Surface pressure charts

The surface pressure charts are the most common types of charts, often seen in the newspaper or on the television. From these charts alone we can make reasonably good surf predictions. If there is a large low pressure system in the ocean, for example, this will produce strong winds blowing over the surface of the water which, in turn, will generate surf. By looking at these charts we can also tell what the local wind conditions will be like for when the swell arrives.

The charts consist of contour lines called isobars. An isobar is a line of equal atmospheric pressure. If the lines are close together, this indicates a big pressure difference over a small area, which means that there will be a strong wind. On the other hand, if the lines are well spaced, this means not much of a pressure difference and not much wind. A good low pressure system will have plenty of closely packed lines, meaning strong winds and big surf. The lines around a high pressure will typically be much more spaced apart, meaning light winds.

The numbers on the charts show how high or low the pressure is, measured in hectarpascals (hPa), which is the same as millibars (mb). The average atmospheric pressure is about 1013 mb, the lowest it normally goes is about 920 mb and the highest is normally about 1050 mb. If the pressure in the centre of a low is lower than about 970 mb, this almost certainly means there will be enough wind blowing around it to produce some kind of surf.

The direction of the wind is more or less along the isobars, although sometimes it points slightly inwards towards the nearest low pressure. In the northern hemisphere the wind blows ANTICLOCKWISE around a LOW pressure and CLOCKWISE around a HIGH pressure. In the southern hemisphere, this is reversed.

Some of the surface pressure charts also contain features called warm fronts and cold fronts . These are attached to the low pressure systems, and mark the division between air with different temperatures. A warm front is shown by a thick black line with round blobs on it, and has colder air in front of it and warmer air behind it. A cold front is shown by a thick black line with triangles on it, and has warmer air in front and colder air behind. Both fronts carry a lot of rain with them. The passing of a cold front often means that the wind will switch from, say, south-west to north-west and suddenly get stronger.

Atlantic low pressure systems are usually ‘born’ just off the east coast of the USA and travel in an east-north-east direction across the Atlantic, until they reach the coasts of the British Isles. If there is a large low in the middle of the Atlantic, then in a couple of days time there should be some surf on the west coasts of Europe. If a high pressure remains over the coast, then the surf will arrive clean.

In the example shown, there is a huge low in the Atlantic, with a central pressure of 945 mb. The wind on the southern part of the low is very strong, blowing from the west. This means that plenty of surf will be generated, which will propagate towards Europe. Over the coast the pressure is high, so the winds are light. On the north coast of Spain, for example, the wind is a light northerly; in Portugal it is a light north-easterly. If it remains like that for about three days then there will be good surf. However, if the low travels across the Atlantic too fast then it might arrive on the coast at the same time as the swell, meaning nasty onshore winds.

Isobaric chart example

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