Weather & Climate- British Isles Climate Case Study

The Great Storm, Southern England, 1987

Causes

• October 1987 started off fine and dry but quickly became unsettled and wet. In the days before the storm low pressure lingered off the west of Ireland, producing spells of wet weather across Britain. 
• Warm air from Africa met cold air from the Atlantic Ocean, causing an intense depression.
• The depression developed over the Bay of Biscay on 15th October and moved northwards.
• The deepening of the depression to 958mb was believed to be the result of a very strong jet stream and warming over the Bay of Biscay which together probably released latent heat energy warming the air even more and reducing pressure even further. 
• Weather forecasters thought it wouldn’t reach England, but by midnight it had changed course and moved towards the south coast.
• It was in fact not a hurricane, but a depression.
• Winds were over 100 km/hour and on the coast and winds reached gale force 11.
• The central pressure was 953 mb.
• There were rapid changes in temperature as the warm front passed over eg. in Farnborough 8.5°C to 17.6°C in 20 minutes.

Impact

Social

• 18 people died in the UK
• Many houses suffered damage leading to homelessness
• Communications were broken with trees falling on railway tracks and roads, and damage to power lines leading to blackouts.
• 5 million homes were without electricity.

Economic

• Numerous small boats were wrecked or blown away, with one ship being blown over and a Channel ferry was blown ashore
• Caravan parks were wrecked
• Cost £2 billion in insurance claims, so premiums went up for everyone next year
• Fire brigade had 6000 calls in 24 hours
• Buildings collapsed
• Losses from storm totalled £1.4 billion in the UK. 1 in 6 households in SE England submitted insurance claims.

Environmental

• 15 million trees were uprooted, including 6 of the famous oaks trees in Sevenoaks
• Loss of habitats and biodiversity but fallen trees provided new habitats for some animals
• Some plants benefited as there was more light on the forest floor allowing them to grow

Prediction

• The storm of 1987 gained an almost mythical status as the storm that proved the weathers forecasters wrong. 
• In reality this ‘1 in a 100 years’ event was very difficult to predict due to the speed of the drop in pressure. 
• An evening weather forecast by the MET office correctly described the current state of the depression & its likely path. 
• The veering of the storm to a more northerly track was noticed too late to allow for effective warning. 
• Most people were asleep so wouldn't have heard the additional warnings.
• Warnings of severe weather had been issued, to various agencies and emergency authorities, including the London Fire Brigade. 
• Perhaps the most important warning was issued by the Met Office to the Ministry of Defence at 0135 UTC, 16 October. It warned that the anticipated consequences of the storm were such that civil authorities might need to call on assistance from the military.

Criticism of the MET Office

• Media reports accused the Met Office of failing to forecast the storm correctly. 
• Repeatedly, they returned to the statement by Michael Fish that there would be no hurricane - which there hadn't been. 
• It did not matter that the Met Office forecasters had, for several days before the storm, been warning of severe weather. 
• The Met Office had performed no worse than any other European forecasters when faced with this exceptional weather event.

• This storm wasn't officially a hurricane as it did not originate in the tropics - but it was certainly exceptional. In the Beaufort scale of wind force, Hurricane Force (Force 12) is defined as a wind of 64 knots or more, sustained over a period of at least 10 minutes. 
• By this definition, Hurricane Force winds occurred locally but were not widespread.

Responses

• Clear up took considerable time with emergency crews being drafted in from Northern regions where damage had been more slight.
• Losses from storm totalled £1.4 billion in the UK. 1 in 6 households in SE England submitted insurance claims.
• MET Office enquiry recommended that observational coverage of the atmosphere over the ocean to the S and W of the UK was improved by increasing the quality and quantity of observations along with refinements made to computer models used in forecasting.
• A significant clean up of fallen trees was criticised by ecologists for removing damaged broad leaf trees that would have recovered with time. 

Weather & Climate- British Isles Climate

Air Masses Affecting the British Isles

• Air masses are large parcels of air with homogenous climate characteristics of temperature and humidity. 
• They form in high pressure regions where air spends a lot of time standing still and where atmospheric conditions are calm. In time though this air begins to move away from these high pressure zones and as it does it interacts with the surface characteristics that it moves over. 
• In Britain there are six air masses that at different times influence the climate and weather. 
• The most dominant air masses influencing the British Isles is the Polar Maritime. The British Isles are quite unique in their location as they stand on the Polar Front. The Polar Front is the line of latitude that marks the path of the Polar Jet Stream. It is along the Polar Front that different air masses collide and the result is a complex and variable climate. 

Depressions

Characteristics:

• Low pressure weather systems- low atmospheric pressure, below 1,000mb
• Represented by closed isobars decreasing towards the centre
• move west to east across the British Isles
• isobars close together producing steep pressure gradient from out to in
• winds strong and blow towards the centre anticlockwise
• Wind 'veer' in south UK- change direction from s/sw to w/nw
• Wind 'back' in north UK- change direction from se/e to ne/e

Formation:

A depression affecting the BI originate in the North Atlantic where two different air masses meet along the polar front.

• Polar maritime- dense, moist, cold
• Tropical maritime- light, moist, warm

The warmer less dense air rises and is removed by strong upper atmosphere winds (jet stream). This rising twisting vortex of air produces a wave to form at sea level in the polar front which becomes more exaggerated as the wave form develops eventually becoming a depression.

1. A wave forms on the polar front. Cloud and rain occur. Pressure falls.
2. Winds blow around depression. Pressure falls. Cold front moves faster than warm front.
3. Cold front catches up with warm front. Pressure rises. Depression starts to die as no warm air to lift near the centre of the low.

Synoptic Charts

Weather & Climate- British Isles Climate

Climate of the British Isles

TEMPERATURE:

• Temperature is reduced towards the north due to the reduction in amount of isolation at higher latitudes. Places further from the sea experience higher summer temperatures, on average as the cooler temps from the sea have less influence inland. 
• In larger land masses this is referred to as ‘continentality’ but is evident in the UK.
• Areas at higher altitudes are cooler as temperatures drop by 6.4 degrees for every 1000m increase.
• The effect of prevailing winds and oceanic currents are evident in the higher winter temperatures in the land bordering the Irish Sea as warmer air associated with the North Atlantic Drift brings warmer Gulf Stream waters to Western Britain.

Average summer and winter temperatures

WIND:

• Most common direction of wind is the South West but this is variable from day to day & winds from other directions are quite frequent. With long spells of easterly or north easterly not unusual in winter. Strongest winds are found in West and North of the country as these areas face the prevailing SW winds. Wind speed increases with height so mountains experience higher wind speeds.  
• A gale is defined as: wind speed of over 63 km/hr for a duration of at least 10 minutes.

PRECIPITATION:

North & West of the British Isles shows greater rain. The key factors affecting this pattern are the direction of the prevailing winds and altitude.

• Relief rainfall occurs when moist air that has been travelling over the sea is forced to rise over upland areas. As it rises and cools, the air reaches dew point (saturated) and condensation occurs leading to rainfall.

In areas such as Keswick in the Lake District rainfall averages around 1500mm per year whereas Tynemouth at similar latitude on the east only receives 660mm of rain falls. This is due to the RAIN SHADOW as air that has lost a lot of moisture over the hills will sink back down, warm up, and as warmer air can hold more moisture it is less likely to generate rainfall.

Britain is affected by FRONTAL RAINFALL:

• This occurs when warmer air is pushed up over a wedge of cooler air where two air masses meet in a frontal system. This occurs where cooler polar air undercuts warmer tropical air. As the warmer air is forced to rise it cools, water vapour condenses and forms cloud and rain over a wide area. Especially in winter months when successive low pressure systems or depressions approach the western shores of Britain. 

CONVECTIONAL RAINFALL:

• Rainfall due to extreme localised heating of the ground. (Summer months). 
• Occurs when air above ground is warmed becomes less dense than the surrounding air & rises. When it reaches dew point condensation occurs & clouds develop but very strong heating produces highly unstable air which continues to rise creating cumulonimbus clouds. Creates intense sudden rainfall affecting South East Britain especially.