On the storm track
Guest blog: With high winds and heavy rain lashing the UK in recent days and weeks, we've turned to the Met Office forecaster Barry Gromett to ask if autumn really does see more of this kind of weather. And if so, what's the reason?
Many factors dictate how and where storms form and move – but it is true that autumn can set up the conditions for a string of storms to arrive at our shores.
A storm's energy comes from temperature differentials. 'Polar fronts' – boundaries between cold northern air and warmer southern air – are especially marked in October and November, as there's no sunlight at all within the Arctic Circle while the Tropics still retain their warmth.
Britain sits in the 'temperate' latitudes between, ready to face whatever the North Atlantic storm track has to throw at us.
It's at a polar front that depressions will begin their lives. The two air masses interact because their densities differ. The warm air rises (lowering the pressure there) and colder air starts to fill in underneath. Forces such as friction and the Earth's rotation govern how the air streams flow. In the northern hemisphere, winds move anti-clockwise around a centre of low pressure.
The swirling clouds of a storm system are apparent in this visible satellite image taken at noon on Thursday 11 November 2010. Courtesy Met Office
Not every depression becomes a weather system with clouds, rain and wind in tow. Upper atmosphere conditions, such as the jetstream winds, will affect the situation. But if the low pressure centre does become established, the weather it brings us will follow a familiar pattern, associated with 'fronts'.
They are boundaries between zones of air with different characteristics and get mentioned in some of the forecasts you read or hear. A cold front (marked on traditional weather maps as a line of blue triangles) and a warm front (red semi-circles) refer to the arrival of that kind of air at ground level.
The pressure pattern and cold/warm fronts at the same time as the satellite image above. Courtesy Met Office
Each front has a cross-section shape that helps explain the weather that it brings. Warm fronts have a broad, sloping boundary and bring layer clouds that prompt light rain. It will take time to pass. The cold front that follows is much more active – torrential rain and thunderstorms are possible. But it moves much more quickly than the warm front, passing through any one location in perhaps just minutes.
Once the cold front catches up with the warm front ahead of it, an occluded front results. In that area, cold and warm air masses are mixing and the storm is usually at its most vigorous.
But it's that very mixing that signals the beginning of the end for any one storm. The temperature gradients will dissipate and with them the winds and the low pressure centre.
Colder, brighter weather may well follow. Unless, of course, there's another storm barrelling our way.