Understanding the Weather
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Chapter 14
Thunderstorms and Tornadoes
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Chapter 14 Overview
– Ordinary Cell Thunderstorms
– Severe Thunderstorms
– Mesoscale Convective
Complexes
– Lightning
– Tornadoes
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Thunderstorms
– A thunderstorm contains thunder and lightning.
• Come in many different shapes and sizes.
– Warm, moist air rises in a conditionally unstable
environment.
• As long as parcel is warmer than environment then it will continue
to rise, it is buoyant.
• Greater the temp difference, the faster the air will rise.
– Rising air must be “triggered”/forcing mechanism.
• Unequal heating, terrain, lifting of air along shallow boundaries of
converging winds.
• Frontal lifting.
• Large scale divergence aloft.
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Ordinary Cell Thunderstorms
– Afternoon storms develop away from fronts.
– Form in a region with limited wind shear – wind speed and
direction do not change with height.
– Form along shallow convergence zones.
– Topography, seabreezes, cold outflow
from a prior thunderstorm
(differences in temp) can
be locations for
development.
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Ordinary Cell Thunderstorms (Cumulus/Growth Stage)
– Warm and humid parcel of air rises, cools, and condenses
into one cloud or a cluster of clouds.
– Gradually moistens the dry air aloft.
– Latent heat release keeps the
parcels warm.
– Must have continuous source of
rising air, the cloud needs to be
constantly fed rising air from below.
– Updrafts suspend the liquid until it
grows large enough to fall.
– This usually does not happen until
ice is present.
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Ordinary Cell Thunderstorms (Mature Stage)
– The entrainment of drier air at the edges of the storm
evaporates some of the liquid which cools the surrounding
air. Evaporative cooling.
– The air, now heavier and cooler
than the air around it, begins to
descend a downdraft. This can be
enhanced by falling precip.
– When this occurs the storm is
mature.
– The updraft and downdraft now
constitute the cell.
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Ordinary Cell Thunderstorms (Mature Stage)
– The most intense stage. Lightning, thunder, rain, and
occasionally small hail.
– Top of cloud reaches a stable region (tropopause) and
spreads out into anvil, up to 40,000 ft (12 km).
– Some updrafts are so strong they penetrate the stable air,
a condition know as overshooting.
– The cold downdraft
forms a gust front.
– The gust front can act to
force more warm, humid
air into the storm.
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Ordinary Cell Thunderstorms (Dissipating Stage)
– Once the storm enters the mature stage, it begins to
dissipate after 15 – 30 mins.
– Downdrafts dominate through much of the cloud and cut
off the supply of warm, moist air.
– Deprived of the rich supply of
warm, humid air, cloud droplets
no longer form.
– As the storm dies, the lowerlevel cloud particles evaporate
rapidly.
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Severe Thunderstorms
– Criteria: A thunderstorm that produces hail at least ¾”
diameter (penny/dime), and/or surface wind gusts of 50
kts, and/or produces a tornado.
– The longer a storm survives, the more likely it will become
severe.
– Forms in same way as an ordinary storm, only the
environment features vertical wind shear.
• The lack of wind shear in an ordinary thunderstorm causes the
precip to fall into the updraft.
• Increased winds aloft push the precip away from updraft.
• The downdraft can still undercut the updraft without the precip
falling into it. This produces a long-lasting multicell storm.
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Severe Thunderstorms (Multicell Storm)
– Most ordinary thunderstorms are multicell storms (storms
with a cluster of cells at various stages of their life cycles),
however many multicell storms are severe.
– The gust front initiates new storms (cells).
– This process may repeat over and over, producing a long
lasting storm.
– Each cell in a different
state of development.
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Severe Thunderstorms (Supercell)
– When the upper level winds are even stronger and also
change direction with height :
• The storm can move in such a way that the downdraft never
undercuts the updraft.
• Can create horizontal spin which may be tilted into a rotating
updraft.
• The result is a supercell.
– A supercell modifies its own environment.
– The vertical wind shear creates a storm structure that
allows the storm to continually move towards the area of
warm moist air.
• Supercells move to the right of the mean flow.
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Severe Thunderstorms (Supercell)
– The supercell thunderstorm is typically a large
thunderstorm that consists primarily of a single rotating
updraft.
– The internal structure is organized in a way so that the
storm may maintain itself as a single entity for hours on
end.
– Storms of this magnitude can
produce updrafts that exceed
90 kts, hail the size of
grapefruit, damaging surface
winds, and large, long lasting
tornadoes.
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Mesoscale Convective Complex
– A large, convectively driven system that is made up of
many individual thunderstorms. Many times, as much as
1000 times larger than an individual thunderstorm cell.
– Within the MCCs, the individual thunderstorms work
together to generate a long-lasting weather system that
moves slowly and quite often for periods exceeding 12 hrs.
– Typically about the size of Ohio.
– Often forms during summer
nights.
– Major source of rainfall for Plains
states in the summer.
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Lightning
– Lightning is simply a discharge of electricity (giant spark)
which usually occurs in mature thunderstorms.
• Cloud to cloud, cloud to ground, cloud to air, within cloud.
• Can heat the air to 54,000°F!! (5 times hotter than sun)
– The extreme heating causes the air to rapidly expand,
initiating a shock wave that becomes a sound wave, called
thunder.
•
•
•
•
Sound travels 330 m/sec vs. 300 million m/sec for light.
Sound travels faster in warm air than in cold air.
It takes sound about 5 seconds to travel 1 mile.
If you see lightning and hear thunder 15 seconds later, the
lightning stroke occurred 3 miles away.
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Lightning (Electric Charge)
– In normal fair weather, the electric field of the atmosphere is
characterized by a negatively charged surface and a positively
charged upper atmosphere.
– For lightning to occur, separate regions containing opposite
electrical charges must exists within the cloud.
– Separation of charge is brought about by precipitation
processes.
– There is a net transfer of positive ions (charge) from a warmer
object to a colder object.
• Ice crystals are colder than hailstones (graupel).
• Hailstones become negatively charged, ice crystals positively.
• Charge is then distributed by weight.
– Ice and a strong updraft are necessary for lightning to occur.
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Lightning (Exchange of Charge)
– When the negative charge near the bottom of the cloud is large
enough to overcome the air’s resistance, a flow of electrons (stepped
leader) rushes towards the earth.
– As the electrons approach the ground, a region of positive charge
moves up into the air through any conducting objects (trees, buildings,
even humans).
– When the downward flow of
electrons meets the upward
surge of positive charge, a
strong electric current (bright
return stroke) carries the
positive charge up into the
cloud.
– Takes 3 or 4 return strokes to
fully exchange the charge.
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Tornadoes
– A tornado is a rapidly rotating column of air that blows
around a small intense area of low pressure.
• Circulation must reach the ground as either a cloud or area of
swirling debris.
• Funnel cloud is a tornado that does not reach the ground.
• The majority of tornadoes rotate counterclockwise.
– Diameter 300 – 2000 ft, some > 1 mi.
– Usually move NE at 20 – 40 kts, up to 70 kts.
– Most tornadoes usually last only a few minutes and have
an average path length of 4 miles.
• But some can last many hours and travel hundreds of miles.
– Most are found in the Plains (tornado alley).
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Tornadoes (Occurrence)
–
–
–
–
Top number: Number of tornadoes in 25 years.
Bottom number: Number of tornadoes per 10,000 square miles.
Tornado alley is from north Texas through Nebraska.
Can occur in any state.
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Tornadoes (Watch vs Warning)
– Tornado watch. Tornadoes are likely to form within the next 4
to 6 hours somewhere in the region (issued by Storm Prediction
Center).
– Tornado warning. A tornado has been spotted or indicated on
Doppler radar (issued by the local weather service office).
– Take shelter in the basement in
an interior room. In a structure
without a basement, the safest
place is in an interior room on
the lowest floor.
– In a dorm or apartment building,
move to an interior hallway and
lie flat with your head covered.
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Tornadoes (Life Cycle)
1. Dust Whirl Stage
2. Organizing Stage
3. Mature Stage
4. Shrinking Stage
5. Decay Stage
2.
1.
3.
4.
5.
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Tornadic Thunderstorms
– Not everything is known about the formation of a tornado.
– It is known that tornadoes tend to form with intense
thunderstorms and that a conditionally unstable
atmosphere is essential for their development.
– Tornadoes form with both supercell thunderstorms and
non-supercell thunderstorms.
– The most intense tornadoes form with supercell
thunderstorms. Thunderstorms with a strong, single
rotating updraft that develop in a region of strong vertical
wind shear.
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Tornadic Thunderstorms (Mesocyclones)
– The rotating updraft of a supercell thunderstorm is called a
mesocyclone.
– Change of wind speed and direction with height is responsible
for the rotating updraft.
• Creates horizontally rotating vortex tubes.
• The strong updraft will tilt the tube and draw it into the storm.
• This creates a mesocyclone 5-10 km wide.
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Tornadic Thunderstorms (Bounded Weak Echo Region)
– The updraft is so strong in a supercell that precipitation cannot fall
through it. Southwesterly winds aloft usually help to blow the
precipitation northeastward.
– Large hailstones that have remained in the cloud for some time,
usually fall just north of the updraft.
– If a mesocyclone is strong and persistent, the precip can be wrapped
around the updraft.
– This swirling area of precip shows up
on the radar, where the area inside the
mesocyclone does not.
– This is called the bounded weak echo
region, an area that is bounded by
precip. On radar, it can appear as a
hook on the southern side of the storm.
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Tornadic Thunderstorms (Tornado Formation)
– The mesocyclone can be compared to a small low
pressure system, like those that we have studied.
– At this point, the updraft, counterclockwise swirling precip,
and the surrounding air interact to form the rear flank
downdraft (southwestern side of supercell).
– When the downdraft strikes
the ground, it may interact
with the region of inflow to
help initiate the tornado.
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Tornadic Thunderstorms (Tornado Formation)
– As air rushes into the low-level core of the mesocyclone,
the air expands, cools, and if moist, condenses into a
cloud (funnel cloud).
– As the air beneath the funnel cloud is drawn into its core,
the air cools rapidly and condenses, and the funnel
descends to the surface.
– As the funnel reaches the ground, it usually picks up dirt
and debris, making it appear dark.
– While the air along the outside of the funnel is spiraling
upward, in the most violent tornadoes, the air is
descending towards the extreme low pressure at the
ground (sometimes 100 mb lower than surrounding air).
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Tornadic Thunderstorms (Final Thoughts)
– Not all supercells produce tornadoes; in fact, only around
15% do.
– However, studies reveal that supercells are more likely to
produce tornadoes when they interact with a pre-existing
boundary.
• Outflow boundary from previous convection.
• Warm front
– When supercells interact with these boundaries, low-level
wind shear is enhanced. This can lead to a stronger and
deeper mesocyclone.
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Fujita Scale
– Classifies wind speed based on damage. Tornado winds
are estimated based on the damage caused by the storm.
– Studies indicate that the majority of tornadoes are F0 and
F1, and only a few % are above F3.
• On average, only 1 or 2 F5’s occur a year.
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Fujita Scale (F0 and F1)
F0
F1
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Fujita Scale (F2 and F3)
F2
F3
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Fujita Scale (F4 and F5)
F4
F5