GEOG 1112: Weather and Climate

Violent Weather

Midlatitude Cyclone• Well-organized low pressure system that

migrates across a region as it spins

• Develops along polar jet as it swings north and south in process called Cyclogenesis

• Both upper-level and surface conditions drive cyclogenesis

• Southerly swing of polar jet creates an upper-level trough supporting cyclogenesis

Upper-level convergencepushes down on the surface, creating high pressure

Upper-level divergence allows air to rise from surface, creating low pressure

Upper-levels and Cyclogenesis

Midlatitude Cyclone

Classic Midlatitude Cyclone

L

Midlatitude Cyclone MigrationFollows path of polar jet

1) Air mass thunderstorms (ordinary thunderstorms): self-

extinguishing; localized short lived phenomena; limited vertical wind

shear.

Two Types of Thunderstorms

2) Supercell

Supercell (Severe Thunderstorms): self-propagating

Mesoscale Convective ComplexesMesoscale Convective Complexes

Squall Line Thunderstorms

Cloud-to-cloud lightning:

The most frequent type of lightning

Occurs within a particular cloud or between clouds.

Also called sheet lightning: the sky is typically uniformly lit while

the stroke is buried within the cloud.

Cloud-to-ground lightning:

Begins when negative charges build in a cloud base.

These negative charges are eventually discharge onto the positively

charged ground.

Thunder

The rapid expansion of air associated with a lightning stroke

causes thunder. The slower speed of sound, with reference to light,

causes a lag between the stroke and the resulting thunder

Rumbling thunder is typically caused by sound echoing off

topographic features and buildings

Tornadoes• Small, intense cyclone

• Spawned by severe supercell thunderstorms

• Most destructive atmospheric phenomenon

• Mesocyclone within the supercell

• Gravity Waves

• Rated on Enhanced Fujita Scale EF0 (40-72 mph) – E F6 (319-379 mph)

• Tornado rotation detected by Doppler radar

Formation of a Tornado

Updrafts

Tornadoes

Tropical Cyclone

• Called hurricane (Atlantic & NE Pacific), typhoon (NW Pacific) or cyclone (SW Pacific & Indian)

• First Easterly Wave – trough in tropical easterlies

• May become Tropical Depression – rotation starts

• Next Tropical Storm – sustained winds over 39 mph

• Finally, Hurricane – sustained winds over 73 mph

Tropical Cyclone Strength

•Storm Surge #1 cause of death and damage in tropical cyclones

•Debate : recent increase in severe tropical cyclones – Is it global warming or a natural cycle?

Atlantic Hurricanes

Official season June 1-Nov 30

Peak seasonMid-Aug – Mid-Oct

Hurricane tracks

Conditions Necessary for Hurricane Formation

Hurricanes form only over deep water layers with

surface temperatures in excess of 27 oC (81 oF)

Coriolis force is an important contributor, and as such,

hurricanes do not form equatorward of 5o latitude

Strong vertical shear must be absent

Tropical Cyclone Structure

Destruction by Hurricanes

Winds and surge are typically most intense in the right front

quadrant of the storm. Why? Combination of wind speeds and the

speed of the storm’s movement.