How is weather influenced in Britain?
Britain is an island. It is influenced by weather from the sea, continental Europe, and its own rugged landscape. When weather systems move into Britain from these different places, they influence our weather in different ways. Forecasters split these into six different “air masses”, and differentiate between them based on wind direction:
– Northerly (Arctic Maritime) is generally cold and showery weather, with snow in winter
– Easterly (Polar Continental) is warm and dry in summer, and very cold with snow showers in winter
– Southerly (Tropical Continental) is hot air from North Africa
– South Westerly (Tropical Maritime) is most common, and is cloudy, wet and mild
– Westerly (Returning Polar Maritime) is cloud and showers from Greenland, via the Atlantic
– North Westerly (Polar Maritime) is cold and showery, with sleet in winter
The jet stream also flows near to, or over Britain. These air masses “compete” with one another continuously, and create a very unstable atmosphere, which is difficult to predict accurately.
When does snowfall occur in St Nicholas?
It is worth noting that snow showers occur more in St Nicholas than surrounding areas due to the St Nicholas Ridge Slope Effect, which is caused by orographic lift. As the air ascends up the slope, it cools and this results in a temperature drop.
When weather comes from the north, cold conditions without snow occur. This is because St Nicholas is in the shadow of the Brecon Beacons. Most snowfall that occurs comes from the north-east to easterly direction, as this polar continental air is influenced less by the sea.
Weather patterns from the other directions are too warm to result in snowfall, unless the ground temperature is cold for a prolonged period of time and 850hPa temperature is below 0C, in which case frontal snow can occur for a short period when precipitation arrives.
This image is intended to provide a visual representation of when snowfall occurs. It is clear that systems from the north pass over the Brecon Beacons.
Why are thunderstorms and snow showers hard to predict?
Numerical weather models are computer models which analyse the conditions of the atmosphere in very high detail. They can predict the weather at up to three days in advance at reasonably high accuracy. These are behind weather apps, and are best used to get a general idea of conditions.
However, they do not have high enough resolution and are not run at a high enough frequency to predict isolated thunderstorms and snow showers, which usually develop on the day, sometimes within a few hours. These are best tracked by radar.
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