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Transient Atmospheric Flows and Disturbances Chapter 7

Midlatitude Cyclones and Hurricanes

The Impact of Storms on the Landscape

• Short term impacts • Floods, windblown damage, dark skies

• Long term Impacts • Accelerated erosion, flooded valleys, destroyed

buildings • Promote diversity in vegetative cover, increase size of

lakes and rivers, stimulate plant growth

How do storms form?• Atmosphere is made up of large air masses

• Large - 1000 miles across, several miles deep • Uniform properties of temperature, humidity and stability • Travel as one and distinct from surrounding air

• Can form anywhere • Causes distinct properties

North American Air Masses

Fronts• Boundary zone between unlike air masses • Narrow zone where air properties change

rapidly • Normally defined by a difference in temperature • Can be stationary but more likely constantly in

motion • Four types of fronts

– Warm – Cold – Stationary – Occluded

• Warm front • Brings warm air into an area • Clouds and precipitation • Gentle transition • No convective activity

• Cold Fronts • Brings cold air by displacing warm air mass • Rapid lifting of warm air • Unstable, violent weather • Thunderstorms, convection cells

Atmospheric Disturbances

• Unsettled and violent atmospheric conditions • Smaller than general circulation • Migratory and transient • Relatively brief duration • Relatively predictable weather conditions

• Midlatitude disturbances • Tropical Disturbances • Localized Severe Weather

Midlatitude cyclones• Most significant of all atmospheric disturbances • Large migratory low pressure systems • Move with the westerlies • Responsible most precipitation • Oval shaped, about 1000 miles in diameter

• Low pressure system • Counterclockwise rotation • Cool air from the north • Warm air from the south • Precipitation along both fronts

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Tropical Cyclones• Intense rain drenched migratory storms • 100-700 miles in diameter • Circular shape • Pulls warm moist air in to fuel

• Air rises causing condensation and increased instability

• Three levels • Tropical Depression (winds up to 38 mph) • Tropical Storm (winds from 39 to 73 mph) • Hurricanes (winds above 74 mph)

• Named when it becomes a tropical storm • Significant storm names are retired

•Eye of the storm •Non-stormy center between 10 and 25 miles in diameter

•No cloud formation •Eye Wall

•Barrier between eye and storm •Winds reach highest speed here

Formation• Only forms over warm water • North or South of the Equator – Never within 3o – Rarely within 8-10o

• Exact mechanics of formation unknown – Preexisting disturbance

• August through October is Hurricane season

• Last from one week to a month • Movement over continents where fuel is unavailable • Movement into midlatitudes where it is cooler

• Can become midlatitude cyclone

Movement

• Most Common Pacific Basin and North Atlantic • Very predictable once formed

• East to west with little latitudinal shift • East to west and curve poleward • Some exceptions

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Damage and Destruction

• Low pressure of hurricane causes ocean to bulge upwards

• Majority of damage from storm surge – Wind driven water – Up to 25 feet above normal tide – Floods low lying areas – Causes 90% of hurricane related deaths

• Damage depends on strength of hurricane and location hit

• Stafford Simpson Hurricane scale – Classifies hurricanes (1 to 5)

Storm Surge

Notable United States Hurricanes

• Hurricane Wilma (2005) – Category 5 – Lowest atmospheric pressure ever recorded

from an Atlantic hurricane at 882 millibars

• Galveston Hurricane (1900) – Category 4 • Greatest hurricane disaster • 8000 deaths (1/6th of the island’s population) • New York City 65mph winds

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• Hurricane Katrina (2005) Category 5 • Costliest hurricane in US History ($108 billion) • 1800 deaths • Displaced over 1 million people • Population drop of 219,000 • 7 million barrels of oil

• 2005 most active Atlantic season in history • 28 named storms • 3 of the most powerful

hurricanes ever • Why did it happen?

• Maybe global climate change • Maybe cycle of hurricane

intensity

• August 23rd formation and crosses Florida then gains strength in Gulf of Mexico • August 25th becomes category 5 hurricane (902 millibars) • August 29th landfall just south of New Orleans • Weakens to a category 3 and that evening is downgraded to a tropical storm

New Orleans in a bowl

Localized Severe Weather• Local significance • Can be destructive • May be associated with larger storm systems

Thunderstorms• A violent convective storm accompanied by

thunder and lightning. • Common where there is high temperature,

humidity and instability • Localized and short-lived • Produces towering clouds

Tornados• One of the most destructive of all atmospheric

disturbances • Deep low pressure cell surrounded by a whirling

cylinder of air (as much as 100 millibar difference) • Less than 1200 feet across • 300 mph wind • Damage from high wind and debris

• Transports girls from Kansas to Oz!

• Formation • How they form is unknown • Along cold front • Severe thunderstorms • 90% in the United States - Optimal conditions