Lecture 25.Thunderstorms.
Stages of an ordinary thunderstorm. Severe
thunderstorms. Electrification of clouds and
lightning. Lightning safety. Reading: Ch. 10,
257–262, 271–275.
Thunderstorm
• a storm that produces lightning and thunder
• Air-mass (ordinary) thunderstorm:
 develops in warm, moist air masses
 usually not associated with weather front
• Severe thunderstorm:
 associated with weather fronts
 produces severe weather
Life cycle of an ordinary
thunderstorm
Mature stage
Dissipating stage
Cumulus (growth)
stage
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Comments
• Cumulus stage:
 cumulus cloud
 no precipitation
 no lightning or thunder
• Mature stage:
 the entrainment of dry air into the updraft results
in evaporation of some of the raindrops and cooling
of the air, which sets up a downdraft
 updraft and downdraft constitute a cell, the region
between updraft and downdraft is most turbulent
 heavy rain, gusts, anvil shape, lightning and thunder
• Dissipating stage:
 updrafts cut off, precipitation dies out, cloud evaporates
Severe thunderstorm
• A severe thunderstorm is different from an ordinary
thunderstorm in that it has a tilted updraft in a mature stage
(due to a strong wind shear)
• the downdraft cannot cut off the updraft and storm continues
to intensify
Formation of hail in a severe
thunderstorm
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Lightning and thunder
• Lightning — a massive discharge of
electricity, a giant spark
• Lightning can heat the air through which
it travels to as high as 30,000oC
• Air is being heated very quickly and
expands explosively; the resulting
booming shock wave is called thunder
How far is the thunderstorm?
• Speed of light is 300,000,000 m/s
• Speed of sound in the air ~ 300 m/s
• So if you see a lightning and 5 seconds
passes before you hear a thunder,
the thunderstorm is 5 " 300=1500m≈1 mi
away
• Close thunder bangs, and distant thunder
rumbles, as it comes from different areas
!
of the stroke
Different forms of lightning
• within the cloud (majority of strikes)
[from one part of cloud to another]
• cloud-to-cloud
[from one cloud to another]
• cloud-to-ground (20% of strikes)
[from a cloud to the ground]
• ground-to-cloud
[from the ground to a cloud]
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Electrification of clouds
Positive charges
on the tip of the rod
Negative charges at the base
of the cloud
Lightning safety
DO
• go inside a building,
if possible
• crouch down and
minimize your contact
with the ground
! people in a car are usually
unharmed, as the lightning
goes to the ground through
the outside metal casing of
the vehicle
DON’T
• stand under an
isolated tree
• go near a body of
water
• go near elevated
places
• lie down
Lightning safety (cont’d)
• Signs that lightning is about to strike
nearby:
 Your hair begins to stand on end
 Your skin begins to tingle
 You hear clicking sounds
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