1. Discuss the various uses of the term cyclone.
2.Compare middle-latitude cyclones, tornadoes,
and hurricanes in terms of size and expected wind
speeds.
3.Outline and describe the stages in the
development of a thunderstorm.
4.List and briefly contrast two or more types of
thunderstorms.
3. Thunderstorms
• A thunderstorm is simply a storm that generates
lightning and thunder.
• It frequently produces gusty winds, heavy rain
and hail.
• A thunderstorm may be produced by just a single
cumulonimbus cloud and may influence only a
small area.
• It may be associated with clusters of
cumulonimbus clouds and influence a large area.
4. Thunderstorms
• Thunderstorms form when warm humid air
rises in an unstable environment.
• Various mechanisms can trigger the upward
movement of air required to create the
cumulonimbus clouds.
• One mechanism, the unequal heating of the
Earth’s surface, significantly contributes to the
formation of air-mass thunderstorms.
5. Thunderstorms
• These storms are associated with the
scattered puffy cumulonimbus clouds that
commonly form within maritime tropical air
masses and produce scattered thunderstorms
on summer days.
• Such storms are usually short-lived and
seldom produce strong winds or hail.
6. Thunderstorms
• In contrast, there is a second category of
thunderstorms that not only benefit from the
uneven heating of the Earth’s surface but are
associated with the lifting of warm air.
• The kind of lifting that occurs along a weather
front or along a mountain slope.
• Moreover, diverging winds aloft frequently
contribute to the formation of these storms
because they tend to draw air from levels upward
beneath them. (Like a stirring up of the air).
7. Thunderstorms
• Some of the thunderstorms in this second
category may produce high winds, damaging hail,
flash floods and tornadoes.
• These storms are described as severe!
• At any given time, there are an estimated 2000
thunderstorms in progress across the globe.
• The greater proportion of these occur in the
tropics where there is plenty of warmth,
moisture, and instability much of the time.
9. Thunderstorms
• About 45,000 thunderstorms take place each
day around the world, and 16 million occur
annually.
• The lightning from these storms strikes the
Earth 100 times each second.
• Annually, the United States experiences about
100,000 thunderstorms and millions of
lightning strikes.
10. Thunderstorms
• The following diagram shows that thunderstorms
are most frequent in Florida and the eastern Gulf
Coast region.
• Here, activity is recorded for between 70 and 100
days each year.
• The region on the east side of the Rocky
Mountains in Colorado and New Mexico has the
next highest number occurrences of
thunderstorms with 60 to 70 occurring annually.
11.
12. Thunderstorms
• Why do you suppose these two regions have
such a high frequency of thunderstorms in
comparison to the rest of the country?
13. Thunderstorms
• Most of the rest of the nation experiences
thunderstorms between 30 and 50 per year.
• The western margin of the United States has
little thunderstorm activity. (Why?)
• The same is true for the northern tier of the
United States and Canada. (Why?)
• Warm, moist, unstable mT air rarely
penetrates these regions.
14. Air Mass Thunderstorms
• In the United States, air mass thunderstorms
frequently occur in maritime tropical (mT) air
that moves northward from the Gulf of
Mexico.
• These warm and humid air masses contain
abundant moisture in their lower levels and
can be rendered unstable when heated from
below or lifted along a front.
15. Air Mass Thunderstorms
• Because mT air becomes most unstable in
spring (like now) and summer as well when it
is warmed from below by the heated land
surface.
• It is during these seasons that air-mass
thunderstorms are most frequent.
• They occur most often in the mid-afternoon
when the surface temperatures are highest.
16. Air Mass Thunderstorms
• Because local differences in surface heating
aid in the growth of air-mass thunderstorms,
they generally occur as scattered, isolated
cells instead of being organized in relatively
narrow bands or other configurations.
17. Stages of Development
• The Thunderstorm Project
– Involved the use of radar, aircraft, radiosondes
and an extensive network of surface instruments.
– Produced a three staged model of the life cycle of
a thunderstorm.
18. Stages of Development
• Cumulus Stage
– Air mass thunderstorms are a product of uneven
heating of the Earth’s surface.
– Initial cumulus development is important
because it moves water vapor from the surface
to greater heights.
– Ultimately, the air becomes sufficiently humid
that newly forming clouds do not evaporate but
instead continue to grow vertically.
19. Stages of Development
• The development of the cumulonimbus tower
requires a continuous supply of warm, moist
air.
• Release of latent heat allows each new surge
of warm air to rise higher than the last.
• This adds to the height of the cloud.
• This stage, the cumulus stage, is dominated by
updrafts.
20. Stages of Development
• Once clouds pass beyond the freezing level,
the Bergeron Process begins producing
precipitation.
• Eventually the accumulation of precipitation in
the cloud is too great for the updrafts to
support.
• The falling precipitation causes drag on the air
and initiates a downdraft.
21. Stages of Development
• The creation of the downdraft is further aided by
the influx of cool, dry air surrounding the cloud a
process called entrainment.
• This process intensifies the downdraft because
the air added during entrainment is cool and
therefore (WHAT????)
• Possibly of greater importance, it is dry.
• This causes some of the falling precipitation to
evaporate thereby cooling the air within the
downdraft.
22. The Mature Stage
• As the downdraft leaves the base of the cloud,
precipitation is released marking the
beginning of the cloud’s mature stage.
• At the surface, the cool downdraft spreads
laterally and can be felt before the
precipitation actually reaches the ground.
• We spoke about this last week.
• Sharp, cool gusts of wind are indicative of
downdrafts aloft.
23. The Mature Stage
• During the mature stage, downdrafts and
updrafts exist in the cloud side by side.
• When the cloud grows to the top of the
unstable region, often located at the base of
the stratosphere, the updrafts spread laterally
and produce the characteristic anvil top
(Yuka!)
• Ice laden cirrus clouds make up the top and
are spread downwind by strong winds aloft.
25. The Mature Stage
• This is the most active stage of the
thunderstorm
• Gusty winds, lighting, heavy precipitation and
sometimes small hail are experienced.
26. Dissipating Stage
• Once the downdrafts occur, the vacating air and
precipitation encouragement more entrainment
of the cool, dry air surrounding the cell.
• Eventually, the downdrafts dominate
throughout the cloud and initiate the dissipating
stage.
• The cooling effect of falling precipitation and
the influx of colder air aloft mark the end of the
thunderstorm activity.
27. Dissipating Stage
• Without a supply of moisture from the
updrafts, the cloud soon evaporates.
• Only about 20% of the moisture that
condenses in an air-mass thunderstorm
actually leaves the cloud as precipitation.
• The remaining 80% evaporates back into the
atmosphere.
28. Thunderstorms
• When you view a thunderstorm, you may
notice that the cumulonimbus cloud consists
of several towers.
• Each tower may represent a separate,
individual cell that is in a somewhat different
part of its life cycle.
29. Thunderstorm Stages
• The cumulus stage
– Updrafts dominate throughout the cloud.
– Growth from a cumulus to a cumulonimbus cloud
begins.
• Mature Stage
– The most intense phase. Heavy rain and possibly
small hail. Downdrafts are side by side with
updrafts.
31. Occurrences
• Mountainous regions, such as the Rockies in the
west and the Appalachians in the east experience
more air mass thunderstorms than the Great
Plains do.
• The air near the top of the mountain is heated
more intensely than the air at the same elevation
over the lowlands.
• A general upslope movement develops over the
daytime that can generate thunderstorm cells.
32. Occurrences
• These cells may remain almost stationary
above the slopes below.
• Although the growth of thunderstorms is
aided by high surface temperatures, many
thunderstorms are not generated solely by
surface heating.
• For example, many of Florida’s thunderstorms
are triggered by the convergence of sea-to-
land airflow.
33. Occurrences
• Many thunderstorms that form over the eastern
two-thirds of the United States occur as part of a
general convergence and frontal wedging that
accompany passing mid-latitude cyclones.
• Near the equator, thunderstorms commonly form
in association with the convergence along the
equatorial low – this is also called the inter-
tropical convergence zone.
• Most of these thunderstorms are not severe and
their life cycles are like that of the three stage
model we just described.
34. Severe Thunderstorms
• Capable of producing heavy downpours and
flash floods.
• Strong, gusty, straight-line winds, large hail,
frequent lightning, and perhaps tornadoes.
• Must be declared by the National Weather
Service
35. Severe Thunderstorms
• Must have winds in excess of 93 kilometers
(58 miles) per hour.
• OR produce hailstones with diameters larger
than 1.9 centimeters (.75 inches).
• OR generate a tornado.
• Of the estimated 100,000 thunderstorms that
occur annually in the United States,
approximately 10 percent (10,000) storms
reach severe status.
36. Severe Thunderstorms
• Remember, air mass thunderstorms are
localized.
• Relatively short lived and they dissipate after a
brief, well-defined life cycle.
• As you have seen, they extinguish themselves
once the downdrafts cut off the moisture
supply necessary to maintain the storm.
• For this reason, they seldom produce severe
weather.
37. Severe Thunderstorms
• By contrast, other thunderstorms (the kind
that is not considered air-mass) do not
dissipate quickly and may remain active for
several hours.
• Some of these larger storms tender to live
longer and reach severe status.
38. Severe Thunderstorms
• Why do some thunderstorms exist for hours?
• A key factor is the existence of strong vertical
wind shear – changes in wind direction and/or
speed at different heights.
• When these conditions exist, the updrafts that
provide the storm with moisture do not
remain vertical, but become tilted.
39. Severe Thunderstorms
• Because of this, the precipitation that forms
high in the upper parts of the cloud falls into
the downdraft rather than the updraft as it
does in air-mass thunderstorms.
• This allows the updraft to maintain its
strength and continue to build upward.
• Sometimes the updrafts are sufficiently strong
that the cloud top is able to push its way into
the stable lower stratosphere.
40. Severe Thunderstorms
• This situation is called overshooting.
• Beneath the cumulonimbus tower where
downdrafts reach the surface, the more
dense, cooler air spreads out across the
ground.
• The leading edge of this cooler air acts like a
wedge, and this forces warm, moist air into
the thunderstorm.
• This further feeds the thunderstorm.
