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Unit 2, topic 1: EXTREME WEATHER
Extreme weather: Severe, unexpected or at record levels. A
dramatic difference to the average.
Classifications: - Immediate( eg. Tornado)
- Consequential(eg. Flood)
- long-term(eg. Drought)…read more

Slide 2

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· Low pressure systems
· Anti-clockwise direction
· Form when strong jet
stream in troposphere
brings tropical maritime air
into contact with polar air
· Vortex of unstable rising air
along the polar front leads
to rapid temp. changes,
strong wings, pulses of
rain.…read more

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Depressions in the UK
Event, date and situation Features Impacts
Great storm, October 1987. Very low air pressure with 12 deaths; 14 million trees
Air from Hurricane Floyd winds of hurricane force lost; ferries ran aground;
over Bay of Biscay meets £2 billion insurance bill
cold front.
Towyn surge, Feb. 1990. Strong on-sure winds and Towyn sea wall and railway
Deep depression, gale- massive waves embankment breached;
force winds and high spring extensive flooding; 2,800
tide. properties damaged
Boscastle floods, Aug. Torrential rain (88mm in Property damage to most
2004. Heavy rainfall over one hour) caused flash premises in Boscastle and
already saturated steep- floods emergency evacuation of
sided valleys. 120 people required; no
Severn floods, July 2007. June rainfall (134 mm) was Significant property
deflected (southerly) twice average; July rainfall damage; 250,000 people
Jetstream led to a (234 mm) was four times without clean water;
continuing series of average insurance bill over £3.25
depressions, billion…read more

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Hurricane development
· Originate in tropics
· Reach wind speeds of over 120km per hour
· Most form in ITCZ( low-pressure area north of equator between
July-October) sea temp. here is 26+ degrees and 75% humidity
· Unstable, moist air rises and results in thunderstorms and thus
high-energy hurricanes
· Falling air pressure= upward spiral of wind speed
· Water vapour rises, cools rapidly and creates wall of cloud around
· Condensation wind speeds, pushing air up to 10km in altitude
· At the top cooling air spreads a thick canopy of cirrus clouds up to
1,500 km across
· Strong winds, torrential rain and massive storm waves are
produced…read more

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Case studies for hurricanes:
· Katrina 2005
· Mitch 1998…read more

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