A document which features high and low pressure systems, the Hadley, Ferrel and Polar cells as well as anticyclone blocking. A very useful revision resource for the geography topic of climatic hazards.

1. MatthewMortimer
High Pressure
Whenthe airis cold,the moleculesare packedtightlytogether;the airbecomesdenserandbegins
to sink.The air nowpressesonthe Earth’s surface,creatinghighpressure.Highpressureoften
bringsgood,clearweather.Highpressure areasare normallycausedbya phenomenoncalled
subsidence,meaningthatas the air at a higheraltitude coolsitbecomesdenserandmovestowards
the ground.Pressure increaseshere because more airfillsthe space leftfromthe low.Subsidence
alsoevaporatesmostof the
atmosphere'swatervapour
so highpressure systems
usually display clearskiesand
calm weather.
Low Pressure
Whenthe air warms,the
moleculesflyfurtherapart;
the air becomeslighterand
rises,creatinglowpressure.
Low pressure drawsmoisture
fromthe groundcreating
clouds,rainand storms.Duringlowpressure (depression),airisrisingandblowsinan anticlockwise
directionaroundthe low(northernhemisphere).Asitrisesandcools,watervapourcondensesto
formcloudsand sometimes precipitation. Thisiswhylow pressure systemsare oftenassociatedwith
rainfall.
Global WeatherPatterns
In eachhemisphere thereare three cells namedthe Hadleycell,Ferrel cell andPolarcell inwhichair
circulatesthroughthe entire depthof the troposphere.The troposphere isthe name giventothe
vertical extentof the atmospherefromthe surface,rightupto between10and 15 km high.Itis the
part of the atmosphere where mostof the weathertakesplace.
Hadley cell
The largestcellsextendfromthe equatorto
between30and 40 degreesnorthandsouth,
and are namedHadleycells,afterEnglish
meteorologistGeorge Hadley.The trade winds
blowtowardsthe equatorand thenascendnear
the equatoras a brokenline of thunderstorms.
From the topsof these storms,the airflows
towardshigherlatitudes,where itsinksto
produce high-pressure regionsoverthe
subtropical oceansandthe world'shotdeserts,
such as the Sahara desertinNorthAfrica.

2. MatthewMortimer
Ferrel cell
The middle cellsare namedthe Ferrel cells.Atthese cellsairconvergesatlow altitudesandfalls
alongbetweenthe cool polarairand the warm subtropical air,generallyfoundbetween60and 70
degreesnorthandsouth.Thisoftenoccurs aroundthe latitude of the UK whichgivesusour
unsettledweather.The Ferrel cell movesinthe opposite directiontothe twoothercells.The spinof
the Earth inducesa motiontothe rightinthe northernhemisphere andleftinthe southern
hemisphere.Thisdeflectioniscausedbythe Corioliseffectandleadstothe prevailingwesterlyand
south-westerlywindsoftenexperiencedoverthe UK.
Polar cell
The smallestandweakestcellsare the Polarcells,whichextendfrombetween60and 70 degrees
north andsouth,to the poles at eachendof the globe.Airinthese cellssinksoverthe highest
latitudesandflowsouttowardsthe lowerlatitudesatthe surface.
AnticyclonicBlocking
Anticyclonicblockingoccurswhenalarge,slow-moving
anticyclone becomesestablishedoverthe continent,
disruptingthe normal westerlycirculation.Blocking
anti-cyclonesforce mildandhumidAtlanticairfurther
north or southof its usual position andmay lastfor up
to several daysorweeks.Withblocking,the airflow
becomesmore meridional, withnortherlyorsoutherly
airstreamsoftenbringingextremetemperatures.
Prolongednortherlyflowsbring polarandarctic
airmasses,andgive below-average temperaturesatall
seasons.Onthe otherhand a southerlyflow brings
tropical continental airmassesfromnorthAfricaand
above-average temperatures.Summerheatwavesare
oftencausedinthiswayand longperiodscancause droughts.