We give two wind speeds, the first is the steady speed and the second the gust speed. Our experience in the water tells us that this gust speed is really the one to pay most attention to. The arrow shows the direction the wind is blowing towards. Hovering your mouse over the arrow also shows the exact direction and whether the wind is onshore or offshore.
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.
Typically most surfers will opt to skip onshore surf sessions, although the same rationale makes them an attractive option for anyone wanting to surf without crowds. Pro surfers keen on aerial manouveurs will often opt for light onshores which provide a more sectioning wave to launch from and which can assist with sticking the board to the feet in the air.
Offshore winds remove (at moderate speeds at least) surface chop and creating more groomed faces. Offshore winds also slow up wave breaking, this means the wave will generally move into shallower water and have a steeper face before it breaks, improving it's surfing potential.
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.
The wind model also works on a grid, again it won't take into effect local conditions, the shelter of a cliff or pier or the funnelling of the wind down a narrow valley. As with the swell forecast you will have a very good idea of the general conditions but need to use some local experience to fine tune the forecast.
The gusting wind can regularly blow to twice the steady strength. It's important to use a forecast that gives both the average steady strength but also some idea of the gust you could expect. a 9mph onshore is probably worth a look if the swell is good. A 9mph gusting to 18mph wind will make for very confused and difficult conditions.
In many climates the sea temperature is different to the air and as importantly the sea will absorb heat from the sun more slowly than the land. Heat the land and the air will rise, creating low pressure, if the sea is colder air will move from the sea to the land making a local wind. These effects are so local (often only a mile or so from the coast) they are most normally missed by wind models but they are fairly easy to estimate. They tend to be at their worst in spring. When the sea is still cold from the winter but the first warm sunny days are heating the land. Typically in this situation you'll get an offshore breeze in the morning (from the cold land overnight falling below sea temperature) building to a strong onshore breeze in the afternoon (as the land heats much faster than the sea) and calm conditions in early evening (as the land cools again). It's unlikely that your wind forecast is taking this perfectly into account and you'll need to be aware of it. As a very general rule of thumb expect the forecast to be under estimating the winds mid afternoon.