Windy states very clearly that winds are arrived at using all relative, applicable and resourceful data and they are defined. The arrows represent wind velocity, (relative speed and direction), within a certain volume suspended above the surface. There may be a billion directions and speeds in that volume, but the one chosen represents the speed and direction expressing the central or typical value in a set of data, in particular the mode, median, or, (most commonly), the mean.****
IMO windy is a great tool-very discerning engineering.