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