Experiment 2: Bathtub Drain
The bathtub drain experiment is something you’ve probably done and don’t even realize it. When you pull the plug or open the drain in your bathtub and watch it swirl down the drain, you are actually witnessing the angular acceleration of water due to gravity. While this might sound really technical, it just means that the speed at which the water goes down the drain increases the longer there is water to flow. While this doesn’t have much to do with the rotation of the Earth, it does have a lot to do with inertia.
To do this experiment properly you need to fill a tub with water and a few small pieces of floating material (wood, ivory soap, etc.) and let it sit until there is little to no motion in the tub from it being filled. Once the motion has stopped, very slowly open the drain. Watch as the water drains down.
For the first few minutes you probably won’t see a vortex form. The water will flow straight down the drain. Then, something interesting will start to happen. A small water tornado will form and rotate in a counter clockwise direction. This will happen wherever you do it in the world (although it was commonly believed that toilets and bathtub drains would work in the opposite direction in the Southern hemisphere).
The water in the tub accelerates because as the pressure on the outside of the vortex gets lower, the Coriolis force gets stronger creating a feedback loop. The pressure gets lower as there is less water pushing on the system. The less pressure on the system, the faster the water drains.
So what does this have to do with hurricanes? The same physics apply here. The lower the pressure on the outside of the vortex, the faster it will rotate. A stronger hurricane with higher winds will be produced by a storm with a very low center of pressure in the eye.
When the pressure outside of the eye of the storm starts to rise, the rotation of the storm will slow. The reason that hurricanes can become so devastatingly powerful is because of this difference in pressure on the inside and outside of the central vortex of the storm.