Transcript GY205 Weather and Climate - University of Mount Union

GY205 Weather and
Climate
Lecture 6
•
•
•
•
•
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
• 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
• 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
• Numerous individual cells arranged in a line,
~300 miles long
• Form along and ahead of fast-moving cold fronts
and dry lines
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
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
• 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
•
•
•
•
•
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
• 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:
• 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
•
•
•
•
•
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
• 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
Supercell Tornado
Tornado Occurrence
Where
When
Tornado Paths
• Most travel from SW toward NE
Ranking Tornadoes
• Tornadoes ranked based on the damage they
caused
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
• 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
•
•
•
•
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
• 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
• 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
• 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
• 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 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
• 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
• 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
•
•
•
•
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
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
• The front right-hand side of a hurricane has the
most intense winds and storm surges
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
• The Saffir-Simpson Scale is based on max
sustained winds
GY205 Weather and
Climate
End of Lecture 6