Thursday, 14 January 2010

Introduction to extreme weather

This post aims to answer the enquiry question: What are extreme weather conditions and how and why do they lead to extreme weather events?

What are extreme weather events?
They are severe and usually unanticipated weather conditions which cause chaos and disruption to our lives.

Why do they occur?
They occur for several reasons from global warming to natural changes in Earth e.g. El Nino.

Lets look at some extreme weather events...


A storm with widespread snowfall accompanied by strong winds. This is formed when there is a ridge of high pressure. It is when a region of high pressure interacts with an area of low pressure. It forces the warm high pressure air to rise up and cool and condense. As a result, snow falls with strong winds.


A long period of abnormally low rainfall, especially one that adversely affects growing or living conditions. This is caused by high pressure systems causing a blocking high. This means that the same hot and dry conditions in an area last for day upon end.


If temperatures get too high this can initiate forest fires and other fires throughout the country. e.g. in the European Heatwave 2003, Portugal experiences many forest fires. 


A downpour of precipitation which causes water stores to over flow. 


heat wave is prolonged period of excessively hot weather, which may be accompanied by high humidity.


It is any product of condensation that occurs in the atmosphere that falls on the Earth's surface; it can include sleet, ice, snow, rain, etc.

Summer Anticyclone

An area of high pressure in the summer which may lead to a heatwave. 

Temperate storms 

Any storm that happens in a temperate area i.e. in the middle latitudes rather than tropical or sub-tropical.It is also another name for a depression.(Depression: An area of low atmospheric pressure.)


A violently rotating column of air, falling from cumulonimbus cloud, and nearly always observable. They form like depressions in areas of low pressure. 

Tropical cyclones (hurricanes)
They are low pressure weather systems which have very strong sustained winds over 120 km/h are bring torrential rainfall. To find out more about how hurricanes form please look at my post on hurricanes

Winter anticyclone 

An area of high pressure in the winter that may lead to snow or ice and below freezing temperatures.

Air masses

They are large bodies of stable air that have acquired the characteristics of the areas over which they have been resting.

There are 4 types: 

Polar Continental 

Large bodies of air rest of cold areas of land. For example in the UK this air mass comes from Scandinavia, Siberia and Eastern Europe. As it travels down it warms and becomes more unstable. In the UK this brings snow, ice, cold snaps of air.

Polar Maritime 

Large bodies of air rest in cold areas of water e.g the ocean or lakes.  For example in the UK it comes from the Arctic water. As it travels down it warms up and becomes unstable. In the UK it brings precipitation, frost and fog.

Tropical Continental 

Large bodies of air rest in hot areas of land e.g. Southern Europe and America. As it travels it cools and becomes more stable. In the UK it brings hot, dry and often dusty conditions. We sometimes experiences red snow with this air mass.

Tropical Maritime 

Large bodies of air in hot areas of water e.g. Atlantic Ocean. As it travels it cools and becomes more stable. In the UK it brings warm, humid and sunny weather. Along with showers, fog and cloud cover.

Pressure Systems 

Air pressure is force pushing down on the Earth. If it is high then air is sinking and tends to move is a clockwise direction and an area with high pressure is known as an anticyclone. If air pressure is low it tends to rising and this forms clouds and an area of low pressure is know as a depression.  Air pressure can be measured with a barometer; see my post on weather instruments for further detail.

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