The wind direction at the coast has a strong effect on the location of the break point and the breaking-wave height. Onshore (offshore) winds
(a) move the break-point location farther offshore (onshore) and
(b) decrease (increase) the breaking-wave height-to-depth ratio γb.
Changes in break-point location are as large as 40% and changes in γb are up to 100% for the range of wind speeds considered.
Take Waimea Bay Beach at Honolulu as an example.
Onshore wind blows from the sea towards the beach. Generally associated with messy, confused conditions this wind creates chop and small waves that mix in with any incoming swell. It also tends to make waves break earlier, this means the wave face is less steep. These negative effects are particularly noticable on gently sloping beaches where onshore winds may cause messy, slow and slack surf to break a long way from shore. Steeper beach breaks or reef breaks are a little more able to handle these effects. The stronger the wind the greater these effects and as the wind swings across the shore they're mitigaged to some extent. Local cliffs or headlands or even a harbour wall can provide productive shelter in these conditions. |
With offshore wind not only do waves break closer to shore, but they have larger breaking-wave heights. Offshore winds remove (at moderate speeds at least) surface chop and creating more groomed faces. Also the wave will have a steeper face before it breaks.
Very strong offshore winds (gusts of perhaps 25mph/40kph+) will make it harder to paddle into waves and, on weaker swells, harder to generate speeds. Considerably stronger still and even a good day can become hard to surf. |