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Chapter 9

Chapter 9 - Atmosphere and Severe Weather

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Wilfrid Laurier University
Rob Milne

Risks and Disasters – Chapter 9  Hurricane Katrina o Most anticipated disaster in American history o 3 reasons  Hurricanes common along coastlines  95% of new Orleans is below sea level  Louisiana lost wetlands and barrier islands that lower magnitude o Category 5 in the gulf of Mexico  Category 3 hurricane when it hit Mississippi Delta  New Orleans experienced Category 1 or weak category 2 o Storm was tracked since its inception over the Bahamas o  Energy o Atmospheric processes involve huge amounts of energy o Expressed in terawatts tW (1 tW = 1 trillion W) o 120 000 tW of energy absorbed by earth surface each year o Types of energy  Three main types of energy  Potential energy – stored energy  Kinetic energy – energy of motion  Heat energy – energy of random motions of atoms and molecules o Two types  Sensible heat – heat that can be measured  Latent heat - heat that is absorbed or released when substance changes state o Heat transfer  Three major heat transfer methods  Conduction o Transfer of heat through a substance through molecular interactions o Moves from area of high heat to low heat  Convection o Transfer of heat by movement of fluid or air o Heat rises up, pushes cold down, creating a circular loop (convection cell)  Radiation o Wave-like energy emitted by substance that produces heat  Earth’s energy Balance o Equilibrium between incoming and outgoing energy o Energy from sun is absorbed in atmosphere, ocean, land and living things o 100% of the energy leaving the sun and hitting earth is lost back to space after  Atmosphere o Thin envelope of gases that surround earth o Responsible for weather and keeps earth habitably warm o Composition of the atmosphere  Mainly made up of nitrogen and oxygen, small amounts of argon, water vapour and carbon dioxide  Water vapours important for cloud formation and atmospheric circulation  Humidity – amount of water vapour in atmosphere at specific temperature  Relative humidity – ratio of water vapour present to maximum amount that could be there  Almost all water vapour is evaporated from the earth’s surface  Giant cycle o Structure of the atmosphere 1 Risks and Disasters – Chapter 9  Entire atmosphere lies below altitude of 100 km  Four major layers  Troposphere o Ground to altitude of 8-16 km o Rapid decrease in temperature as you go upwards o Clouds are formed here  Stratosphere  Mesosphere  Thermosphere  Types of clouds  Cumulus – pieces of floating cotton, puffy fair-weather  Cumulonimbus – clouds which release tremendous amounts of energy during thunderstorms  Weather processes o Atmospheric pressure  Also known as barometer pressure  Weight of column of air at a point on/above earth’s surface  Greater at sea level than top of mountain where there’s less air  Also differs over distances of tens to hundreds of km across earth’s surface  Strongly affect weather  Low pressure – air rises and cools, clouds condense and precipitation forms  High pressure – drier air slowly descends, clear skies  Air flows horizontally from high pressure to low pressure (convergence)  Divergence – when air flows out of a region, reduction in pressure o Driving force for wind  Combined effects of temp and air movement produce low-pressure centres (L) and high- pressure centres (H)  Less dense air rises in areas of low pressure and diverges in upper troposphere  Can also go from high to low pressure o Vertical stability of the atmosphere  Can understand vertical movement by examining parcels of air  Atmospheric stability refers to ability of parcel of air to remain still  Stable – parcel resists vertical movement or return to original position after they moved  Unstable – lighter warm air overlain by denser cold air  Air turbulence and severe weather associated with unstable atmospheric conditions o Coriolis effect  Air moving from high to low pressure area moves in straight path  Coriolis effect – due to earth rotating, causes wind to take up a curved path  Deflect to right in N. hemisphere, deflect to left in S. hemisphere o Fronts  Boundary between cold and warm air masses  Stationary front – boundary that does not move much  Storm develops when fast-moving cold front comes into contact with slow-moving warm front  Occluded front – when a cold front comes into contact with another cold mass  Hazardous weather o Cyclones  Large cells of moisture laden air that rotate around low pressure area  Clockwise in N. Hemisphere , counter clockwise in S. Hemisphere 2 Risks and Disasters – Chapter 9  Tropical cyclones – form over warm tropical or subtropical ocean waters, not associated with fronts and have warm centres  Create high winds, heavy rain, seawater surges and tornados  Extratropical cyclones – develop over land or water in temperate regions, associated with fronts and have cool centres  Same effects, snowstorms and blizzards (during winter)  Characterized by intensity – sustained wind speeds and lowest atmospheric pressure  Associated with most severe weather conditions in NA  Storms differ in source of energy and structure  Tropical – energy from warm ocean water and latent heat released as rising air condenses to form clouds  Extratropical – fed by cold air at surface and flow of cool, dry air  Classification  Variety of terms to describe cyclones in different parts o Hurricane – Caribbean word (Atlantic and eastern Pacific ocean o Typhoons – Chinese word (western pacific ocean and north of equator ) o Cyclones – based on Greek word (pacific ocean south of equator)  Classified by wind speed on damage-potential scale o 5 categories Category Wind speed Storm surge 1 119-153 km/h 1.2 to 1.5 m 2 154-177 km/h 1.8 to 2.4 m 3 178-209 km/h 2.7 to 3.7 m 4 210-249 km/h 4.0 to 5.5 m 5 249 km/h + 5.5 m + o Category 3-5 considered to be major hurricanes  Naming  Small percentage of cyclones given name to identify where they form or to track movement  All hurricanes given individual names by government forecasting centres o Assigned once maintained wind speeds exceed 63 km/h o 1978 – male and female names given o 1979 – practice extended to hurricanes in Atlantic ocean and gulf of Mexico  List is previously agreed upon  Six standardized lists, alternatiing male and female – English/Spanish and French o Used in rotation  Names associated with intense/destructive hurricanes retired and replaced  Cyclone formation  To be a hurricane, winds must exceed 119 km/h  Form only in oceans warmer than 26c  Start out as tropical disturbances o Large areas of unsettled weather that are 200-600 km in diameter, thunderstorms, more than 24 hours o Elongated area of low pressure, rotates weakly  Form in variety of ways o Lines of convergence similar to squall lines o Upper-level troughs of low pressure o Remnants of cold fronts o Easterly waves of converging and diverging winds that develop 3 Risks and Disasters – Chapter 9  Tropical disturbances develop into tropical depression if winds increase and rotate around area o Moist air drawn into depression and spins faster  Once depression upgraded to tropical storm, name is given  Few tropical storms develop into hurricanes  Mature hurricanes about 500km in diameter, consists of counter clockwise spiralling clouds o Internal speeds are the strongest, outer area averages >50 km/h  Rain bands – clouds around hurricane  Most intense winds and rainfall in the innermost bands of clouds (eyewall) o Constantly changes as storm progresses, some develop double eyewalls  Eyewalls surround the eye – circular area with calm conditions and broken clouds 5- 60 km  Rising warm and moist air spirals upward around eye wall  Movement of hurricane controlled by Coriolis effect o Average speed of 19-27 km/h  Means it is a multiple day event  End of life will increase to 74-93 km/h  Extratropical cyclones  Formation o Strong temperature gradient in air near surface  Generally strongest along cold, warm or stationary front o Strong winds in upper troposphere  Jet stream  N. hemisphere has 2 jet streams o Polar jet stream o Subtropical jet stream  Most start as low-pressure centre along frontal boundary o Cold front developing on SW side of cyclone and warn front on E side o Circulates the air in a counter clockwise motion o Cyclone matures and merges with warm front to become occluded front  Storm intensifies and then completely dissipates within a few days  Predicting birth, development, movement and death very challenging o Can develop within 12-24 hours o Movement steered by winds in middle of troposphere  Movement half speed of steering winds  Regions at risk  Serious threat to Atlantic coast of Canada and Atlantic and gulf coasts of US in summer and early fall o Hurricane season starts June 1 to Nov 30  most occur in august, September and October o S. hemisphere – season starts Jan-April  US Atlantic and Gulf coasts experience 5 hurricanes each year o Form off coast of Africa o Follows 3 tracks  W across Caribbean, passing over islands, then moves NE into Atlantic without making landfall  W over Cuba and into gulf of Mexico where US Gulf coast is threatened  W across and E Caribbean and NE along Atlantic coast 4 Risks and Disasters – Chapter 9  Threatens coast from Florida to NY o Few continue as far N as New England and Atlantic Canada  Some form in gulf of Mexico, Caribbean sea and east pacific ocean o Threaten same areas (except pacific)  More hurricanes form in pacific and Indian ocean than in N. Atlantic  Geographic region at risk from extratropical cyclones in NA higher than tropical cyclones o Create strong winds in winter months, produce heavy snowstorms and blizzards o Severe thunderstorms and tornadoes east of Rockies  Cyclone effects  Damages, injuries deaths caused by storm surges, winds and flooding generated by cyclone  Storm surges cause greatest damage o Rapid local rise in sea level o Surges more than 3m and 12m + have been recorded o Two causes for storm surge  Stress exerted by wind  Low atmospheric pressure, raises water level o Affected by shape of coastline o Not advancing wall of water, but steady increase in sea level o Erodes beaches, islands and roads o Overwash – sand eroded from surges  Wind damages more obvious because of the larger affected area o Wind speeds diminish quickly once they make landfall  Drop by half within seven hours  Some do not lose all their strength and transition into extratropical cyclones  May maintain or increases speed  may occur if dying hurricane merges with upper-level extratropical cyclone, cold front or moves from warm to cold water o strong winds and heavy rain cause most severe weather on Pacific coast of NA  responsible for high winds in blizzards and tornadoes  inland flooding o releases about 4 trillion litres of water each day o heavy rain and extensive flooding a result o four factors affect extent  storm’s speed  terrain over which it moves  interaction with other weather systems  amount of water in soil o Thunderstorms  Occur all over the world all the time  Thunderstorms require special set of conditions  Water vapour must be present in troposphere to feed clouds as storm forms  Temperature gradient must exis
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