One rating for the sports that need wind: sailing, kitesurfing, windsurfing and wing foiling. More wind is better here, up to a point. GoDay also checks the wind direction against your beach, and finds the best 2 hours of each day.
Also on the page, but not in the score: wind direction, which way the beach faces.
A thunderstorm holds the score at 1, and one due within two hours at 3. Wind under 8 km/h (5 mph) holds it at 3, gusts over 65 km/h (40 mph) at 3, and offshore wind of 12 km/h (7 mph) or more at 4, because it blows you away from the beach.
Daylight counts too: at dusk the score is held at 4, and once it's dark at 1.
GoDay's sweet spot is 20 to 40 km/h (12 to 25 mph, about 11 to 22 knots). Under 12 km/h (7 mph) there isn't enough to ride, and past 48 km/h (30 mph) it's overpowered for most riders.
Offshore wind blows from the land out to sea. If your gear fails or the wind drops, it carries you away from the beach. Cross-shore wind is best, and onshore is safe but messy.
Gusty wind swings between too little power and too much. GoDay compares the gusts with the average wind: gusts up to 30% stronger count as steady, and more than 80% stronger as very gusty.