Transcript Severe Weather: Thunderstorms and Tornadoes

Severe Weather:
Thunderstorms and Tornadoes
EQ: What can we learn from severe
weather to help prevent other
disasters? (What can we do to keep
safe?)
Thunderstorms
Needed to form:
• Source of Moisture
– Humid Air, Less Dense
– Water vapor condenses into clouds
• Lifting Air Mass
– Releases heat into air as water condenses
• Unstable Atmosphere
– Clouds grow upward
Development
1. Cumulus Stage
– Air rises vertically, clouds visible
2. Mature Stage
– Precipitation falls, downward drafts
produce wind (Caused by convection
current)
3. Dissipation Stage
– Storm loses energy (heat), storm ends
Development
Thunder
• Thunder – superheated air rapidly expands
and contracts
Lightning
• Lightning occurs when
there is a charge
separation in the cloud.
• As a thunderstorm grows,
electrical charges build up
within the cloud, with
positive charges at the
top, and negative charges
at the bottom.
• Oppositely charged
particles gather at the
ground below the cloud.
• Sound waves travel slower than light waves,
so we see lightning then hear thunder, but
both are formed at same time.
Lightning Safety
• Outside: If you feel hair stand on end, Get
low to ground
• Make yourself the smallest target possible
• Sheds, isolated trees, and convertibles are
hazardous.
• Do not use electrical equipment and stay
away from water
Tornadoes
• Violent whirling column
of air in contact with the
ground.
– Called a funnel cloud
if it does not make
contact with ground
Development
• Wind speed and direction change suddenly
with height (Shearing)
• Cooler air given horizontal rotation
• If close to updraft from a T-storm, can tilt
vertically
• Rotation accelerates , low pressure center
• Large pressure gradient causes winds
Classification
• Fujita Tornado Intensity Scale
– Ranks tornadoes according to path of
destruction, wind speed and duration
– F0 (118 km/h) – F5 (500 km/h)
– 1% of tornadoes reach F4 or F5
How are Tornadoes Measured?
Distribution
• Occur most often in Spring in late
afternoon/evening
– This is when temp contrast is greatest
• Central US (cP and mT air masses collide)
• “Tornado Alley” – part of US that has greatest
amount of tornadoes
Watches and Warnings
• Watch: Issued when the weather conditions
are good for a tornado to form
• Warning: Issued when a tornado is sighted or
indicated on radar