The rose diagram describes how frequently and how strongly the wind blows from different directions through a typical northern hemisphere summer. The largest spokes point in the directions the wind most commonly blows from and the shade of blue implies the strength, with dark blue strongest. It is based on 5064 NWW3 forecasts of wind since since 2006, at 3hr intervals, for the closest NWW3 model node to Llantwit Major, located 41 km away (25 miles). There are not enough recording stations world wide to use actual wind data. Without question some coastal places have very localized wind effects that would not be predicted by NWW3.
According to the model, the most common wind at Llantwit Major blows from the WSW. If the rose diagram shows a close to circular outline, it means there is no strong bias in wind direction at Llantwit Major. On the other hand, dominant spokes illustrate favoured directions, and the more dark blue, the stronger the wind. Spokes point in the direction the wind blows from. Over an average northern hemisphere summer, the model suggests that winds are light enough for the sea to be glassy (pale blue) about 12% of the time (11 days each northern hemisphere summer) and blows offshore 20% of the time (1 days in an average northern hemisphere summer). During a typical northern hemisphere summer winds stronger than >40kph (25mph) are expected on 2 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.