Anticyclones

Anticyclones

1. What Are They

   • High Pressure Systems (above 1020mb)
   • Stable weather conditions
   • Very Large
     • Typically 3000km+ wide
   • Winds blow clockwise in N. Hemisphere
   • Also know as Highs
   • Winds blow anti-clockwise in S. Hemisphere
   • Remain for several days
   • Less frequent then depressions
   • Larger then Depressions

2. Formation

   • Air Molecues descend from high in the troposhpere, usually where Hadley cell meets the Ferrel Cell (30 Degrees N or S )
   • As the air descends the pressure increaes
   • The warming air picks up any moisture from the atmsomspere.
   • The lack of condensation and weak presure gradient results in few clouds
   • The provides the underlying area with....
     • Dry
     • Sunny
     • Bright
     • Clear
     • Still
       • Weather
   • Can lead to temperature inversions
     • Air thats heated during day rises as sits at top of antmospehere
       • This warm air is above the air that is cooled during the night due to heat loss from the ground
         • Can cause fogs and frost depending upon the season and can casue atmospheric pollutants to build up in the atmosphere

3. Case Study : Western Europe, August 2003

   • 20-40% less rainfall then normal on average
     • Rivers and lakes fell to lowest levels for a long time
       • Revieled WW2 Bobme and tanks for the first time
       • . E.g River Danube in siveria fell to lowest levels in 100 years
     • Resevoirs and river caused public water supply and HEP to run low
   • Large anticyclone over western Europe. Air is moving around the high in clockwise direction, bringing hot, dry, tropical contonental ar mass to the area
   • The cyclone lasted around a month, bringing abnormally high pressure, little cloud and warm conditions
   • 10% of alpice glacier masses melted
     • Swelling rivers and lake to close to bursting point
   • 35,000 deaths in 5 nations
   • 1/3 of deaths in UK caused by poor air quality
   • Hottest day on record (38.1 degrees) in Kent
     • Forrest fires hit portugal destroying 330,000 + hectare
   • UK Beach resorts were packed
     • Motorways at standstill as people fled for cooler places
   • Tran speed restruction to prevent buckled rails in the heat
   • 10% of wheat crops failed
   • Heat caused grapes to ripen quicker

4. Types of Anticyclones

   • Summer Anticyclones
     • There bring hot and sunny weather due to the clear skies
     • Days are long and sun is high in the sky
     • Heats up the land sigificantly so when rain occurs there is a lot of humidity
   • Winter Anticyclones
     • Bring clear, cloudless crisp conditions
     • Sun is low in the sky and days are short
     • Heat from the ground is lost due to absence of clouds, which allows frost to form
     • The UK is surrounded by sea so air is moist. The rapidly cooling ground causes air to condense, forming droplets that freeze in cold temps - frost
     • Fogs formed when clear winter nights. The ground looses heat, cools the air above it causing moisture to form around dust particles
   • Blocking Anticyclones
     • The ridges of high pressure can block the passage of the jet stream
       • Leads to long periods of stabe weather