What is the concept of north wind level 4-5?

Level 4 breeze, dust flying, paper scraps flying. Level 5 breeze, waves in the stream, small trees shaking.

Level 4 wind is Beaufort wind, with a wind speed of 5.5-7.9m/s and 20-28km/h, also called gentle wind, with a wind pressure of 18.9-39Pa, which can blow up dust and paper on the ground, and the twigs of trees also move slightly, so the wind force is generally small.

Level 5 wind, also known as breeze, refers to the wind speed of 8.0- 10.7m/s, which is equivalent to 29-38 km/h, and the waves in the sea are getting higher and higher, foaming at the peaks and splashing occasionally. Small trees with leaves are swaying on the land, and there are small waves on the inland water.

Extended data

When Beaufort table was invented, it was a classification method based on observing sea surface phenomena. The number of grades is divided according to sea conditions or wave conditions, and no relevant wind speed is specified.

1830, the Royal Navy took the beaufort scale as the recording standard. Starting from 1850, the Beaufort scale is also used for general navigation, and then the wind speed is measured by the wind cup anemometer. The anemometer is standardized in 1923, and the Beaufort table is slightly modified to facilitate meteorological use. However, the classification adopted in the Maritime Meteorological Service Manual of the World Meteorological Organization is only 0 to 12, and the extended classification from 13 to 17 is not recommended.

Up to now, Beaufort scale 0 to 12 is still used to describe different wind speeds in navigation, but many western countries have given up the wind scale in ocean forecasting and changed it to every 5 knots (kt) (Britain and France are a few exceptions) because wind meters can measure wind speeds more accurately.