How much wind is a strong wind?

Level 8. Meteorology can be divided into 12 grades according to the size of wind, and the wind of grade 0 is called windless. Level 2 wind is called breeze, the leaves are slightly noisy, and people feel windy. Level 4 wind is called breeze, and the twigs of trees shake, which can blow up dust and paper on the ground. Level 6 wind is called strong wind, branches are shaking, wires are whistling, and it is difficult to walk with an umbrella. Grade 8 wind is called gale, which is tree-shaped.