Arrifana Swell Statistics, Autumn: Surf with Light or Offshore Winds
This image shows only the swells directed at Arrifana that coincided with light winds or offshore conditions over a normal northern hemisphere autumn and is based upon 7252 predictions, one every 3 hours. The direction of the spokes show where quality surf generating swell comes from. Five colours illustrate increasing wave sizes. Blue shows the smallest swells, less that 0.5m (1.5 feet) high. Green and yellow illustrate increasing swell sizes and highest swells greater than >3m (>10ft) are shown in red. In each graph, the area of any colour is proportional to how frequently that size swell happens.
The diagram suggests that the dominant swell direction, shown by the biggest spokes, was NW, whereas the the dominant wind blows from the NNW. The chart at the bottom shows the same thing but without direction information. For example, swells larger than 1.5 feet (0.5m) coincided with good wind conditions 28% of the time, equivalent to 25 days. Open water swells exceeding >3m (>10ft) only arise 1.2% of the time in a typical northern hemisphere autumn, equivalent to just one day but 10% of the time we expect swell in the range 2-3m (6.5-10ft) 10%, equivalent to (9 days). Taking into account the fraction of these swells that coincided with forecast offshore winds, and given the fact that Arrifana is slightly protected from open water swells, we think that that clean surf can be found at Arrifana about 28% of the time and that surf is blown out by onshore wind 36% of the time. This is means that we expect 58 days with waves in a typical northern hemisphere autumn, of which 25 days should be surfable.
IMPORTANT: Beta version feature! Swell heights are open water values from NWW3. There is no attempt to model near-shore effects. Coastal wave heights will generally be less, especially if the break does not have unobstructed exposure to the open ocean.