of cooling,
saturation, condensation, and gives up more latent heat to the air, which again
causes it to rise. The process keeps repeating itself. It is almost like a run-awayengine. The greater the amount of water vapor in the air the sooner the air will
become saturated. Hence the base of the cumulonimbus cloud will be lower for very
moist air, and higher for not so moist air. This point where the condensation begins,
and the base of the cloud is found, is usually referred to as the level of free
convection (LFC). If there is little or no moisture in the air, no clouds will form, but
quite often you will get clear air turbulence here.
8.2 The Life Cycle Of The Thunderstorm Cell
Individual thunderstorm clouds are rarely larger than 10 miles in diameter
and their life cycle is about 20 minutes to about 2 hours. Severe thunderstorms
usually consist of two or more cells. A thunderstorm goes through a life cycle. That
is, it is born, grows to maturity, and then dies. There are three stages in the life
cycle of a thunderstorm. They are:
(1) the Cumulus or Growth Stage,
(2) the Mature Stage, and
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Chapter 8: Thunderstorms and Tornados
(3) the Dissipating Stage.
Each of the three stages last about 10 minutes. The Life cycle of the thunderstorm
is shown in figure 8.2.
ice
snow
Up drafts Down drafts
Hail if any
rain
(a) Cumulus Stage
(b) Mature Stage
Anvil
ice
snow
Down drafts
rain
(c) Dissipating Stage
Figure 8.2 The life cycle of a thunderstorm.
(1) The Cumulus Stage
The initial stage of thunderstorm is always a cumulus cloud. The main feature of
the thunderstorm is the updraft which may extend from near the surface to several
thousand feet above the cloud top, figure 8.2(a). The cloud droplets start out very
small but grow into rain drops as the cloud builds upward. By definition, no
precipitation occurs during this stage because the water droplets and ice particles
are being carried upward by the updraft.
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(2) The Mature Stage
By definition, the beginning of the rain at the earth’s surface initiates the mature
stage of the thunderstorm, figure 8.2(b). The rain droplets and ice particles have
grown too heavy to be supported by the updrafts and down they fall. Usually there
are very heavy rain showers. This occurs 10 to 15 minutes after the cloud rises
above the freezing level. On the average, the tops of the thunderstorm clouds range
anywhere from 25,000 to 35,000 ft and occasionally to as much as 50,000 to 60,000
ft.
As the rain drops begin to fall, they drag the adjacent air down with them.
This is a major factor in the formation of a down draft which is a characteristic of
the mature stage of the thunderstorm. This air that is dragged downward by the
falling raindrops is cooler than its surroundings and being unstable, its rate of
downward motion is accelerated. These downdrafts develop in the middle of the
cloud, and their speed may be as high as 15,000 ft/min. When these strong
downdrafts hit the ground below, they cause a large horizontal outflow of air that
produces strong gusty winds and are usually accompanied by a sharp drop in
temperature and a sharp rise in pressure. Since these thunderstorms usually occur
when the weather is very warm, these cool winds are usually very delightful.
If hail is to occur, it will occur during this phase of the thunderstorm. The
size of the hailstone varies from the size of a small pea to the size of a large
grapefruit.
(3) The Dissipating Stage
In this final stage of the life of the thunderstorm, there is nothing but downdrafts,
figure 8.2(c). Because of the heating and drying of the air produced by the
downdrafts, the rainfall gradually ceases and the thunderstorm cell begins to
dissipate. (Since there are no more updrafts, there is no more energy, supplied by
the latent heat of vaporization, to keep the system going, and the system dies.) The
base of the thunderstorm becomes stratiform and the top becomes anvil shaped.
8.3 Classification of Thunderstorms
Thunderstorms are classified according to where the thunderstorms are formed.
The different classes of thunderstorms are:
(1) Frontal Thunderstorms
(2) Squall Line Thunderstorms
(3) Air Mass Thunderstorms
(1) Frontal Thunderstorms
Frontal thunderstorms, as the name implies, are thunderstorms that are associated
with the different frontal systems.
(a) Warm Front Thunderstorms. Warm front thunderstorms are
associated with warm fronts. In general they are usually few and the least
severe of all the thunderstorms.
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Chapter 8: Thunderstorms and Tornados
(b) Cold Front Thunderstorms. When the cold front approaches, it has a
very steep slope and hence pushes the warm moist air in advance of it
upward rather rapidly. This rapid rise of the warm moist air which is usually
unstable, triggers the birth of the thunderstorm. The thunderstorms
associated with a cold front are usually the most severe anywhere except in a
squall line.
(c) Warm Front Occlusion. The thunderstorms associated with the warm
front occlusion, are usually associated with the cold front aloft of the
occlusion.
Frontal thunderstorms are more numerous than air mass thunderstorms.
(2) Squall Line Thunderstorms
A squall line is a line of thunderstorms that develops 50 to 300 miles ahead of and
roughly parallel to a fast-moving cold front, as seen in figure 8.3. The most severe
conditions, such as heavy hail, destructive winds, and tornados are usually
associated with these thunderstorms.
cold air
cP
L
cP
cool air
Squall line
mT
warm air
Figure 8.3 A Squall Line.
(3) Air Mass Thunderstorms
An air mass thunderstorm is a thunderstorm that occurs in the middle of a large air
mass. The characteristics of an air mass thunderstorm are: (1) the air mass is
usually a warm, moist air mass, and (2) the thunderstorms are usually widely
scattered over a large area. The air mass thunderstorms can be caused by:
(a) Convection - Heating of the air at the surface causes the air to
rise. These thunderstorm are most active over land in late afternoon
and early evening.
(b) Oragraphic lifting - Air rising over a mountain barrier is cooled
to condensation, and the cloud then forms.
(c) Nocturnal Cooling - This occurs when night time cooling
increases the instability of the air.
Associated with all thunderstorms is rain, hail, turbulence, lightning, thunder, and
icing in clouds. Lightning is an electrical discharge between two charged bodies.
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Chapter 8: Thunderstorms and Tornados
The lightning stroke can occur from cloud to ground, cloud to cloud, and from
ground to cloud, as shown in figure 8.4. Sheet lightning is a lightning flash that
occurs within a cloud system that is hidden from view. All that is seen is the bright
light, not the stroke.
Cloud-to-cloud
+
+
+
-
-
-
-
+ + + + +
Cloud-to-ground
+
+
-
-
+
-
Ground-to-cloud
Figure 8.4 Lightning.
8.2 Tornados
Tornados are violent, rotating columns of air that descend with a familiar
funnel or tube shape from a thunderstorm cloud systems, figure 8.5. The tornado
Figure 8.5 A Tornado.
vortex is usually several hundred yards in diameter and whirls in a
counterclockwise direction. The maximum winds in the most severe tornado is
greater than 260 mph. Tornados are found mostly in the United States, Australia,
the United Kingdom, New Zealand, Italy, and Japan. In the United States the
greatest frequency is in the spring when there is the greatest contrast between the
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Chapter 8: Thunderstorms and Tornados
cP and mT air masses, and also in the middle of the late afternoon on a very hot day
in the summer. In the United States tornados are found mostly in Kansas, Texas,
and Oklahoma. Here in the northeast there have been about 30 to 50 tornados
observed over the last 50 years.
Tornados usually form in the warm sector of a cyclonic system just ahead of
the cold front and in the leading edge of an intense thunderstorm. Tornados also
occur in hurricanes. Thunderstorm activity, rain or hail, and lightning accompany
most tornados. Low level convergence and abnormally strong convective turbulence
in moist, unstable air presumably generate the vortex. The paths of tornados are
usually parallel to the cold front, figure 8.6. The rate of travel at the ground may
cold air
cP
L
cP
cool air
mT
warm air
Figure 8.6 Path of tornados.
reach 50 knots or more or may remain stationary for short periods. Some make
contact with the ground and then lift, only to strike again at a distance of several
kilometers. Others travel only a few meters before rising and dissipating. At sea,
tornados are called waterspouts. They are usually smaller in diameter. When in
contact with the surface a water spout picks up some spray, but its funnel is
composed primarily of condensed water vapor in the low pressure vortex. Tornados
usually travel in “families” and last from a few minutes up to 6 hours in duration.
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