THUNDERSTORMS AND TORNADOES
Vicki Drake
Department of Earth Sciences
Santa Monica College
I. Thunderstorms
A. Storms associated with thunder, lightning, hail, strong winds, gust fronts,
hail and heavy ppt.
B. May be isolated events, a cluster of storms, or line of Cb clouds extending
for 100 km.
II. Thunderstorm Creation
A. Warm humid air rising in an unstable environment
a. One of four uplift mechanisms at work
B. Diverging upper-level winds (Jet Stream) divergence and enhancement
III. Thunderstorm Types
A. Air Mass Thunderstorm
a. Summer storms with warm mT air – formed away from fronts
b. Short-lived
B. Severe Thunderstorms
a. Formed along fronts, collision zones of air masses
b. Strong vertical wind shear (speed and directional shear)
c. Associated with high winds, gust fronts, tornadoes, etc.
d. Long-lived, self-perpetuating
IV. LifeCycle of Thunderstorm (Air Mass or Severe)
A. Cumulus Event
a. Warm humid air rises, cools, condensation – release of Latent Heat
b. No ppt in this stage – all energy into cloud-making and growth
c. No thunder or lightning at this stage
d. All updrafts – no downdrafts
B. Mature Stage
a. Release of latent heat continuing along with cloud growth
b. Beginnings of downdrafts of cold air and droplets
c. Thunder and lightning at this stage – most intense part of storm
life
d. Heavy ppt
e. Anvil shape begins as cloud tops troposphere – hits more stable
stratosphere
f. Convective Instability and Electrical Polarity within cloud leads to
more activity – possible tornado activity
C. Dissipating Stage
a. Little release of latent heat – warm air “cut off”
b. Downdrafts overcome updrafts – more cold air and droplets falling
than warm rising air can replace
c. Heavy precipitation as cloud slowly “wrings” itself out
d. Strong vertical wind shear may lead to tornado development
V. Gust Fronts
A. Boundary at base of cloud separating cold downdrafts from warm updrafts
B. Wind shirts – strong/cold/gusty
C. Acts as wedge to push up warm air – prolonging life of storm
D. Associated with roll clouds in front of main storm cloud (Cb)
© Vicki Drake
SMC – F-2000
1
VI. Microburst
A. Localized downdrafts of cold air – (~4 km area) – hit ground and spread
out horizontally
B. Winds speeds of >75 meters/sec (146 knots/hour or ~168 mph)
C. May evolve into Gust front
D. Responsible for knocking down trees and associated with damage usually
attributed to tornadoes
a. 1982 – 727 over New Orleans knocked down
b. 1985 – L1011 over Dallas-Fort Worth airport
VII. SuperCells
A. Enormous Thunderstorms with balance updrafts/downdrafts – sustainable
for long periods of time
B. Capable of producing surface winds >90 mph, hail size of grapefruits
(Texas and Florida), and large long-lasting tornadoes
VIII. Squall Line Thunderstorms
A. A line of severe thunderstorms – form along leading edge of cold front –
or 100-300 km out in front
B. Formed as result of air aloft flowing over cold front and developing into
“waves” – upward wave encourages cloud formation in unstable
environments.
XIV. Tornadoes
A. Rapidly rotating winds, focusing rotation around small areas of intense low
pressure
B. First sign – Slowly rotating “Wall Cloud” dropping out of bottom of severe
thunderstorm
C. Second sign – Rapidly rotating Funnel Cloud dropping out of Wall Cloud
D. Third Sign – Funnel cloud touches ground – becomes “tornado”
E. Diameter – 330 feet to over 1 mile
F. Average rotation speed >200 mph (clocked at 230-250 mph)
G. Traveling speed on ground – 20-40 knots moving from SW to NE across
USA Midwest
a. Tornado Alley – Central Plains of USA
b. Most tornados than any where in world
c. Topographically and meteorlogically set up for tornadoes
d. Collision Zone between cP and mT air masses
e. Greatest seasonal and time occurrences Late afternoon – Spring
H. Fujita Scale
a. F0 – weak
b. F5 – most destructive
X. Thunderstorm Activities
A. Hail – Frozen “balls” of ice
a. Formed from a nucleus swept into clouds – dust, leaf, insect
b. Rides updrafts/downdrafts – freezing on way up, melting on way
down, refreezing, remelting, growing and adding layers until cloud
cannot hold any longer
c. Can be thrown out the top of cloud or dropped out bottom
B. Lightning
a. Superheated air created by polarity
b. Temps ~45,000-54,0000F
c. Cloud bottoms – “negatively-charged” – cloud tops – “positively
charged”
d. Most common lightning strikes are within cloud or cloud-to-cloud
© Vicki Drake
SMC – F-2000
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e. Cloud-to-ground results from induced charge (from negative cloud
base to positive Earth surface….trees, buildings, poles, airplanes,
humans)
f. Stepped Leader – first from cloud as “charged particles then from
ground up….then from cloud base to ground – human eye cannot
detect original motion – so see the strike as “wiggly”.
g. Return Stroke - large number of electrons racing to ground and
then back upward to cloud – a bright channel of light
C. Thunder
a. Superheated air expands explosively – creating sound waves
b. Travel ~ 700 mph (1 mile per five seconds or 1100 feet per
second)
c. Close lightning strikes – “Thunderclap
d. Farther away lightning strikes – “Thunder roll or rumble”
© Vicki Drake
SMC – F-2000
3