This image shows only the swells directed at Zanzibar that coincided with light winds or offshore conditions through a typical June. It is based on 1594 predictions, one every 3 hours. The direction of the spokes show where quality surf generating swell comes from. Five colours represent increasing wave sizes. Very small swells of less than 0.5m (1.5 feet) high are shown in blue. Green and yellow represent increasing swell sizes and highest swells greater than >3m (>10ft) are shown in red. In both graphs, the area of any colour is proportional to how commonly that size swell occurs.
The diagram implies that the prevailing swell direction, shown by the longest spokes, was ESE, whereas the the dominant wind blows from the S. 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 81% of the time, equivalent to 24 days. Open sea swells exceeding >3m (>10ft) are unlikely to happen in a normal June but 15% of the time we expect swell in the range 2-3m (6.5-10ft) 15%, equivalent to (4 days). Taking into account the ratio of these swells that coincided with forecast offshore winds, and given the fact that Zanzibar is exposed to open water swells, we calculate that clean surf can be found at Zanzibar about 81% of the time and that surf is blown out by onshore wind 6% of the time. This is means that we expect 26 days with waves in a typical June, of which 24 days should be clean enough to surf.
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.