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Wave prediction charts.

These charts give you a ‘snapshot’ of an area of ocean, showing different colours according to the wave height in different places. Areas of big waves correspond to areas of strong winds around low pressure systems. But the charts also show swell propagating away from the storm centre, which is very useful for predicting waves arriving on our coasts.

The main features of these charts are the colour contours indicating wave height. A scale is given showing which heights correspond to which colours. Some charts give heights in feet and some in metres. Other details include arrows showing wave direction, and arrows showing wind direction. It is important to recognise which is which.

The example below gives wave height contours, with the numbers on the chart corresponding to the wave height in feet, and also wave direction, indicated by the small arrows. We can see that there are waves of about 12 foot arriving on the coast of Portugal. We know by the arrows that the waves are coming in from the west, but on this chart we have no information about the local wind direction.

The next example below shows a slightly different kind of wave prediction chart, giving wave height contours and numbers in feet, but instead of wave direction it gives wind direction. The wind direction is shown by small lines with ‘barbs’ on them. For example, on the chart we can see that the wind off the west coast of Ireland is from the south-west, and in Portugal it is from the north. The more ‘barbs’ the lines have on them, the stronger the wind. We also know that there is are 8 foot waves off Ireland, but, since we don’t have any wave direction arrows, we don’t know from what direction they are coming.

Beside the wave height contour charts, you will also find charts with colour contours on them representing period. This is basically how far apart the waves are (similar to wavelength), and tells us whether we are looking at a choppy sea or a nice clean groundswell. The charts also contain small arrows indicating wave direction. When a new swell arrives, the wave period and direction change sharply, which you can see clearly on these types of charts. A thick black line is sometimes shown which is the ‘swell front’, and from which we can tell if a new swell is about to arrive on our coasts. In the example below, there is a new swell from the north-west about to arrive at the Canary Islands, replacing an old swell from the north-east. The colours show that the new swell is long-period, well lined-up groundswell, and the old swell is short-period wind chop, probably generated by local trade winds.

There are many more variations of wave prediction charts available, and it is important to know what you are looking at. Some charts show both wave direction and wind direction, for example, and some show different coloured arrows for wind-sea and groundswell. The most important things to look out for are to make sure you know whether the heights are in feet or metres, and to make sure the arrows you are looking at are for wave direction or wind direction.