NAS 125: Meteorology
Thunderstorms and Tornadoes
Tornado outbreak, part 1
• On 3 May 1999, more than 70 tornadoes struck an
area stretching from northern Texas to south-central
Kansas.
– Twenty-six tornadoes struck in or near Oklahoma City.
• The most intense tornado, F5 on the Fujita scale, tore
through Moore and Bridge Creek, claiming 28 lives.
– An F4 tornado killed six at Haysville, Kansas, a few hours
later.
Rev. 30 April 2006
Thunderstorms and Tornadoes
2
Tornado outbreak, part 2
• Rawinsonde data from Norman, Okla., indicated a
shallow layer of warm, humid air near the surface
with dewpoints near 21 °C.
• The warm, humid layer was capped by a temperature
inversion at about 1,500 m with much drier air above.
– The inversion layer kept convection currents in check, but
as the temperature and humidity contrast grew during the
day, deep convection began, leading to the developmen tof
severe thunderstorms.
Rev. 30 April 2006
Thunderstorms and Tornadoes
3
Tornado outbreak, part 3
• The rawinsonde indicated a vertical wind shear, with
southerly winds at the surface to westerly or westsouthwest winds aloft.
– The wind shear contributed to the development of severe
weather.
• The arrival of a jet streak in the afternoon lifted the
capping inversion, allowing the development of
massive supercell thunderstorms.
Rev. 30 April 2006
Thunderstorms and Tornadoes
4
Rev. 30 April 2006
Thunderstorms and Tornadoes
5
Rev. 30 April 2006
Thunderstorms and Tornadoes
6
Rev. 30 April 2006
Thunderstorms and Tornadoes
7
Rev. 30 April 2006
Thunderstorms and Tornadoes
8
Rev. 30 April 2006
Thunderstorms and Tornadoes
9
Rev. 30 April 2006
Thunderstorms and Tornadoes
10
Rev. 30 April 2006
Thunderstorms and Tornadoes
11
Rev. 30 April 2006
Thunderstorms and Tornadoes
12
Thunderstorms, part 1
• Thunderstorms are violent convective storms
accompanied by thunder and lightning; they are
usually localized and short-lived.
– Vertical air motion, considerable humidity, and instability
combine to create towering cumulonimbus clouds, so
thunderstorms are always associated with this combination.
– They frequently occur in conjunction with other kinds of
storms (hurricanes, tornadoes, fronts [especially cold
fronts]) in midlatitude cyclones, and orographic lifting.
– They are associated with other mechanisms that can trigger
unstable uplift.
Rev. 30 April 2006
Thunderstorms and Tornadoes
13
Thunderstorms, part 2
• Thunderstorms (continued):
– Thunderstorms are virtually unknown poleward of 60˚ of
latitude.
– Thunderstorms often occur in clusters.
Rev. 30 April 2006
Thunderstorms and Tornadoes
14
Rev. 30 April 2006
Thunderstorms and Tornadoes
15
Life cycle, part 1
• Thunderstorms have a life cycle that consists of there
stages: cumulus stage, mature stage, and dissipating
stage.
• In the cumulus stage, updrafts prevail and clouds
grow.
– Cumulus clouds quickly surge to altitudes of more than
8,000 m.
– Cumulus clouds also merge horizontally, such that the
lateral size may reach 15 km.
Rev. 30 April 2006
Thunderstorms and Tornadoes
16
Life cycle, part 2
• Cumulus stage (continued):
– Two types of convection drive storm development: free
confection and forced convection.
• Free convection is caused by intense heating of the Earth’s surface,
but is not usually adequate to generate a thunderstorm.
• Forced convection is driven by frontal or orographic lifting or by
converging surface winds; most thunderstorms result from forced
convection.
– Latent heat is released by condensation of water; this extra
heat adds to the buoyancy of saturated air parcels, which
rise and cool at the moist adiabatic rate.
Rev. 30 April 2006
Thunderstorms and Tornadoes
17
Life cycle, part 3
• Cumulus stage (continued):
– Two types of cumuliform clouds form:
• Clouds with significant vertical development that resemble a huge
cauliflower are called cumulus congestus.
• Cumulus congestus clouds that continue to develop vertically such
that precipitation, lightning, and thunder are produced, are called
thunderstorm clouds – cumulonimbus clouds.
– No precipitation falls during the cumulus stage because
updrafts are strong enough to keep water droplets and ice
crystals suspended in the upper reaches of the cloud.
Rev. 30 April 2006
Thunderstorms and Tornadoes
18
Life cycle, part 4
• The mature stage begins when precipitation reaches
the ground.
– It is the most active and largest stage, with cloud tops
reaching 18,000 m; they often have an anvil shape where
high-altitude winds distort cloud shape.
– The weight of water droplets and ice crystals becomes too
great for them to remain suspended by the force of the
updrafts.
– As the water droplets and ice crystals fall, they drag
adjacent air with them to form a downdraft, which
descends along the updrafts building the storm.
Rev. 30 April 2006
Thunderstorms and Tornadoes
19
Rev. 30 April 2006
Thunderstorms and Tornadoes
20
Life cycle, part 5
• Mature stage (continued):
– Unsaturated air at the edge of the cloud is dragged into the
cloud in a process called entrainment.
• Entrainment causes some of the water mixes with the saturated air,
causing water droplets and ice crystals to vaporize, which in turn
weakens the updrafts and strengthens the downdrafts.
– The downdraft exits the base of the cloud, hits the ground
and spreads out across the surface in a strong, cool wind
resembling a mini cold front called a gust front.
Rev. 30 April 2006
Thunderstorms and Tornadoes
21
Rev. 30 April 2006
Thunderstorms and Tornadoes
22
Life cycle, part 6
• The mature stage begins when precipitation reaches
the ground.
– Unusual clouds are associated with the gust front.
• Roll clouds are tube-shaped clouds, not connected to the
cumulonimbus cloud, that form behind the gust front; they appear
to roll horizontally.
• Shelf clouds – or arcus clouds – are low, elongated, wedge-shaped
clouds that are attached to the cumulonimbus cloud; they form at
the edge of the gust front.
• In the dissipating state, downdrafts dominate, and
turbulence ceases.
• Adiabatic warming in the downdrafts contributes to the drying up
of moisture that fuels the storm.
Rev. 30 April 2006
Thunderstorms and Tornadoes
23
Rev. 30 April 2006
Thunderstorms and Tornadoes
24
Classification, part 1
• Single-celled thunderstorms are weak, isolated
systems that form anywhere in warm, humid air
masses.
– They usually form long some boundary within the air mass.
• Multi-cellular thunderstorms consist of more than one
storm cell, each of which may be at different stages of
the thunderstorm life cycle.
– A squall line is an elongated cluster of thunderstorm cells.
• They typically form along frontal boundaries, thus individual
storms are called frontal thunderstorms.
Rev. 30 April 2006
Thunderstorms and Tornadoes
25
Rev. 30 April 2006
Thunderstorms and Tornadoes
26
Rev. 30 April 2006
Thunderstorms and Tornadoes
27
Classification, part 2
• A mesoscale convective complex (MCC) is a nearly
circular cluster of interacting thunderstorm cells.
– MCCs may be 1000 times larger than individual storms.
– MCCs occur mostly in the warm season, and are not
associated with a front.
• Supercell thunderstorms are long, lived, large, intense
storms, composed of a single cell, with an updraft
that may exceed 240 km per hour.
Rev. 30 April 2006
Thunderstorms and Tornadoes
28
Rev. 30 April 2006
Thunderstorms and Tornadoes
29
Distribution, part 1
• Thunderstorm formations require three conditions:
– Humid air in the low- to mid-troposphere;
– Atmospheric instability;
– A source of uplift.
• Most form in maritime tropical (mT) air masses.
• Most form when mT air is lifted:
– Along fronts;
– Up mountain slopes;
– Via horizontal convergence of surface winds.
Rev. 30 April 2006
Thunderstorms and Tornadoes
30
Distribution, part 2
• Thunderstorm day: A statistic used to express
thunderstorm frequency; defined as a day in which
thunder is heard.
– Thunderstorm days/month
– Thunderstorm days/year
• Globally, thunderstorm days are highest:
– In humid tropics;
– Along ITCZ.
Rev. 30 April 2006
Thunderstorms and Tornadoes
31
Distribution, part 3
• In North America, thunderstorm days are highest:
– In Central Florida;
– Along Colorado Front Range.
• Thunderstorms are common along coastal areas.
Rev. 30 April 2006
Thunderstorms and Tornadoes
32
Rev. 30 April 2006
Thunderstorms and Tornadoes
33
Rev. 30 April 2006
Thunderstorms and Tornadoes
34
Severe thunderstorms, part 1
• Severe thunderstorms are (by convention)
accompanied by locally damaging winds, frequent
lightning, or large hail.
– According to the National Weather Service, Severe
thunderstorms must meet one or more of the following
criteria:
• Surface winds stronger than 93 km/hr (58 mph);
• Tornadoes or funnel clouds;
• Hailstones 1.9 cm (0.75 in) or more in diameter.
Rev. 30 April 2006
Thunderstorms and Tornadoes
35
Severe thunderstorms, part 2
• The taller the storm, the greater the potential for it
becoming severe.
– Vertical wind shear (change in horizontal wind speed or
direction) is key to the development of severe
thunderstorms.
• Most severe storms develop over the Great Plains.
– They are usually associated with synoptic-scale cyclones.
– They usually form ahead of a squall line within the warm
sector, ahead of and parallel to a fast-moving cold front.
Rev. 30 April 2006
Thunderstorms and Tornadoes
36
Severe thunderstorms, part 3
• Feature of typical Southern Plains storm outbreaks:
– Mature cyclone centered over Western Kansas;
– Well defined cold front trailing into West Texas;
– Interaction between the westerly polar front jet aloft and a
southerly low-level jet blowing up from Gulf of Mexico
create vertical wind shear and instability; effect is enhanced
if subtropical jet is near
– Jet streak induced horizontal divergence and convergence
aloft, which in turn triggers cyclogenesis under the leftfront quadrant of the streak and subsidence under rightfront quadrant;
Rev. 30 April 2006
Thunderstorms and Tornadoes
37
Severe thunderstorms, part 4
• Southern Plains storm outbreaks (continued):
– Compressional warming in area of subsidence leads to
formation of a capping inversion;
– Persistent capping inversion intensifies differences between
air masses;
– Updrafts eventually break through capping inversion,
allowing explosive convection;
– Updrafts may penetrate the stratosphere, thus creating a
severe thunderstorm.
Rev. 30 April 2006
Thunderstorms and Tornadoes
38
Severe thunderstorms, part 5
• Southern Plains storm outbreaks (continued):
– Dry lines typically form between eastern edge of cold front
and western edge of mT air;
– Mammatus clouds, formed from blobs of cold, cloudy air,
often appear at the base of mature cumulonimbus clouds.
Rev. 30 April 2006
Thunderstorms and Tornadoes
39
Rev. 30 April 2006
Thunderstorms and Tornadoes
40
Rev. 30 April 2006
Thunderstorms and Tornadoes
41
Rev. 30 April 2006
Thunderstorms and Tornadoes
42
Thunderstorm hazards
•
•
•
•
Lightning
Downbursts
Flash floods
Hail
Rev. 30 April 2006
Thunderstorms and Tornadoes
43
Lightning, part 1
• There are more than 8.5 million lightning bolts daily
in world.
• Lightning most frequently occurs as exchanges
between adjacent clouds or between the upper and
lower portions of the same cloud; it also occurs as an
electrical connection of ionized air from cloud to
ground.
Rev. 30 April 2006
Thunderstorms and Tornadoes
44
Lightning, part 2
• The sequence that leads to lightning discharge is
known, but the mechanism for electrification is not.
– Large cumulonimbus cloud experiences a separation of
electrical charges:
• Positively charged particles are mostly high in cloud, while
negatively charged particles tend to concentrate in base.
– A growing negative charge in base attracts a growing
positive charge on Earth’s surface immediately below
cloud.
• An insulating barrier lies between cloud base and surface.
Rev. 30 April 2006
Thunderstorms and Tornadoes
45
Lightning, part 3
• Sequence (continued):
– Growing negative charge (continued):
• The contrast between cloud base and surface builds to tens of
million volts and overcomes the insulating barrier.
• A finger of negative current flicks down from cloud and meets a
positively charge darting upward from the ground, causing
lightning.
• The cause is unknown; theories differ.
– Most popular theory is that updrafts carry positively
charged particles to the top of the clouds, while falling ice
pellets gather negative charges and transport them
downward.
Rev. 30 April 2006
Thunderstorms and Tornadoes
46
Lightning, part 4
• Cause (continued):
– Scientists are investigating if cosmic rays from deep space
may be involved.
• Thunder is an instantaneous expansion of air caused
by the abrupt heating that lightning bolt produces.
This expansion creates a shock wave that becomes a
sound wave.
– One can time the distance that lightning is away because of
the different rates thunder and lightning travel at (speed of
sound vs. speed of light).
• A three-second interval equals about a kilometer.
Rev. 30 April 2006
Thunderstorms and Tornadoes
47
Rev. 30 April 2006
Thunderstorms and Tornadoes
48
Downbursts
• Downbursts are exceptionally strong downdrafts that
diverge horizontally after they reach the ground.
• Macrobursts cause destruction over distances of more
than 4 km with winds that may exceed 210 km/hr.
– They last up to 30 minutes.
• Microbursts are smaller and of shorter duration, but
wind speeds may reach 270 km/hr.
– Microbursts are hazardous to aircraft.
• Derechos are straight-line downburst winds that may
span hundreds of kilometers.
Rev. 30 April 2006
Thunderstorms and Tornadoes
49
Rev. 30 April 2006
Thunderstorms and Tornadoes
50
Flash floods, part 1
• Flash floods are short-term, localized, difficult to
predict rises in stream level, usually forming in
response to high precipitation amounts in watershed
areas.
– Flash floods typically rise and fall within 6 hours of rain
event.
– Often form under stationary or slow-moving
thunderstorms.
– Typically form under conditions of weak vertical wind
shear.
Rev. 30 April 2006
Thunderstorms and Tornadoes
51
Flash floods, part 2
• Big Thompson Canyon flood, 31 July 1976:
– Canyon located in Colorado Front Range below Rocky
Mountain National Park.
– Persistent thunderstorm over headwaters of Big Thompson
River dropped 25 to 30 cm of rain; about 20 cm fell in 2
hours.
– Floodwaters poured into narrow canyon, raising the river as
much as 6 m.
– 139 killed; 418 houses and 197 motor vehicles destroyed;
$35.5 million in property damage.
Rev. 30 April 2006
Thunderstorms and Tornadoes
52
Rev. 30 April 2006
Thunderstorms and Tornadoes
53
Rev. 30 April 2006
Thunderstorms and Tornadoes
54
Hail, part 1
• Hail is precipitation in the form of balls or chunks of
ice more than 5 mm in diameter.
• Hail almost always falls from tall cumulonimbus
clouds with strong updrafts and abundant supercooled
water droplets.
• Largest hailstone recorded near Coffeyville, Kansas,
on 3 September 1970: 766 g, 44.5 cm.
• The form when ice pellets fall, get picked in updrafts,
and grown as additional layers of water freeze on the
surface.
Rev. 30 April 2006
Thunderstorms and Tornadoes
55
Hail, part 2
• Hail falls to the ground when updrafts are no longer
able to keep it suspended.
• Hail may cover ground in long, narrow stripes called
hailstreaks.
– Hailstreaks may reach widths of 2 km and lengths of 10
km.
Rev. 30 April 2006
Thunderstorms and Tornadoes
56
Rev. 30 April 2006
Thunderstorms and Tornadoes
57
Rev. 30 April 2006
Thunderstorms and Tornadoes
58
Tornadoes, part 1
• Tornadoes are localized, typically cyclonic, lowpressure cell surrounded by a whirling cylinder of
wind spinning so violently that partial vacuum
develops within the funnel.
– They have the most extreme pressure gradients known (as
much as 100-millibar difference between tornado center
and air immediately outside funnel).
– Extreme pressure differences produce winds of
extraordinary speed.
Rev. 30 April 2006
Thunderstorms and Tornadoes
59
Rev. 30 April 2006
Thunderstorms and Tornadoes
60
Tornadoes, part 2
• Tornadoes (continued):
– How fast are the winds? No one knows, because tornadoes
blow to bits the anemometer (and instrument for measuring
wind speed). Maximum estimates range from 320 to 800
kilometers.
– The exact mechanism of formation is unknown.
– They usually develop in warm, moist, unstable air
associated with midlatitude cyclones.
– They most often develop along a squall line that preceded a
rapidly advance cold front, or along the cold front.
Rev. 30 April 2006
Thunderstorms and Tornadoes
61
Tornadoes, part 3
• Tornadoes (continued):
– The spring and early summer are favorable for
development because there are considerable air-mass
contrasts present and greater instability in the lower
troposphere at those times.
– They most frequently occur in midafternoon, at the time of
maximum heating.
– Less than 1 percent of thunderstorms produce tornadoes.
– Waterspouts occur over ocean and lakes; they have less
pressure gradient, gentler winds, and reduced destructive
capability.
Rev. 30 April 2006
Thunderstorms and Tornadoes
62
Tornadoes, part 4
• Tornadoes (continued):
– More than 90 percent of all reported tornadoes occur in the
United States. This reflects optimum environmental
conditions:
• The relatively flat terrain of the central and southeastern U.S.
provides uninhibited interaction of Canadian cP and Gulf mT air
masses.
• Tornado alley runs from east Texas and Texas Panhandle north into
southeastern South Dakota.
• The United States can expect between 700 and 1100 tornadoes,
with the record being 1389 in 1998.
Rev. 30 April 2006
Thunderstorms and Tornadoes
63
Rev. 30 April 2006
Thunderstorms and Tornadoes
64
Rev. 30 April 2006
Thunderstorms and Tornadoes
65
Tornadoes, part 5
• The deadliest tornado was the Tri-State tornado of 18
March 1925, which claimed 695 lives.
– The tornado lasted 3.5 hours, moved at a speed of 118
km/hr, and wreaked havoc along a path 350 km in length.
• Tornado hazards:
–
–
–
–
Extremely high winds
Strong updrafts
Subsidiary vortices
Abrupt drops in air pressure
Rev. 30 April 2006
Thunderstorms and Tornadoes
66
Tornadoes, part 6
• Tornado intensity is measured using the Fujita scale.
– The Fujita scale is a six-point scale rating tornado strength
and damage to structures.
– Weak tornadoes
• F0: estimated wind speed 65-118 km/hr; 40-73 mph
• F1: estimated wind speed 119-181 km/hr; 74-112 mph
– Strong tornadoes
• F2: estimated wind speed 182-253 km/hr; 113-157 mph
• F3: estimated wind speed 254-332 km/hr; 158-206 mph
– Violent tornadoes
• F4: estimated wind speed 333-419 km/hr; 207-260 mph
• F5: estimated wind speed 420-513 km/hr; 261-318 mph
Rev. 30 April 2006
Thunderstorms and Tornadoes
67
Rev. 30 April 2006
Thunderstorms and Tornadoes
68
Tornadoes, part 7
• Tornado intensity (continued):
– Tornado intensity can fluctuate over the course of its life
cycle.
• The Wichita-Andover (Kansas) tornado of 26 April 1991 began as
an F1, worked up to an F5 (as it struck a mobile home park in
Andover, Kansas, where it destroyed 241 mobile homes), then
decreased to an F1 before it ended.
• Most tornadoes are spawned from supercell
thunderstorms.
– Supercell thunderstorms have updrafts with speeds
exceeding 240 km/hr.
Rev. 30 April 2006
Thunderstorms and Tornadoes
69
Tornadoes, part 8
• Tornadoes and supercell thunderstorms (continued):
– Tornadoes develop in the interaction between supercell
updrafts and horizontal winds.
• Horizontal winds exhibit strong vertical shear in terms of both
speed and direction.
– Wind strengthens and turns clockwise with altitude.
• Horizontal cylinder of rolling air is tilted upright by the updraft;
and vertical shear within cloud intensified rotation – typically in the
counterclockwise direction.
– The spinning thunderstorm cloud is called a mesocyclone.
• Wall clouds typically accompany mesocyclones, forming in the
zone of the strongest uplift.
• Wall clouds do not normally form tornadoes.
Rev. 30 April 2006
Thunderstorms and Tornadoes
70
Tornadoes, part 9
• Tornadoes and supercell thunderstorms (continued):
– Mesocyclones (continued):
• Tornadic wall clouds are generally long-lived, with strong surface
winds blowing in toward the center.
• Mesocyclone circulation is most intense at about 6,100 km.
• Updrafts strengthen as more air converges toward the base of the
supercell.
• The mesocyclone narrows (with an accompanying increase in wind
speed) and builds toward the ground.
• As interior air pressures drop, water vapor condenses to form a
funnel cloud.
Rev. 30 April 2006
Thunderstorms and Tornadoes
71
Tornadoes, part 9
• Tornadoes and supercell thunderstorms (continued):
– Mesocyclones (continued):
• A strong downdraft develops near the back of the supercell, and the
funnel follows the downdraft to the ground.
• Squall lines and mesoscale clusters also spawn very
productive thunderstorms.
Rev. 30 April 2006
Thunderstorms and Tornadoes
72
Rev. 30 April 2006
Thunderstorms and Tornadoes
73
Rev. 30 April 2006
Thunderstorms and Tornadoes
74
Rev. 30 April 2006
Thunderstorms and Tornadoes
75
Monitoring tornadic storms, part 1
• In the early 1980s, storm chasers tried to place
instruments in the path of tornadoes, without much
success.
– Remote platforms, such as portable Doppler radar, are
much more effective tools.
• Radar signs
– Hook echoes, which are detected by reflectivity radars,
often indicate possible tornadoes.
– A tornado vortex signature on Doppler radar signfies the
presence of a possible tornado.
Rev. 30 April 2006
Thunderstorms and Tornadoes
76
Rev. 30 April 2006
Thunderstorms and Tornadoes
77
Rev. 30 April 2006
Thunderstorms and Tornadoes
78
Rev. 30 April 2006
Thunderstorms and Tornadoes
79
Rev. 30 April 2006
Thunderstorms and Tornadoes
80