So, I've thought about posting up some notes on meteorology for a while now but never got around to doing it. I had a discussion with luckyylotus on my personal journal about meteorology in a fictional world and this progressed and I offered to tell her about realistic weather systems for literary purposes and personal interest. So here goes nothing!
Air masses affecting the British Isles:
So basically there are five major air masses that influence British weather, but as the seasons change, so do the dominating influencing air masses. Looking at where the winds are coming from on a news website site, such as the BBC, can be used to try and predict what type of weather is likely due to the characteristics of these air masses and where they have come from. For example, the forecast for my postcode has the wind coming from the North North East which suggests that the temperature will be cold (about 8°C) and there is a chance of rain. I can gather this basic information about what the weather conditions might be like by looking at the map above and seeing that the air is coming from the Arctic which means it has passed over the Arctic Ocean and will be carrying some moisture that could be later deposited as rain.
Characteristics of the five main air masses that affect the weather of the British Isles:
Polar maritime (Pm): It is a very common air mass over the British Isles and it brings cool conditions throughout the year as the air is coming from the North Pole. It warms slightly as it passes over the Atlantic Ocean and causes it to become unstable in its lower layers. It gives heavy showers as it crosses the highlands, separated by bright intervals. Associated with cumuloform clouds. Good visibility. Often strong winds and gales occur after the passing of a cold front.
Arctic maritime (Am): Very cold conditions in Winter; cold in Spring (today and Saturday); rare in Summer. Slowly heats up as it crosses the sea, picking up some moisture and becoming unstable in its lower layers. Snow in Scotland; hail in Spring, often in heavy showers. Usually good visibility. Often lasts several days. Temperatures in London may be just above freezing point in Winter and precipitation is limited. Winds often strong in the North.
Polar continental (Pc): Gives very cold temperatures in Winter (London well below 0°C.) Begins stable, but warms slightly crossing the North Sea to become unstable in the lower layers and giving heavy snow in Eastern Britain (bright and clear on the West coast.) Often lasts several days if blocking anticyclone interrupts prevailing westerlies. Wind chill factor is high. If this air stream occurs in Summer, it brings warm conditions (because the air has come from the continent, it is warmer than the surrounding ocean) and is more stable.
Tropical continental (Tc): Occurs mainly in Summer, but has been know to influence weather in Autumn and Spring (the late September heatwave of 2011 is a good example of a Tropical continental air mass affecting the British Isles in a season other than Summer) when sub-tropical high pressure moves North. The air is coming from North Africa/Sahara Desert. Heatwave conditions have been known (1976, 1989) Very stable in lower layers (drought), though upper layers may become unstable with thunderstorms (convectional uplift.) Gentle winds, dusty haze. North-west Scotland can be cloudy and wet.
Tropical maritime (Tm): A common air mass over Britain. Air is coming from the Tropics/Equator. Occurs during the warm sector of a depression. Very mild and wet in Winter, with a thick cloud cover. Often stratus cloud giving hill and coastal fog. Warm in Summer, though not hot due to the moisture that has been picked up whilst travelling over the Atlantic Ocean. Lower air is stable but if forced to rise over hills the upper layers can become conditionally unstable to give thundery showers. Winds usually moderate to fresh.
Next time in Meteorology in minutes!: pressure systems, fronts, depressions, ocean currents, atmospheric circulation and tropical weather!