The rose diagram describes how commonly and how strongly the wind blows from different directions through a typical northern hemisphere spring. The largest spokes point in the directions the wind most commonly blows from and the shade of blue suggests the strength, with the strongest winds shown by the darkest shade of blue. It is based on 4849 NWW3 forecasts of wind since since 2007, at 3hr intervals, for the closest NWW3 model node to Llantwit Major, located 41 km away (25 miles). There are too few recording stations world wide to use actual wind data. Invevitably some coastal places have very localized wind effects that would not be predicted by NWW3.
According to the model, the prevailing wind at Llantwit Major blows from the WSW. If the rose plot shows a nearly round shape, it means there is no strong bias in wind direction at Llantwit Major. Converseley, dominant spokes represent favoured directions, and the more the darkest shade of blue, the stronger the wind. Spokes point in the direction the wind blows from. Over an average northern hemisphere spring, the model suggests that winds are light enough for the sea to be glassy (the lightest shade of blue) about 11% of the time (10 days each northern hemisphere spring) and blows offshore 31% of the time (2 days in an average northern hemisphere spring). In a typical northern hemisphere spring winds stronger than >40kph (25mph) are expected on 4 days at Llantwit Major
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.