GY205 Weather and GY205 Weather and ClimateClimate

Lecture 6Lecture 6

ThunderstormsThunderstorms• 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-stormsSevere 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 ComplexesMesoscale 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 LinesSquall Lines• Numerous individual cells arranged in a line,

~300 miles long• Form along and ahead of fast-moving cold fronts

and dry lines

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

T-storm DistributionT-storm Distribution

LightningLightning• 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!

ThunderThunder

• 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 SafetyLightning 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

MythbusterMythbuster

• 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

TornadoesTornadoes

• 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 FormationTornado 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 PatternsTornado Wind Patterns

Supercell TornadoSupercell Tornado

Tornado OccurrenceTornado Occurrence

Where When

Tornado PathsTornado Paths• Most travel from SW toward NE

Ranking TornadoesRanking Tornadoes• Tornadoes ranked based on the damage they

caused

Tornado SafetyTornado 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

MythbusterMythbuster

• 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.

WaterspoutsWaterspouts• 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 SystemsTropical 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 FormationHurricane 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 DevelopmentHurricane 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 HurricaneFrom 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 PathsTropical 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 HurricaneAnatomy 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 DissipationHurricane 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 DestructionDeath 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 DisastersGreatest Hurricane Disasters

What NOT to do when a hurricane What NOT to do when a hurricane is approaching: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 HurricaneWhere 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 WarningsHurricane 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 HurricanesRanking Hurricanes

• The Saffir-Simpson Scale is based on max sustained winds

GY205 Weather and GY205 Weather and ClimateClimate

End of Lecture 6End of Lecture 6