During summer 23-30°C, and water about 22°C as well. Shorty or even shorts are enough in summer. Besides summer a wetsuit is needed. Surfing all year is possible.
Works best with winds coming from the east. West winds make this a very gusty location. Further south NE is onshore. On the small island off shore any wind direction can be sideshore.
In the center of Bonifacio take the small road left of the surf shop. Keep going on this road until you see parked cars by the side of the road. All the way at the end of the road on the left hand side there's a small beach and a bar at the right hand side (closed from mid September, except for the speed crossing to Sardinia). Walk in front of the bar over the rocks towards the main beach (on the right hand side). Even though it's not very wide, it's long enough to launch a kite with just about any wind direction safely. Alternately, you can walk to the end of the beach and walk over to the small island. You can then start from the beach there (talk to the locals at the surf school before doing so).
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