Birsay Bay Surf Stats

This picture shows the combination of swells directed at Birsay Bay over a normal April. It is based on 3360 NWW3 model predictions since 2007 (values every 3 hours). The wave model does not forecast surf and wind right at the coast so we have chosen the best grid node based on what we know about Birsay Bay, and at Birsay Bay the best grid node is 17 km away (11 miles). The rose diagram describes the distribution of swell sizes and swell direction, while the graph at the bottom shows the same thing but lacks direction information. Five colours show increasing wave sizes. The smallest swells, less than 0.5m (1.5 feet), high are coloured blue. These happened only 22% of the time. Green and yellow show increasing swell sizes and red represents the biggest swells, greater than >3m (>10ft). In both graphs, the area of any colour is proportional to how frequently that size swell occurs. The diagram suggests that the dominant swell direction, shown by the longest spokes, was WNW, whereas the the prevailing wind blows from the SSW. Because the wave model grid is offshore, sometimes a strong offshore wind blows largest waves away from Birsay Bay and away from the coast. We combine these with the no surf category of the bar chart. To keep it simple we don't show these in the rose plot. Because wind determines whether or not waves are clean enough to surf at Birsay Bay, you can select a similar diagram that shows only the swells that were predicted to coincide with glassy or offshore wind conditions. During a typical April, swells large enough to cause surfable waves at Birsay Bay run for about 78% of the time.

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