"Why does the water flow in a toilet in the Northern Hemisphere spin counterclockwise toward the center?" "Because the Coriolis force causes the water to flow downward while turning to the right." This is a topic occasionally heard in daily life. Is this statement correct? What exactly is this "Coriolis force"? This "force" is named the Coriolis force, first proposed by French mathematician and engineer Coriolis. It refers to the force acting on moving objects in a rotating reference frame, causing them to deviate from their original trajectory. The Earth's constant rotation can be considered a rotating reference frame. Coriolis pointed out that in the Northern Hemisphere, moving objects are subject to a force pointing to the right of their motion, while in the Southern Hemisphere, this force points to the left. This explains why water in the Northern Hemisphere always tends to flow to the right.
So, is the Coriolis force responsible for the counterclockwise flow of toilet water? Not necessarily. Due to the Earth's extremely slow rotation, the effects of the Coriolis force are quite minimal. Let's make a simple calculation. Assuming the water flow speed is similar to Liu Xiang's 110m hurdle race (about 10m/s), the Coriolis force would indeed cause a deviation in the water's direction. However, specific calculations show that even if the diameter of the toilet is 1m and the water flows directly from the edge of this "giant" toilet to the center, the deviation caused by the Coriolis force would be less than half a millimeter, making it difficult to produce noticeable effects. Even if there are any effects, they would easily be offset by various disturbances, such as water impacts. In fact, the main reason for the vortices in toilet drains is their own design. Some toilets are specifically designed with vortex-type flushing systems to better clean away waste. No matter where on Earth they are placed, they can only generate vortices in the same direction. For general sewer drains, if you observe them carefully on rainy days, you will find vortices rotating in both directions.
Although the Earth's rotation speed is not high, leading to a relatively small Coriolis force, its effects can still be observed on a large scale and over long periods of time. As the saying goes, "water drips through stone." Rivers that flow non-stop for thousands of years are always deviated to the right (in the Northern Hemisphere) due to the Coriolis force, so the right bank of the river is always eroded, while the left bank has relatively slow water flow, often leading to sand and gravel accumulation. For railways, thousands of tons of trains travel at speeds of hundreds of kilometers per hour in the same direction every day in the Northern Hemisphere. On the right side of the track, the wear and tear is always slightly worse than on the left side. In short, on a large scale, the effects of the Coriolis force are obvious. Looking at satellite cloud maps, all typhoons in the Northern Hemisphere rotate counterclockwise outward, which is also a trick played by the Coriolis force. Typhoons are low-pressure systems, and the air pressure near the center of the typhoon is particularly low, so the wind blows toward the center. Due to the Coriolis force, the airflow toward the center rotates counterclockwise (this has been widely publicized and become common knowledge). When a large amount of air reaches the center of the typhoon, it has nowhere else to go and can only rise to the top of the typhoon along the wall of the eye, forming a strong radiation zone where the wind blows outward from the eye. At this point, the Coriolis force still causes the direction of the airflow to gradually shift to the right, so the clouds at the top of the typhoon form a structure that unfolds counterclockwise outward. The same applies to the Southern Hemisphere.