Why is the water flow in the drain hole of the water tank always swirling in one direction?

Let’s take a look at a strange phenomenon: when the tank is flush, around the tank’s outlet, the water always swirls counterclockwise. Even if you use your hand to make the water swirl clockwise for a short time, the water will swirl more and more slowly; after a while, it swirls counterclockwise.

For what reason? The rotation of the Earth causes it. We know that the Earth spins nonstop. It spins around in 24 hours. By calculation, at any point on the equator, the rotation speed from West to East is 0.46 m / s, while the rotation speed in Beijing is 0.35 m / s. Therefore, the more objects in the Northern Hemisphere, the closer it is to the North, the smaller the Earth’s rotation speed. Suppose there is a flow of water from the North to the South because the West’s rotation speed to the East is relatively low initially. It will deviate to the West; If flowing from South to North, the speed of rotation from West to East is large, it will because of the inertia which maintains relatively fast speed initially and deviates to the East. 

The water around the water tank’s outlet is from four directions, eight directions, the water flowing from the North to the South deviates from the West, from the South flows to the North, it deflects East. Eventually, the water swirls counterclockwise. Just think, what will people who live in the Southern Hemisphere see? The results of their observations are against us; the water around the outlet of the water tank always swirls clockwise.

This phenomenon was first noticed by French physicist Coriolis and conducted a comprehensive study of both experiment and theory. Later life called this type of eddy formation force Coriolis force.
Coriolis force has a certain influence on human activities. In the Northern Hemisphere, the river’s right bank is relatively eroded due to the Coriolis force pushing the river in a horizontal direction. Likewise, when a train runs along the North-South direction, it is always the right rail that gets bigger. Coriolis forces also affect the movement of air on the surface of the Earth. Under Coriolis’s action, the atmosphere will generate eddy currents with great energy; tornadoes are one of those.