REMINDERS:

Problem set 4 is on web site and due this Friday

Thunderstorms

- form in unstable atmosphere

- usually warm moist air near surface forced to rise

• Ordinary (Air-Mass) Thunderstorms - last relatively short time ~ 1 hour - downdrafts overwhelm updrafts and storm weakens

• Severe Thunderstorms - need vertical wind shear to form - updrafts and downdrafts are separate ==> storm can last much longer and intensify SEVERE THUNDERSTORMS

Z 0 WIND 3 Storm motion

4 2

"Shelf Cloud" forms on side with updrafts

6

1 5 (0) Presence of vertical wind shear (1) Updrafts on one side of cloud (2) Due to wind shear, updrafts slant into upper cloud (3) Droplets/crystals grow => fall on other side of cloud => drag induces downdraft (4) Entrainment of dry air => evaporation (cooling) => enhances downdraft (5) Downdraft hits surface => spreads => creates gust front (6) Shelf cloud often forms on side with updrafts

SUPER-CELL THUNDERSTORMS AND TORNADOES

noted for extremely strong updrafts

Favorable Conditions: - Vertical wind shear (change in speed with height) => helps produce severe thunderstorm - Change in wind direction with height => helps rotate updrafts

Conditions often found in US Midwest during Spring just ahead of cold front TORNADOES TORNADOES

Rotating updraft can produce Mesocyclone:

- 5-10 km system within Severe Thunderstorm - may produce Wall Cloud - Tornadoes often form within Wall Cloud

Wall Cloud Wall Cloud

Wall Cloud Video TORNADO FREQUENCY

F2 strength or greater

F4 strength or greater TORNADO FREQUENCY F2 strength or greater

Thunderstorm Frequency

F4 strength or greater Tornado Seasonality

© Jim Leonard Lightning and Thunder Clicker Question Set Frequency to "AD"

Why doesn't Lightning form in shallow Marine Stratus Clouds?

(A) cloud air is too warm

(B) cloud air is too cold

(C) temperature range of cloud air is too narrow Clicker Question Set Frequency to "AD"

Why doesn't Lightning form in shallow Marine Stratus Clouds?

(A) cloud air is too warm

(B) cloud air is too cold

(C) temperature range of cloud air is too narrow

Hurricane Katrina on August 28, 2005

HURRICANE CROSS-SECTIONAL VIEW ANTI-CYCLONIC FLOW OUT OF HURRICANE AT TOP OF STORM CLOUDS

CYCLONIC FLOW INTO EYE FASTEST WINDS IN HURRICANE AT SURFACE THE EYEWALL