Many national research institutes provide a vast selection numerically modelled weather forecast data. Weather services like WeatherOnline or the Met Office are using the data for plotting weather maps, such as the mean-sea-level pressure chart, which is also known simply as surface chart. The main feature of a surface map are the isobars, lines which join points of equal mean-sea-level-pressure. The prefix iso- is from the Greek term isos meaning equal and the term bar stands for a common pressure unit.
On surface maps isobars represent stational pressure readings reduced to sea level to eliminate the effects of the different heights of the reporting stations. The pressure is usually given as hecto-pascals (hpa) or millibars (mbar) as most people know them. Also isobars determine the location and intensity of the weather-producing disturbances: the lows, the highs, the troughs and the ridges. The centre of each system is indicated by the central value and surrounding pressure gradients. By comparing a set of successive charts, the forecaster can determine how they are evolving and moving and what weather should be expected over a certain region.
The terms low, high, ridge, trough and even col are evocative of the features shown on topographical maps. As a matter of fact the terms used on an isobaric surface map explain pressure differences and gradients through topographical land features. For example, troughs can be thought of valleys, ridges as ridges, gradients as slopes, highs as peaks and lows as hollows. An atmospheric col is the equivalent of its namesake, too - a region of weak pressure gradient between two ridges and two troughs. However, highs and lows may be many hundreds or even thousands of miles across.
Analysing and interpreting the isobaric surface map reveals not only these features, but given the strength of the pressure gradient and the alignment of the isobars, also provides information about the surface wind direction and strength. Closely packed isobars indicate a steep horizontal pressure gradient and, thus, strong winds.
The resulting wind usually aligns parallel to the isobars and of course one has to know that on the northern hemisphere winds will blow clockwise around highs and anti-clockwise around lows.
