What is Low Pressure?
Isobars make shapes and patterns. When they enclose an area of low pressure which is called a 'low' or 'depression'.
Its centre is labelled on a weather map with an 'L' and often has a number associated with it indicating pressure.
A low pressure system is like a giant funnel of wind spiralling inwards and upwards forcing warmish air in the centre to rise. As air rises it cools and clouds form.
The central pressure of a shallow low is above 1000 hPa, of a moderate low 980-1000 hPa, and of a deep or intense low below 980hPa. It's deep ones we're interested in as surfers as these generate our best waves.
If there are two or more centres, the low system is said to be 'complex'. If the central pressure is rising the low is said to be 'filling'. If the central pressure is falling the low is said to be 'intensifying' or 'deepening'.
The low pressure systems that affect the British Isles normally form out in the Atlantic and are as a consequence accelerations and decelerations of the strong winds associated with the jetstream at about 30,000 feet above the surface of the Earth.
In the Atlantic, as a new low pressure area or depression starts to form at sea level, the winds which start to blow from high to low pressure are then deflected to the right.
In the northern hemisphere, this is due to the Earth's rotation as winds spiral into the low pressure and is then slowed down and turned by friction.
Winds from low pressure generally blow in an anti-clockwise direction so you can work out what the winds will be doing at your local spot from the position of the low in relation to your particular beach.
