Why do low and high pressure areas rotate?
Why do low and high pressure areas rotate?
Due to the Earth’s rotation, lows in the northern hemisphere rotate counter-clockwise. We ourselves feel nothing of this distraction force. Why then can it have such a strong effect on the air pressure structures?
If you sit on a merry-go-round and try to hit someone opposite you with a water spray gun, you will be amazed to see that the water jet is apparently deflected to the side by the rotating movement and misses its target. Although the jet exits the pistol in a straight line, in the time between exiting the muzzle and failing to reach the target, the carousel has continued to rotate, causing it to arc.
The force that deflects the water jet is a so-called apparent force, which is named after the French mathematician and physicist Gaspard Gustave de Coriolis (1792 to 1843), who first described it mathematically in 1835 and is therefore listed alongside other 71 French greats from the natural sciences and technology immortalized in the Eiffel Tower in Paris.
It is precisely this apparent force that acts on air masses that move over a large area on our rotating planet. However, the Coriolis force effective on earth is quite small. In addition to the mass and the speed of the deflected body, it is also influenced by the so-called angular speed of the rotary movement. The earth rotates only once around its own axis in a day, on a children’s carousel with such an angular speed one would probably nod off quite quickly. Although one races through space on the rotating earth in Germany at over 1000 km/h, the circle that one describes is so wide that one cannot perceive the distracting force on one’s own body.
However, if the Coriolis force acts for hours and days, this leads to a significant distraction. And so the air, which only wants to flow from high to low pressure, is so turbulent on its way there that it would have to circle the pressure center for all eternity if it were not slowed down by ground friction and other effects. This eventually reduces the distracting Coriolis force and allows air to flow into the low and eventually fill it up.
The Coriolis force is strongest at the poles and disappears at the equator because you are perpendicular to the axis of rotation and rotation is no longer a distraction. That is why tropical cyclones cannot form directly on the equator. Only about 5 degrees north or south latitude is the deflection strong enough to allow hurricanes, typhoons and cyclones.
In absolute terms, the direction of rotation in the southern hemisphere is the same as in the northern hemisphere, but you are practically standing upside down on the earth, which reverses the deflection relative to the viewer. That’s why lows rotate clockwise and highs counterclockwise in the southern hemisphere.
The vortices that are created when the bathtub is drained are not caused by the Coriolis force, as it is far too weak and acts far too briefly. Frictional forces are also at work here, which increase towards the edge of the bathtub. Due to the uneven deceleration of the water, it swirls, with the direction of rotation being much more influenced by the shape of the bathtub or the splashing movements.