Earth Sciences 240A - Lecture 30- Severe Weather:
Thunderstorms – Supercells - Tornadoes
Thunderstorms:
Introduction
Mountain barriers
Differential heating
Cold fronts
Thunderstorms: Cell
composition
aligned
with front
each cell
(2-8 km) and short-lived
Stages
cumulus
(warm air rises, cools adiabatically to dew point)
mature
(rapid updraft, rain reaches surface, anvil head)
final
(downdraft cuts off new moisture)
new
cells born as old die
Electric charges
redistributed;
induction
“Derechoes”
Supercells
single
cell
rotating
mesocyclones
may
lead to tornadoes
Tornadoes
“the
smallest, most violent weather disturbance that occurs on Earth”
rope-like funnel descended from a
thunderstorm/supercell; cyclonic motion
speeds in funnel can exceed 500
km/h
funnel diameter: 100-400 m usual
speed: 50 km/h common
duration: normally few minutes /
funnel
Tornado alley
expect
increase with increasing climatic T
Monitoring
“tornado
chasers”
National Severe Storms Laboratory
Lab experiments
Classification
Fujita scale
The Great Ice Storm:
1998
Polar jet stream abnormally far
south
transporting
warm, moist air
Strong Atlantic high pressure
stalled
deflection
of polar jet stream northward
Southern warm air climbed above dry
cold air
heavy
release of moisture = ice
End of storm: cold, dry system
again
-30oC;
no power; 24 people died; >$2B
Next: Hurricanes -
Typhoons - Cyclones