Lecture 6

GY205 Weather and Climate: 

GY205 Weather and Climate Lecture 6

Thunderstorms: 

Thunderstorms Air mass thunderstorms – most common Usually during afternoon, hottest time of day Not associated with fronts Life span <1 hour for individual cell Three stage life cycle

Severe T-storms: 

Severe T-storms Have at least one of the following: Winds >58 mph Hailstones >0.75” Produce tornadoes Larger scale than air mass t-storms ~10% of all t-storms Types: Mesoscale convective complex (MCC) Squall line Supercell

Mesoscale Convective Complexes: 

Mesoscale Convective Complexes Numerous individual t-storms cells organized into a roughly circular cluster Slow-moving, lasting up to 12+ hours Most common in Great Plains

Squall Lines: 

Squall Lines Numerous individual cells arranged in a line, ~300 miles long Form along and ahead of fast-moving cold fronts and dry lines

Supercells: 

Supercells Single, large t-storm cell Up to ~30 mile diameter Last 2-4 hours Most violent tornadoes are a product of supercells

T-storm Distribution: 

T-storm Distribution

Lightning: 

Lightning Kills ~100, injures ~500 Americans/year ~80% cloud-cloud, ~20% cloud-ground 10’s-100’s of millions of volts, 5x hotter than surface of the sun!

Thunder: 

Thunder Caused by explosive expansion of air heated by lightning Count seconds from “flash to bang,” divide by 5 to get your distance from lightning in miles Heat lightning is too far away from viewer to hear its thunder

Lightning Safety: 

Lightning Safety If you can hear thunder, you are in danger Do not be the tallest thing around Do not shelter under a tree Get out of the pool or bath Go into sturdy structure or a car, do not touch metal parts Do not use a corded phone Stay away from windows

Mythbuster: 

Mythbuster Rubber tires on cars DO NOT protect you from lightning. The metal body conducts the current around you and into the ground.

This is a bad sign:: 

This is a bad sign: If you see this, you are about to be struck Run for cover! Crouch down on your tippy toes to make yourself a smaller target

Tornadoes: 

Tornadoes Average ~90 fatalities/year in US Formed by severe t-storms 100 yds.-1/4 mile diameter Last from few minutes to several hours Extremely low pressure in core can create most violent winds on earth

Tornado Formation: 

Tornado Formation Poorly understood, but we know that a mesocyclone often precedes a tornado Only about half of all mesocyclones will actually produce a twister

Tornado Wind Patterns : 

Tornado Wind Patterns

Supercell Tornado: 

Supercell Tornado

Tornado Occurrence: 

Tornado Occurrence Where When

Tornado Paths: 

Tornado Paths Most travel from SW toward NE

Ranking Tornadoes: 

Ranking Tornadoes Tornadoes ranked based on the damage they caused

Tornado Safety: 

Tornado Safety Take shelter in basement or inner room on lowest floor, bathtubs are good Cover yourself to protect from flying debris Avoid trailer parks (good general rule) Do not try to outrun in a vehicle, stop and take cover in a ditch

Mythbuster: 

Mythbuster Do NOT open windows to try to “equalize” air pressure during a tornado. It won’t matter in a direct hit. It lets in the wind to cause more damage if tornado passes nearby.

Waterspouts: 

Waterspouts Caused by cold air over warm water Water heats air, causing instability Rising air produces low pressure, drawing air in Dropping pressure causes condensation “funnel” Waterspout over Lake Erie

Tropical Systems: 

Tropical Systems Hurricanes – most powerful storms on earth Called typhoons in western north Pacific, known as cyclones in Australia and Indian Ocean Path of Andrew, August 1992

Hurricane Formation: 

Hurricane Formation Condensation forms clouds, convection from heat causes low pressure, drawing in moist air to feed the growing storm Requires very warm ocean surface temps, >81°F, which allows lots of evaporation Requires Coriolis effect strong enough to cause rotation Both of these requirements limit hurricane formation to between 5°-20° latitude

Hurricane Development: 

Hurricane Development Hurricanes begin life as tropical disturbances, groups of disorganized thunder storms w/o rotation Tropical disturbances often form off the coast of NW Africa, most do not develop into hurricanes Produced by easterly waves that develop in the Trade Winds Convergence at the surface forces air up, creating weak low pressure and thunder storms

From Disturbance to Hurricane: 

From Disturbance to Hurricane If the low pressure in a disturbance strengthens, more air is drawn in and rotation begins The system is now called a tropical depression, with sustained winds <37 mph If sustained winds intensify above 37 mph, it is called a tropical storm, and given a name Sustained winds >73 mph officially make the storm a hurricane

Tropical System Paths: 

Tropical System Paths Tropical systems migrate westward, driven by the Trade Winds The Bermuda High tends to steer them If they cross over into the Westerlies, they are blown eastward

Anatomy of a Hurricane: 

Anatomy of a Hurricane Average 350 miles across Spiral rain bands – lines of t-storms, spiraling counterclockwise (in N. hemisphere) Eye wall – most intense wind and rain Eye – calm, clear (shrinks as storm intensifies)

Hurricane Dissipation: 

Hurricane Dissipation Hurricanes need a constant source of warm water to stay alive If they drift over cooler waters or land they will die out

Death and Destruction: 

Death and Destruction Most hurricane deaths are causes by drowning Storm surge - a rise in coastal sea level Storm surge is caused by winds and low pressure Inland flooding, caused by heavy rains as storm moves overland Other hazards: inland flooding, flying debris, tornadoes

Greatest Hurricane Disasters: 

Greatest Hurricane Disasters

What NOT to do when a hurricane is approaching:: 

What NOT to do when a hurricane is approaching: Don’t have a hurricane party near the beach They did, they died: Before Camille After Camille’s 25-foot storm surge

Where NOT to be in a Hurricane: 

Where NOT to be in a Hurricane The front right-hand side of a hurricane has the most intense winds and storm surges

Hurricane Watches and Warnings: 

Hurricane Watches and Warnings Few should ever die in a hurricane Modern technology lets us know they are coming several days in advance Hurricane watch – landfall in >24 hours Hurricane warning – landfall within next 24 hours Erratic movement makes pinpointing landfall difficult

Ranking Hurricanes: 

Ranking Hurricanes The Saffir-Simpson Scale is based on max sustained winds

GY205 Weather and Climate: 

GY205 Weather and Climate End of Lecture 6