Birsay Bay Surf Stats

The rose diagram illustrates the range of swells directed at Birsay Bay over a normal October. It is based on 3471 NWW3 model predictions since 2006 (values every 3 hours). The wave model does not forecast wind and surf right at the coastline so we have chosen the optimum grid node based on what we know about Birsay Bay. In this particular case 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 illustrate increasing wave sizes. Blue shows the smallest swells, less that 0.5m (1.5 feet) high. These happened only 23% of the time. Green and yellow show increasing swell sizes and red represents the largest swells, greater than >3m (>10ft). In either graph, the area of any colour is proportional to how frequently that size swell was forecast. The diagram suggests that the prevailing swell direction, shown by the longest spokes, was WNW, whereas the the most common wind blows from the SW. Because the wave model grid is out to sea, sometimes a strong offshore wind blows largest waves away from Birsay Bay and offshore. We lump these in with the no surf category of the bar chart. To simplify things we don't show these in the rose plot. Because wind determines whether or not waves are good for surfing at Birsay Bay, you can view an alternative image that shows only the swells that were forecast to coincide with glassy or offshore wind conditions. During a typical October, swells large enough to cause clean enough to surf waves at Birsay Bay run for about 77% of the time.

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