Anticyclones and Synoptic Charts By the end of this lesson you will: Know what an anticyclone is. Understand how and why it forms. Be aware of the characteristics of an anticyclone and the atmospheric conditions experienced during these events.

Summary Questions: What is a depression? A. An area of low air pressure. B. An area of high air pressure. C. An area of medium air pressure.

Summary Questions: What are the stages in their formation? A. Warm front, cold front, warm front. B. First, Second, Third. C. Juvenile, Mature, Occluded.

Summary Questions: What are the characteristic weather conditions associated with a passing warm front? A. Increasing temperatures, increasing cloud cover, drizzle turning to rain, falling air pressure. B. Falling temperatures, decreasing cloud cover, no rain, high air pressure. C. Settled dry weather.

Summary Questions: What are the characteristic weather conditions associated with the warm sector? A. Falling temperatures, decreasing cloud cover, no rain, rising air pressure. B. Rising temperatures, increasing cloud cover, rain, falling air pressure. C. Warm temperatures, sparse cloud cover, no rain, constant air pressure.

Summary Questions: What are the characteristic weather conditions associated with a passing cold front? A. Rising temperatures, increasing cloud cover, no rain, low air pressure. B. Falling temperatures, rapidly increasing cloud cover, heavy rain storms, rising air pressure. C. Settled dry weather.

What is Air Pressure? Air pressure is the force of the air pressing down on the Earth's surface. More air=more force so pressure is higher at sea level than on top of Everest. Warm, rising air leads to low surface air pressure. This is associated with depressions. Cold, sinking air leads to high surface air pressure. This is associated with anticyclones. Air pressure is constantly trying to equalise so air will move from high to low pressure centres.

So What are Anticyclones? A large stable mass of subsiding (sinking) air which produces an area of high pressure. Anticyclones are much bigger than depressions at up to 3000km in diameter. Stable, slow-moving systems which bring settled weather as descending dry air inhibits cloud formation. They can settle over the UK and strengthen, deflecting Atlantic depressions away to the west and leading to extreme heatwave/ drought (in summer) or freezing conditions in winter. These are called Blocking Anticyclones.

In summer they bring hot dry (‘heatwave’) weather and in winter they bring cold clear weather with frosty nights. Heat escapes rapidly from the Earths surface due to the lack of cloud cover. Anticyclones are made up of a single air mass and so have no fronts. The pressure gradient is gentle and so winds are light and blow outwards from the centre. Winds rotate in a clockwise direction around anticyclones (in the Northern Hemisphere). Met Office Pressure Charts

Anticyclones Compared to Depressions TASK: Complete the table – use page 173 to help Anticyclones Depressions High pressure Clockwise Subsiding air Extreme temperatures Calm Slow moving Simple structure Stable

Synoptic Weather Charts Task: Using the Key and atlas, write down a description of the weather in: 1.Inverness 2.Nottingham 3.Belfast 4.Wick 5.Shrewsbury Then complete the tasks (1-3) on page 174 of the textbook relating to the 2007 summer floods. Synoptic Weather Charts

Blocked and Progressive Conditions Read through page 175 of your textbook and write your own explanation of blocked and progressive phase weather in the UK in table form. This is important as it helps explain our recent run of wet summers and can create extreme weather.

Depressions and Extreme Weather 20% of UK weather is cyclonic (caused by depressions). Only occasionally does this lead to extreme weather: Floods due to heavy rain. UK 2007 July Gales from high winds. Spain/France 24/01/09 Blizzards and Ice Storms. France 8/01/09 Storm surges on coastlines. 1953 Surge UK

Flooding 33% of world’s surface suffers from floods. The most common cause of floods is excessive rainfall related to atmospheric pressure e.g. monsoons and cyclones. Intense rainfall sometimes leads to flash floods which can have a devastating effect El Nino can bring devastating floods as in Mozambique in 2006 Rapid snowmelt can add water to already swollen rivers

Gales Gales are very strong winds (over 30mph) which can cause damage and disruption. UK Great Storm 1987 had winds up to 137mph and killed 18 people. Gales are caused by: Large pressure gradients (shown by tightly packed isobars) in depression systems.

Blizzards Snowstorms with wind speeds of over 56kph. Temperatures often drop to -12ºC with very poor visibility. Blizzards are caused by: Depression systems meeting very cold polar air such as often occurs in the USA/Canada. Strong gales blow the snow around making conditions very difficult.

Ice Storms When a warm front brings rain to an area with very cold dense air at the surface. The rain freezes instantly on impact coating everything with a layer of ice. Canada and NE USA suffers especially from this type of extreme weather. It is costly as it brings down power lines. 1998 was an especially bad year in Canada/USA.

Storm Surge The combined effect of strong winds and low pressure causes waves to ‘pile up’ against coasts. The effect may be intensified by high tides, shallow water or estuaries. This causes coastal flooding often combined with heavy rain (so swollen rivers) lead to lots of flooding. One of the worst natural disasters ever in the UK, the 1953 Surge killed over 300 people.

Summary: Summary Questions: What is an anticyclone? What characteristic weather do they bring? Why is this? Based on what you now know about weather conditions associated with anticyclones and depressions, when do you think the most recent anticyclone was over Nottingham? Independent Learning Links: Read throughthe Key Concept box on page 175 of your textbook – This is quite complex. Print/Highlight Geo Factfile Number 106. Met Office www.metoffice.com/education/higher BBC Weather www.bbc.co.uk/weather