The main influences are Britain's latitiude and it's maritime position - it lies within the cool temperature western maritime climate belt. This means that Britain does not experience extremes of temperature and there is precipitation throughout the year. However, its so unpredictable due to the dominance of low pressure
The mean summer temeprature in the UK is lower than average for the latitude, monthly averages rarely exceed 18dc. This is due to the cooling influence of the Altantic Ocean.
In winter, the average temerpature is above freezing (2-7dc).
In some coastal locations (Cornwall/Pembrokeshire) have relatively high average winter values as a result from the warming influence of the sea and ensure a year-round growing season.
The annual range of temperatures within the CTWM is relatively small but increases with distanec from the west coast
It is experienced throughout the year but varies with relief. In upland areas, in particular those close to western coasts, rainfall totals can exceed 2500mm. A short distance further east, on low land in the shadow of the mountains, annul totals can be as little as 500mm.
Most of the rainfall is brought by frontal systems moving east, releasing moisture as they cross the land. In general, summer is the driest season (followed by winter). This is because high pressure weather systems (anticyclones) are more likely to become established in these seasons and block the approaching fronts, deflecting them to the north or south.
The prevailing wind direction is southwest and this is governed by the general atmospheric circulation system. However, although Britian recieves most air streams from the west, easterly winds do occur. They generally bring spells of dry weather
Winds from the west can be strong and gales are common, particularly during the autumn. These winds are influenced by low pressure weather systems, where rising air in the centre of the low pressure results in a steep pressure gradient on the ground surface, causing air to rush in to replace that which has risen
Gales are a common feature of the CTWM climate. They're generally at their worst during autumn, when the sea temperatures are still warm enoguh to fuel powerful low-pressure cells. Sometimes, the relics of tropical storms from the Caribbean find their way across the Atlantic - by the time they reach Europe, most of their energy has been lost lost because of lower tempertures. The magnitude of glaes is greatest in the exposed western coastal areas of the UK.
They are associated with low pressure weather systems and occur where there is a steep pressure gradient (represented on a weather chart by closely spaced isobars). Severe gales occur when the air pressure drops to a very low level. Storms of this nature are rare in the mid latitude but when they do occur the scale of damage is significant.
The magnitude of the event can never equal that of a major hurricane. This is becuase in the mid-latitude the air pressure at the centre of an intense depression does not fall to such as extreme low leverl as in the tropics. Therefore, the wind speed in a tropical revolving storm can never quite be matched.
Impact of Storm Events
Moderate gale - 50-60km/h, sets whole trees in motion
Fresh gale - 61-73km/h, breaks twigs from trees
Strong gale - 74-86km/h slight damage to buildings ie rooftops/chimneys
Whole gale - 87-100km/h, uproots trees and causes structural damage to buildings
Winds in excess of 100km/h are classed as storms and capable of causnig widespread damage to buildings and infrastructue - these are rare in the UK but they cause widespread disruption for a short period of time.
Wind speeds above 120km/h are unlikely to be experienced in the British Isles. Storms of such a magnitude are classified as hurricanes. Every year gales are expected to result in short term disruption to transport and power supplie but such damage is usually minor
The 1987 Storm of Southern Britain
A low pressure weather system veered northwards and hit southern England. By midnight on 15th October, gusts exceeding 150km/h were recorded over-exposed coastal areas.
The emergency services were stretched to their limits throughout the night, dealing with fallen trees, accidents and power cuts. If the storm had occured 8 hours later, during rush hour the impacts would have been much worse.
- 19 deaths
- damage to 13million homes
- power cuts
- cancelled train and ferry services
- wrecked caravan parks
- boats washed ashore
Insurance claims were estimated to be over £1.5billion. The clear up took months, particualrly where structural damage to buildings had occured. One of the omst significant long-term impacts of this storm was the destruction of some 15million trees, uprooted by the hurricane-strength winds.
Prediction and Response
The storm of 1987 gained an infamous status as the sotmr that proved the weather forecastors wrong. this once in a 100 year event was very difficult to predict because of the speed of the drop in pressure.
An evening weather forecast by the MET office correctly described the current state of the depression and 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 alseep and would have been unaware of additonal warnings had they been avaliable
- The clear up took considerable time, with emergency crews being drafted in from Northern regions, where damage had been more slight
- losses from the storm totalled £1.4billion in the UK
- A MET office enquiry recommended that observational coveraeg of the atmosphere over the ocean to the south and west of the uk was improved by increasing the qualit and quantity of observations along with refinements made to the computer modules used in forecasting
- Significant clean-up of fallen trees was criticised by egologists for removing damaged broad leaf trees that would be recovered with time