Pressure Charts

A pressure, or synoptic, chart can tell a Meteorologist a lot about the weather situation at a glance - where high or low pressure systems are, how close together the isobars are, where weather fronts are and what type of fronts they are - but what does all this mean the weather will be like?

High or low pressure
High pressure, or an anticyclone, generally means the weather will be settled - often fine and cloudless. In summer, though, they can sometimes bring sea and coastal fog. In winter they can also bring a lot of cloud or fog. Winds move around high pressure in a clockwise direction in the Northern Hemisphere.

Low pressure, or a depression, usually means the weather will be unsettled, often with plenty of rain (or snow) and strong winds or gales. Winds circulate around low pressure in an anti clockwise direction in the Northern Hemisphere.

On the TV a number is often shown in the centre of a high or low pressure - this is its central pressure in millibars. You can use this number to set your barometer. Divide the millibars by 33.88 to get inches of mercury.

Isobars
The lines around the high or low pressure centre are lines of equal pressure - or isobars. The closer together these lines are, the windier it is going to be; the wind blowing around the high or low pressure as described above. Isobars on an Atlantic pressure chart are drawn at 4 millibar intervals.

Fronts
These are usually associated with low pressure and there are three types:

• Warm front: red semicircles along a line. These show warmer air moving in, and along the line of the front rain usually falls.
• Cold front:- blue triangles along a line. These show colder air moving in - often with a change in wind direction to a more northerly direction. Again you can usually expect a band of rain, often followed by showers.
• Occluded front:- red semicircles and blue triangles along a line. Again this often means a band of rain is on the way.

So, from a pressure chart you can glean a good deal of information about the weather that is on the way, without necessarily needing to see a symbol chart.