When the arrow on a weathervanepoints west, the wind is blowing from the west.
Explanation: Under the influence of wind, the wind vane rotates around the vertical axis, causing the wind tail to swing downwind and the head to point in the direction of the wind. So the title says "the arrow of the wind vane points east", and the head points to the direction of the wind, so "when the arrow of the wind vane points east, the wind is blowing from the west".
Basic principle:
When the direction of the wind reaches a certain angle with the wind vane, the wind exerts pressure on the wind vane. This force can be decomposed into two wind forces parallel and perpendicular to the wind vane. . Since the wind-receiving area of the wind vane head is relatively small and the wind-receiving area of the tail is relatively large, the wind pressure felt is not equal. The wind pressure perpendicular to the tail produces a wind pressure moment, causing the wind vane to rotate around the vertical axis until the wind vane head is facing the wind. When the weather vane is coming in the direction of the wind vane, the wind vane is stable in a certain direction because the forces on both sides of the wing plate are balanced.
The arrow of the weather vane always points to the source of the wind. The principle is actually very simple: the wind-receiving area of the arrow tail is larger than the arrowhead. If both the arrowhead and the arrow tail are subject to wind, the arrow tail will be pushed back by the wind, causing The arrow moves to the source of the wind.