The to: Am, Pm, Pc, Tm, Tc

The main factor determining the weather in a cool temperate western maritime climate is wind direction, and where the air is coming from
Examine the validity of this statement
Wind direction can come form varying different directions and that is the key to understanding a cool temperate western maritime climate, or any for that matter and can greatly alter the weather conditions experienced in a certain place in quite a short space of time. There are other factors which affect the weather in a cool temperate wester maritime climate which will be discuses in this essay.
If air remains stationary in a certain area for a set amount of time it begins to inherit some of the characteristics of the air and temperature that it is over. A example of this would be the high pressure belts found over the subtropics for example the Sahara or in the higher latitudes of the earth such as Canada and Siberia. These air masses slowly develop and are then known as source regions. There are five of these major air masses that effect the British isles and any CTWM climate. These are abbreviated to: Am, Pm, Pc, Tm, Tc (a description of each can be found below)
Artic Maritime (Am) – This comes from the Artic Ocean and is a northerly air type. It is the air mass that brings very cold conditions in winter, as it crosses the sea it can heat up therefore causing some low level precipitation.
Polar Maritime (Pm) – this comes from north westerly direction. It gives cool conditions but it warms slightly as it crosses the Atlantic Ocean therefore causing instability. It can give heavy showers. It is a very common air mass over Britain.
Topical Maritime Tm – This comes from a south westerly direction and again is a very common air mass over Britain. It is often very mild and wet. It is usually quite stable but can be pushed up by highlands inducing thunder storms.
Tropical Continental (Tc) – This comes from a southerly direction and only occurs when Sub – tropical high pressure moves north. It can cause heatwave conditions (as experienced in 1976). It is very stable however the upper layers can become unstable and cause thunderstorms.
Polar Continental (Pc) – This comes from a easterly direction and can cause very cold temperatures in the winter. When it approaches the North Sea it is very stable, but as it passes it can become incredibly unstable causing large amounts of precipitation. This is the air mass that was accoiated with the 2018 ‘Beast from the East’ phenomenon which was accoiated with several days of heavy snowfall raining havoc on the UK.
It is to be considered that these air masses and there descriptions are the ones directly effecting the UK and not all CTWM climates. Each of these air masses, however, have there own effects on the climate of the British Isles. When the air masses move over certain areas they are altered by the land forms. A example of this would be tropical air that moves northwards, this would then be cooled and it would become more stable, polar air on the other hand moves south and becomes increasingly unstable as it is warmed. These two air masses can mix however and this can form depressions which are responsible for the weather conditions of the UK. In short there are low and high pressure depressions causing rain and clear weather conditions respectively. This is what causes the highly variable weather that is experienced in the British Isles. A example of these pressure systems causing issues would be the ‘great storm of 1987. There was a enormous depression which hovered of the UK for a number of days, winds hit 100km/h on the southern coast and said winds were accompanied by consistent heavy rainfall.
These theory’s are quite specific to CTWM climates however. For example in India you have a monsoon climate which is caused by the Northen/Southern movement of the ITCZ throughout the year. This is not caused by air masses however, unlike the UK, but instead by the convergence (hence the name of the ITCZ – inter tropical convergence zone) of the trade winds of the equator causing the air to be heated by the land forms and then sinking forming thunder storms occurring every afternoon in these regions.
There are other factors however which can attribute to the climate and weather of a CTWM climate however. Take Rossby waves for example, which were discovered when WW1 zeppelin were significantly blown of course. These waves (also known as Jet Streams) form a pattern across the globe and can effect the global weather patterns as they are travelling at such high speeds (excess of 230km/h). These winds therefore can cause the global circulation of ash In a matter of days. The Polar front jet stream however is the one that effects the UK most significantly. It varies between the latitudes of 40 and 60 degrees in both of the Hemispheres of the earth. It forms the division between the Polar and Ferrell cells. It is very responsible in giving either fine or wet weather on the earths surface. In the Northen hemisphere, where the jet stream moves south it helps cold air to descend giving dry and stable conditions associated with those of anticyclones. When the jet stream reverses, it brings with it warm air which rises causing strong winds and heavy rain (depressions). The path over Britain is usually north east, therefore explaining the frequently wet conditions. This path can occasionally be blocked by a blocking anticyclone which will cause the extremes of climate such as hot and dry summers (1976 and 1989).
Lastly and briefly, ocean currents can play a large role in a areas climate. For example a El Niño event (if strong enough) can effect the climate of Britain. For example the strong event of 1997 – 1998 was linked to a very mild and wet winter in the UK and NW Europe. This event is strong enough to have a effect on the majority of the world for example greatly weakening the ITCZ, and causing it to arrive late. This caused drought and forest fires in many Asian states. Closer to home the NAO event effect the British isles more significantly.
I therefore conclude that although air masses can greatly effect the climate and weather experiencing temperate western maritime climate, they are not solely responsible for the weather and climate in these areas and in fact, wether weak or strong there are other factors at play which will effect affect these areas climate and weather patterns.
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