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

Anticylones

Characteristics:

• Anticyclones are characterized by high atmospheric pressure
• Represented by closed isobars with pressure increasing towards the centre
• move slowly and may remain stationary over an area for several days or weeks
• isobars are far apart therefore there is little pressure difference between the centre and edges of the anticyclone
• winds are weak and flow gently outwards in a clockwise direction in the northern hemisphere, anticlockwise in the southern hemisphere

Formation:

• The air that descends from the upper atmosphere is cooler, drier and denser than air at ground level. 
• As it descends it warms and can hold a lot of moisture leading to limited condensation and reduced rain. 
• This stable air can cover several thousand KM and presents a gentle pressure gradient with weak winds blowing clockwise out from the centre of the high pressure area. 
• The passage of anticyclones tends to be much slower than depressions. Weather can be days or weeks depending on the season.

Winter Anticyclones:

• Cold daytime temperatures and freezing night-time temperatures as less isolation is available to warm the British Isles and the clear skies of a high pressure zone mean that the moderate gain of warmth during the day can be more than offset by rapid loss of heat at night.
• Surface cooling frequently gives rise to radiation fog and frost which may persist because of the weak sunshine during the day. 
• High levels of atmospheric pollution in urban areas caused by subsiding and no wind.
• When dry cold polar continental air from central Asia and Europe moves across the North Sea it can bring heavy snowfall to eastern regions. 

Summer Anticyclones:

• Hot daytime temperatures and warm night-time temperatures as clear skies allow maximum isolation producing temperatures of over 25 during the day time.
• Generally clear skies, hazy sunshine in some areas.
• Formation of dew & mist which clear quickly in the morning.
• The east coast of Britain may have sea frets caused by onshore winds.
• Thunderstorms when the air has high humidity.

Where blocking anticyclonic cells exist for days over Europe the deflection of warmer depressions away from Britain can bring dry freezing conditions in winter and heatwaves in summer.

Fog

Radiation fog:

• Radiation fog usually occurs in the winter, aided by clear skies and calm conditions. 
• The cooling of land overnight by thermal radiation cools the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation and fog to occur. 
• Radiation fogs usually dissipate soon after sunrise as the ground warms. An exception to this can be in high elevation areas where the sun has little influence in heating the surface.
• Radiation fog is common under temperature inversion which often occur in valleys:

Valley fog:

• Valley fog forms where cold dense air settles into the lower parts of a valley condensing and forming fog. 
• It is often the result of a temperature inversion with warmer air passing above the valley. 
• Valley fog is confined by local topography and can last for several days in calm conditions during the winter.

Advection fog:

• Advection fog occurs when moist air passes over a cool surface and is cooled. 
• A common example of this is when a warm front passes over an area with snow cover. 
• It is also common at sea when moist tropical air moves over cooler waters. If the wind blows in the right direction then sea fog can become transported over coastal land areas; a sea fret or haar.

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.

Weather & Climate- Atmosphere

The general atmospheric circulation system

Planetary surface winds

Horizontal air movement is known as wind. But it can move vertically too. Winds are caused by differences in air pressure. They move from high to low. Air pressure decreases with increasing height. When air temperature increases it becomes warmer and less dense and will rise, leaving low pressure beneath. Isobars which are close together show high pressure.

In the North Hemisphere wind blows anticlockwise into a low pressure zone and clockwise outwards away from a centre of high pressure as a result of the effects of the Coriolis force & friction.
High pressure occurs where air is descending and is associated with dry weather. This is because air warms as it descends, leading to the evaporation of most water vapour.

Low pressure occurs where air is rising. It is generally linked to precipitation and windy conditions. As it ascends, it cools adiabatically and cannot hold as much water vapour. The water condenses into droplets, which become clouds at condensation level.

Coriolis Force: an effect that causes anybody that moves freely with respect to the rotating earth to veer to the right in the N hemisphere and the left in the S hemisphere.

ITCZ- inter tropical convergence zone: result of the heating of part of the Earth’s surface caused by concentrated insolation. Hot air rises. This draws in cooler air that flows across the surface to replace rising air. Air streams are drawn in from N and S where they meet.

Position of the ITCZ:
Changes according to the seasons. The sun is located directly above the TROPIC OF CANCER on 21st June which pulls the ITCZ North of the Equator whereas on the 21st December it is just over TROPIC OF CAPRICORN and the ITCZ moves into Southern Hemisphere.

FRONT : Boundary between a warm air mass and cold air mass results in frontal rainfall. 

Hadley, Ferrel & Polar Cells

• It starts in the Doldrums, an area of intense low pressure found at the equator where the intense heating (be convection) of the earth’s surface forces air to rise through the Troposphere. 
• This area is known as the Inter Tropical Convergence zone (ITCZ). 
• As this air rises it cools and condenses forming a belt of clouds. Some of this air migrates northwards in the upper Troposphere to equalise out the temperature and insolation differences of our globe. 
•  As this air migrates north it cools relative to the air around it, becomes denser and sinks to the Earth’s surface at around 30°N and S of the Equator, creating a band of high pressure. 
• Some of this air migrates (because of Pressure gradient force) back to the low pressure area at the equator to complete the first cell of the system, the Hadley cell.
• Some of the air continues towards the poles to continue equalising the temperature differences. 
• When this air reaches 60°N and S it reaches cold polar air that is migrating south. This is our second convergence zone where 2 surface air streams meet. This causes the warmer, less dense tropical air to rise through the atmosphere again creating an area of low surface pressure. 
• It is this zone where we find the mid-latitude weather systems that blight British weather. 
• Some of this air migrates back towards the Equator where it eventually sinks at 30°N and S to form the middle cell of the model, the Ferrell cell. The rest of the air migrates to the pole, where it cools and sinks creating high pressure in the Polar Regions and completing a weak polar cell. 
•  Near the Tropopause at 30°N and S and 60 °N and S we find the high speed jet stream winds.

This model has many applications and limitations. The model fails to accommodate other major transfers of energy, such as the El Nino and La Nina models of circulation from West to east or Vice Versa across The Pacific Ocean. It also fails to acknowledge the presence and impact of Geomorphological features such as the Himalayas which complexly disrupt the movement of jet streams and surface level winds within the Hadley cell on a yearly basis. 

 However, it does offer people a starting point for understanding atmospheric circulation, ad does allow for some level of prediction of the weather that affects billions of people around the globe.

Oceanic circulation 

• The second major way that heat is redistributed around our planet is by oceanic circulation or ocean currents. 
• The globes ocean currents are interlinked into a global system, which is commonly known as the Thermohaline conveyor. 
• Warm less salty water travels at the surface of our oceans driven by surface winds that blow over the top of those oceans. This water cools as it travels north and south from the Equator and increases in salinity as the salt is left behind during evaporation of the warm water. 
•  This water, now cooler and salt laden, sinks and returns to the equator as another method of balancing out the Earth’s heat budget. This mechanism is hugely important for the people of Western Europe, as a warm ocean current called the Gulf Stream brings warm ocean waters which warm Western Europe well beyond what it should be given its latitude. 

Weather & Climate- Atmosphere

The atmospheric heat budget

The Earth and the atmosphere are heated by energy from the sun. The atmospheric heat budget of the Earth depends on the balance between insolation and out going radiation. This budget has remained constant over the last few thousand years. However there is evidence of global warming in recent decades.

The amount of energy received from the sun is determined by:

• The solar constant- varies slightly and affects longer term climate rather than short term weather variations.
• The distance from the sun- the eccentric orbit of the Earth can cause a variation of up to 6% in the solar constant.
• The altitude of the sun in the sky- the equator receives more energy as solar radiation strikes the Earth head on, whereas at 60 N or 60 S the angle creates twice the area to cover and increases the amount of atmosphere to go through.
• The length of day and night

The Earth receives energy from the sun as insolation. Some is lost as it passes through the atmosphere but overall the surface has a net gain of energy, the exception being the polar regions. Only about 24% of this insolation reaches the surface as it is either absorb, reflected or scattered.

The atmosphere in contrast has a net deficit of energy. Because of this difference, heat is transferred from the surface to the atmosphere by radiation, conduction and by the release of latent heat

Heat budget by latitude

There are variations in energy and heat between latitudes. 

• Low latitudes have a net surplus of energy, mainly because of their relative proximity to the sun. 
• The high latitudes (pole wards of 40 N and 40 S) have a net deficit. As the tropics are not heating up and the poles are not cooling down, a transfer of heat must occur.

This occurs by:

• Horizontal heat transfers: air movement (winds, 80%, including the jet streams, hurricanes and depressions) and water movement (ocean currents).
• Vertical heat transfers: energy is transferred from the warm surface vertically by radiation, conduction and convection. Latent heat also helps to transfer energy, e,g, when water is evaporated. This energy is released when condensation occurs in the upper atmosphere.

Factors affecting insolation and the heating of the atmosphere:

Longer term effects:

1) Altitude of the land- insolation heats the surface of the land which warms the air above by conduction & convection. As higher land is further away from heat source, the main mass of land heated by insolation is cooler. Density of air decreases with height adding to cooler effect.

2) Altitude of the sun- At higher latitudes the heat energy from sun has to pass through more atmospheres so more heat energy is lost to absorption or scattering.

3) Proportion of land & sea- land and sea react differently to insolation. Land heats up quicker than sea. Water has a greater specific heat capacity which means it requires twice as much energy to raise 1kg of water by 1 degree than it does for land. But oceans retain heat for longer.

4) Prevailing winds- As the temperature of an air mass is determined by the area which it originated from and the surfaces it passes over, wind from the sea is cooler in summer and warmer in winter, than wind that has travelled over land.

5) Ocean Currents- Surface ocean currents are caused by influence of prevailing winds blowing across the sea. Warmer ocean currents migrate pole wards, away from the equator & colder currents replace them by moving towards equator in a circulatory motion known as a gyre. Clockwise in N hemisphere and anticlockwise in S hemisphere. The rotation of the Earth causes water to move westwards. North eastwards movement of warmer waters is a key factor in raising air temp that deliver mild winters and cool summers to Britain. Labrador Current off the N E of N America can reduce summer temps. 

6) Ocean Conveyor belt- The transfer of cold water at depth of Polar Regions to the equator is like a conveyor belt. As water cools at the poles the formation of ice, leaves the remaining water- saltier and denser. Denser water sinks. It sweeps the Antarctic continent. These motions are reciprocated by movement of the less salty and less dense surface temp which moves north towards N Atlantic from India & pacific oceans. These warmer waters have a significant effect on temp of N Atlantic. North Atlantic is warmer than North Pacific. 

Short Term Factors:

1) Seasonal Changes- Insolation is distributed equally in each hemisphere at the spring and Autumn equinoxes, when the sun is at the equator. In the summer around 21st June and winter around 22nd December 22nd solstices when the sun is directly overhead at the tropics, maximum insolation is experienced in the N hemisphere and the S hemisphere respectively.

2) Diurnal Range- the length of day & night varies in all locations away from the equator. At the poles there is no insolation during winter months when the regions are tilted away from the sun and there are up to 24 hours of daylight in summer when they are tilted towards the sun.

3) Local

• Aspect- slopes alter the angel at which the sun strikes Earth. South facing slopes in the N hemisphere receive more of the available insolation than N facing. This has impact on agriculture.
• Cloud cover- clouds may reflect, absorb and scatter incoming radiation, but can also act as an insulating blanket, keeping the heat in the lower atmosphere. Therefore when there are clear days temperatures rise more rapidly as more insolation reaches the surface but when there are clear nights they fall quickly as terrestrial radiation reduces the surface temp. Conversely when it is cloudy temperatures do not rise as high or fall as low. 
• Urbanisation- Urban surfaces tend to absorb more heat than natural surfaces during the day and radiate more at night creating urban heat island effects. 

Weather & Climate- Atmosphere

Structure of the atmosphere

The atmosphere is made up of several gasses;

• 78% Nitrogen
• 21% Oxygen
• ~0.01% Water Vapour, Carbon Dioxide, Ozone, Methane, Argon, Helium etc

The Earth's atmosphere contains several different layers that can be defined according to air temperature or chemical composition. The atmosphere can be split into distinguishable layers, called the Troposphere, the Stratosphere, the Mesosphere and finally closest to space the Thermosphere. 

Troposphere	It contains all of the elements vital to life, plus most of the water vapour in the atmosphere, most of the clouds, dust and pollution. Most weather happens here. The temperature decreases rapidly with increasing distance from the Earth’s surface. This is largely because the Earth is heated from the surface outwards, as it receives incoming solar radiation as UV rays which is converted to heat at the surface. Pressure also decreases with height in this zone, and the amount of Oxygen available decreases. The top of this layer is marked by the TROPOPAUSE, where temperatures remain constant, (isothermal).
Stratosphere	Temperatures increase with height as ozone absorbs the UV. The layer is free of dust and cloud and ozone not only absorbs but filters out UV.  The upper layer is known as the STRATOPAUSE another isothermal layer. The Stratosphere does have winds, but these are light in the lower layers and increase in strength with height.
Mesosphere	Temperature decreases to -90. No water vapour or dust to absorb radiation. Very strong winds at 3000 km/hr. Topped by another isothermal layer, the MESOPAUSE.
Thermosphere	Temperatures increase to 1,500 C due to the absorption of UV radiation by atomic oxygen.