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Thunderstorms and
Tornadoes
Chapter 14
Thunderstorms
• Thunderstorm: A storm containing
lightning and thunder.
• Convective storms
• Warm, moist air starts moving
upwards towards cold, dry air aloft
• Vertical wind shear causes abrupt
change in wind.
• Severe thunderstorms: has either
large hail with a diameter of at least
1 inch, and/or surface wind gusts of
50 knots (58 mph) or greater, or
produces a tornado.
Thunderstorms
• Scattered thunderstorms
(ordinary cell thunderstorms or
air-mass thunderstorm)
• ‘Simple’ thunderstorm
– Limited vertical wind shear (no
abrupt change of wind speed
and wind direction vertically)
– Stages: 1. cumulus, 2. mature,
3. dissipating
Cycle of Development of
Ordinary Thunderstorm
Cumulus stage (growth stage)- as a parcel of warm,
humid air rises, it cools and condenses into cumulus cloud
(or multiple clouds)
• Transformation of water vapor into cloud droplets
releases latent heat, keeping the cloud warmer than the
surrounding air
• Continues to grow from rising air (vertical development)
• No lightning or thunder yet
• Entrainment: dry air is drawn in, causing some
raindrops to evaporate, which chills the air.
• Colder air then descends in a downdraft.
Cycle of Development of
Ordinary Thunderstorm
Mature Stage- cell of clouds now have downdrafts AND
updrafts.
• Can last for less than 30 minutes
• Thunderstorm is most intense
• Top of the cloud can take anvil shape from upper-level
winds spreading top ice crystals
• Can extend up to an altitude of 12km with several km of
diameter at base
• Updrafts and downdrafts cause turbulence in middle of
cloud
• Lightning and thunder are present
• Heavy rain, small hail
• Cold downdraft at surface creates gust front of wind
An ordinary thunderstorm in its mature stage. Note the
distinctive anvil top.
Cycle of Development of
Ordinary Thunderstorm
Dissipating stage – when updrafts weaken as the gust
front moves away from the storm
•Downdrafts dominate which destroys humid updrafts
•Cloud droplets stop forming as no more warm, humid air
comes up in updrafts
•Storm dies down with only light precipitation
•Lower level droplets evaporate, leaving only cirrus anvil
•May go through all stages in one hour or less.
Figure14.3 A dissipating thunderstorm near Naples, Florida.
Most of the cloud particles in the lower half of the storm have
evaporated.
Figure14-3 p383
Figure14.2 Simplified model depicting the life cycle of an ordinary cell
thunderstorm that is nearly stationary as it forms in a region of low wind shear.
(Arrows show vertical air currents. Dashed line represents freezing level, 0°C
isotherm.)
Figure14-2 p381
Multi-Cell Thunderstorms
• Multi-cell Thunderstorms
– Thunderstorms that contain a number of
convection cells, each in a different stage of
development,
– Moderate to strong wind shear
– Can become severe thunderstorm, lasts longer
than simple thunderstorm
– Rising air can intrude into stratosphere,
producing an overshooting top, can produce
mammatus clouds
– Cold, dense air can create small, shallow area of
high pressure called mesohigh
This multicell storm complex is composed of a series of cells
in successive stages of growth. Middle: mature stage. Right:
cumulus stage.
Figure14-4 p383
Diagram of intense multicell thunderstorm that has a tilted updraft.
The severity depends on the intensity of the storm’s circulation
pattern.
Figure14-5 p384
When a thunderstorm's downdraft reaches the ground, the air
spreads out forming a gust front, similar to a cold front.
Figure14-6 p384
A swirling mass of dust forms along the leading edge of a gust
front as it moves across western Nebraska.
Figure14-7 p385
A shelf cloud can form as warm, moist air rises along the gust
front. A dramatic example of a shelf cloud (or arcus cloud)
associated with an intense thunderstorm in the picture below.
Figure14-8 p385
Roll clouds form behind gust front, which are ominous-looking
clouds. In the picture, a roll cloud forms behind a gust front.
Figure14-9 p385
Multi-Cell Thunderstorms
– High winds in straight-line are called straightline winds
– Outflow boundary- gust fronts merging into
huge gust front, can generate new
thunderstorms.
– Downburst- downdrafts that hit the ground
and spread horizontally (smaller is called
microburst, larger is macroburst).
Radar image of an outflow boundary.
Figure14-10 p386
Thunderstorms
• Downburst (or microburst) and planes
– Headwind lifts airplane and climbs up, pilot may
correct by directing downward
– Powerful downdraft pushes down and tail wind
redirects
– DFW airport August 1985, aircraft encountered
severe horizontal windshear beneath intense
thunderstorm, dropped down and crashed, killing over
100 passengers.
– Many airports now use Doppler to find horizontal wind
shear.
Plane flying into a microburst. Can cause plane crashes and
damage on poorly built structures or sailing vessels.
Figure14-12 p387
Dust clouds rising in response to the outburst winds of a
microburst north of Denver, Colorado, can come from virga
where rain evaporates before reaching the ground. Can also have
blinding rain.
Figure14-11 p386
Multi-cell Thunderstorms
– Pre-frontal squall-line thunderstorms in
middle latitudes can precede a cold front and
have huge thunderstorms along a long length.
– Bow echo- a bow-shaped squall line. At bow
echo the release of latent heat leads to low
pressure area that spins counterclockwise.
– Straight line winds in a bow echo are called a
derecho windstorm, can cause lots of
damage. Similar damage to a tornado.
Doppler radar display superimposed on a map shows a pre-frontal squall line
extending from Texas into Oklahoma and Arkansas during February, 2011. Some of
the thunderstorms embedded within the squall line (dark red and orange color)
produced high winds, heavy rain, and large hail.
Figure14-13 p387
Pre-frontal squall-line thunderstorms may form ahead of an
advancing cold front as the upper-air flow develops waves
downwind from the cold front.
Figure14-14 p387
A side view of the lower half of a squall-line thunderstorm with the
rear-inflow jet carrying strong winds from high altitudes down to
the surface. These strong winds push forward along the surface,
causing damaging straight-line winds that may reach 100 knots
(derecho).
Figure14-16 p388
The strong thunderstorms (red and orange in the image) are
producing damaging straight-line winds from bow echo
over a wide area. Damaging straight-line winds that extend
for a good distance along a squall line is called a derecho.
Figure14-17 p389
Thunderstorms
•
Mesoscale Convective
Complex
• a number of individual
multi-cell thunderstorms
grow in size and organize
into a large circular
convective weather
system
• Can be 1000 times larger
than individual ordinary
cell thunderstorm and can
cover an entire state,
around 10,000km2
• Usually forms in summer.
Thunderstorms
• Supercell thunderstorms
– Large, long-lasting thunderstorm with a single
rotating updraft
– Strong vertical wind shear creates a horizontal
spin
– Can produce high speed updrafts, damaging
surface winds, and large tornadoes.
– Updrafts can allow hailstones to grow, up to
grapefruit size, can push them out of the side of
the cloud (hail in clear skies)
– Storm can be as high as 18km and width of
40km
Thunderstorms
• Supercell thunderstorms, three types:
– Classic (CL)- heavy rain, large hail, high
surface winds, majority of tornadoes
– High precipitation (HP)- Flash flooding,
hard to see tornadoes though heavy rain
– Low precipitation (LP)- tornadoes and large
hail, vertical rotation, cloud looks like
corkscrew.
A supercell thunderstorm with a tornado sweeps over Texas.
Figure14-20 p391
Some of the features associated with a classic tornadobreeding supercell thunderstorm
Figure14-21 p391
Wall cloud- low-level humid air drawn into updraft created
a rotating cloud (tornado can come down from these). A
wall cloud descending from a storm:
Figure14-22 p392
Thunderstorms
• Thunderstorms and the Dryline
– Sharp, horizontal change in moisture at the
dryline
– Thunderstorms form just east of dryline
– Spring and early summer in Texas, Oklahoma,
and Kansas
• Floods and Flash Floods
– Flash floods rise rapidly with little or no advance
warning; many times caused by stalled or slow
thunderstorm, stationary front
Surface conditions that can produce a dryline with intense
thunderstorms.
Figure14-25 p394
Thunderstorms
• Floods and Flash Floods
• Great Flood of 1993
•
•
•
Stationary front (alternating warm and cold front)
stayed across the upper Midwest for days,
Lower than normal jet stream provided uplift.
Warm, humid air from Gulf of Mexico provided
moisture for storms along Midwest, more than 60%
of levees along Mississippi River were destroyed,
area larger than Texas was flooded.
The heavy arrow represents the average position of the upper-level jet stream
from mid-June through July, 1993. The jet stream helped fuel thunderstorms
that developed in association with a stationary front that seemed to oscillate
back and forth over the region as an alternating cold front and warm front.
Figure14-26 p395
Here, floodwaters near down-town Des Moines, Iowa, during July, 1993,
inundate buildings of the Des Moines water-works facility. Flood-contaminated
water left 250,000 people without drinking water.
Figure14-27 p395
The average number of days each year on which
thunderstorms are observed throughout the United States.
Figure14-28 p398
The average number of days each year on which hail is observed
throughout the United States.
Figure14-29 p398
Thunderstorms
• Lightning and Thunder
– Lightning: discharge of electricity in mature
storms (within cloud- most lightning, cloud to
cloud, cloud to ground)
– Can heat the air to 54,000°F, 5 times hotter
than the sun.
– Thunder: explosive expansion of air due to
heat from lightning, travels outward in all
directions
Figure14-30 p399
Thunderstorms
– Lightning is instantaneous, travels at speed of
light
– Thunder travels at 330m/sec, much slower.
– It take 5 seconds for sound to travel 1 mile (3
seconds for 1km)
– For example thunder is 15 seconds after lightning,
lightning is 3 mi or 5km away.
– When lightning is very close (100m), sounds like a
clap or crack followed by loud bang
– When lightning is far away, sounds like rumbling.
– Sound travels faster in warm air than cold air, and
can refract upward sometimes so no thunder is
heard.
Figure14-31 p399
Lightning Strike
• The Lightning Stroke
– Positive and negative charges, leaders vs.
return strokes, usually lasting less than a
second, can look like it flickers.
Leader
Return stroke
When the downward flow of electrons meets the upward surge of
positive charge, a strong electric current — a bright return stroke —
carries positive charge upward into the cloud.
Figure14-34 p401
Figure14-35 p402
Lightning Strike
• Observation: Apple tree
– DO NOT seek shelter during a thunderstorm
under an isolated tree.
– Average yearly death toll from lightning in the
U.S. is 100, most in Florida. Many victims in
open places like golf course, sailing, or
working on farm.
– Cardiac arrest- stops heart. Those who
survive can have personality changes,
depression, or chronic fatigue.
Figure14-37 p403
The four marks on the road surface represent areas where
lightning, after striking a car traveling along south Florida’s
Sunshine State parkway, entered the roadway through the tires.
Figure14-39 p404