Roughly on Beufort scale

Levels 01-02 - 20-25 mph - Small trees sway
Levels 03-04 - 26-30 mph - Umbrellas hard to use
Levels 05-06 - 31-40 mph - Difficulty walking
Levels 07-08 - 41-45 mph - Twigs broken on trees
Levels 09-10 - 46-55 mph - Damage to roofs
Levels 11-12 - 56-65 mph - Trees uprooted
Levels 13-14 - 56-60 mph - Moderate damage
Levels 15-16 - 61-65 mph - Major damage
Levels 17-18 - 66-70 mph - Severe damage
Levels 19-20 - 71-75 mph - Apocalyptic damage

At level 5, a Wind Powered character can fly as if they were a level 1 flier.