A presentation on high and low pressure weather systems. A great revision resource for the geography topic of climatic hazards.

1. MatthewMortimer
High & Low Pressure Weather Systems
Low Pressure
A depressionisalowpressure systeminmidandhighlatitude.Itiscausedbythe mixingof coldand
warm air massesmeet.The warmairmass islighterandtherefore risesabove the densercoldair
mass.This formsa centre of lowpressure.There are manyhazards associatedwithlow pressure
weathersystems.These includeheavysnowfall andblizzards.
In 2003 Britainwasbadlyaffectedbyheavysnow andArcticwinds.InScotland,the conditions
severedtransportlinkswithroadsblockedandferriescancelled.InWhitby,NorthYorkshire,the
windswhippedstickyfoamfromthe seaintothe
streets,whichcausedveryhazardousconditions.
Major roads inEnglandsuch as the M4 andM40
were litteredwithaccidents.Upto23cm of snow
fell nearHull.The M11 queuedfor20 hourswith
a 64km tailback,andthe police hadto senda
helicoptertodropfoodparcelstothe people.41
Londonundergroundstationswere close anda
total of 225 flightswere cancelledatHeathrow.
High Pressure
Highpressure systemsare oftenreferredtoasanticyclones.Ananticyclone isalarge massof
subsidingair,whichproduceshighpressure atthe surface.Airmovingtothe polesfromtropical
regionssinkstoformthe subtropical highpressure belt.Highpressure systemsare opposite tolow
pressure inthe conditionsthattheyproduce.Highpressure systemscreate hot,sunny,drycalmdays
inthe summertime andcold,sharp,crispdaysinthe winter.The reasonfornightsbeingcoldisthe
lack of cloudcoverallowsheatto escape.Frostandfog are common.Large stationaryanticyclones
have beenknowntoformnear the BritishIsles.These are mostcommoninthe spring.These
anticyclonesare responsibleforheatwaves.
In 2003 the UK and much of Europe experienceddrought.The large heatwavewasgreatforthe UK’s
tourismindustry.Transportlinkswere damagedbythe heat.Inthe Essex village of GreatLeighsthe
roads melted.The Londonundergroundreached37°C,hotterthanthe legal limitfortransporting
animals.Itwascalculated that the cost of people
takingtime off workto enjoythe heatwave was
between£7.5millionand£10 millionperday. The
heatwave alsoincreasedlow-level ozone,
particularlyoverSouthEngland.Asmanyas 900
people dieddue tothispoorairquality. InPortugal
a state of emergencywasdeclaredasthey
experiencedthe worstforestfiresfor30 years.Up
to 35,000 hectaresof forestand farmlandscrub was
burned. InLisbon,the temperature soaredto43°C.
Over1300 deathsoccurredin the firsthalf of August.