A low pressure system, or depression as it is sometimes known, is an area of warm air. The description ‘warm’ is relative to the air around it and so don’t expect scorching temperatures as a result!
As we know, warm air rises and cold air falls. Therefore, the air within a depression is rising and convecting from the equator to the poles. Along the way, that air cools and falls to earth. That cool air is creating a heavier air mass as it falls, referred to as a high pressure system. Again, this is all relative to the air around it.
Because of the effect of the Earth spinning on its polar axis, the direction that the air flows across the earth’s surface during convection is affected by coriolis. The spinning of the earth’s surface means that warm air (a low pressure system), in the northern hemisphere, spins in an anticlockwise direction. Cold air within a high pressure system spins in the opposite direction. In the southern hemisphere the opposite is true.
Air pressure is usually measured on vessels by a barometer. This enables us to detect if the air around us is imposing more or less pressure and the relative speed of the change in air pressure enables us to predict likely wind speed, weather and even wind shifts if we use our skills and experience. As a rule of thumb, anything below 1000 mb is quite a deep depression and likely to bring gales and wet weather. Anything over 1013 mb is becoming what we’d usually refer to as a high pressure system. But remember, it’s all relative to the air masses around the point of measurement.
In Western Europe, especially the UK, the weather is largely governed by the effect of the Gulf stream and the warm, moist air that rolls in from the Atlantic Ocean. As the air moves north it pushes against colder air and the warm air slides over the heavier colder air causing two fronts to form. A warm front and a cold front. This mass of warm and cool air spins out of the Atlantic as a depression or low pressure system, regularly referred to as an Atlantic Low.
In the summer months high pressure tends to dwell over the Atlantic and causes clear blue skies and settled weather. As autumn approaches the frequency of low pressure systems increases over the UK and with these low pressure systems comes unsettled, windy weather and rain. Of course, for those of you that live in the British Isles, the weather associated with South Westerly winds and Atlantic depressions will be all too familiar.