1. Warm Up 4/2/08
How does surface air flow in a middle-latitude cyclone in the Northern Hemisphere?
a. convergent and counterclockwise
b. divergent and clockwise
c. convergent and clockwise
d. divergent and counterclockwise
Which of the following best describes the pressure in a middle-latitude cyclone?
a. Pressure decreases toward the center.
b. Pressure increases toward the center.
c. Pressure remains the same everywhere.
d. The pressure is not predictable.
On a weather map, which type of front is shown by a line with triangular points on one side?
a. cold c. occluded
b. warm d. stationary
Answers: 1) a. 2) a. 3) a.

2. Severe Storms
Chapter 20, Section 3

3. Thunderstorms
Thunderstorm – a storm that generates lightning and thunder; frequently produce gusty winds, heavy rain, and hail
Thunderstorms may be produced by a single cumulonimbus cloud, or it may be associated with clusters of cumulonimbus clouds along a cold front
At any given time, there are an estimated 2000 thunderstorms in progress on Earth (~45,000 a day and 16 million a year); the U.S. experiences ~100,000 a year
Thunderstorms form when warm, humid air rises in an unstable environment
The life span of a single cumulonimbus cell within a thunderstorm is only about an hour or two, but as the storm moves, it is constantly getting fresh supplies of warm, humid air

5. U.S. Distribution of Thunderstorms

6. Stages in the Development of a Thunderstorm

7. Concept Development
Descirbe the stages in the development of a thunderstorm.
During the cumulus stage, strong updrafts supply moist air that causes the cloud to grow vertically. Usually within about an hour of the initial updraft, heavy precipitation is released from the cloud. Gusty winds, lightning, and sometimes hail also are experienced during this stage. Eventually, downdrafts dominate throughout the cloud and the storm dies down.

8. Tornadoes
Tornadoes – violent windstorms that take the form of a rotating column of air called a vortex; the vortex extends downward from a cumulonimbus cloud
In the U.S., ~770 tornadoes are reported each year; greatest occurrence between April and June
Most tornadoes form in association with severe thunderstorms
An important process in the formation of many tornadoes is the development of a mesocyclone (see diagram)
Pressures within some tornadoes can be as much as 10% lower than the surrounding area, causing air near the ground to be “sucked” into the vortex
One scale used to measure tornado intensity is the Fujita tornado intensity scale

10. Tornado Suction Vortices

11. Formation of a Mesocyclone

12. U.S. Tornado Incidences

14. Concept Check How does a tornado form?
Most tornadoes form in association with severe thunderstorms.

15. Hurricanes
Hurricanes – whirling tropical cyclones that produce winds of at least 119 kilometers per hour (also known as typhoons, cyclones, and tropical cyclones)
At sea, they can generate 15-meter high waves capable of destruction hundreds of kilometers away
A hurricane is a heat engine that is fueled by the energy given off when huge quantities of water vapor condense
Hurricanes develop most often in the late summer when water temperatures are warm enough to provide the necessary heat and moisture to the air

16. Eye Wall – doughnut-shaped wall that surrounds the center of the storm, where the greatest wind speeds and heaviest rainfall occur
Eye – center of the storm where precipitation ceases and winds subside
The intensity of a hurricane is described using the Saffir-Simpson scale
Storm Surge – a dome of water about 65 to 80 km wide that sweeps across the coast where a hurricane’s eye moves onto land
A hurricane weakens when it moves over cooler ocean water and land

17. Hurricane Cross-Section

19. Hurricane Katrina

20. Concept Check How does a hurricane form?
A hurricane develops when water temperatures are warm enough to provide the necessary heat and moisture to the air.

21. Assignment Read Chapter 20, Section 3 (pg. 571-577)
Do Ch. 20 Assessment #1-37 (pg )
Study for Chapter 20 Quiz!