If you live on or near the East coast of the US and you've been wondering why it's been so cloudy lately, here's the reason:
That's a low-pressure system that's been squatting over the Great Lakes region for a few days, as seen by NASA's Aqua Earth-observing satellite [click to centrifugenate]. It stretches clear across the country north-to-south; you can see the Gulf of Mexico at the bottom of the picture.
Why is it comma-shaped? Because the Earth rotates. Seriously.
The Earth spins once per day, and is about 40,000 km (24,000 miles) around at the Equator. That means someone standing there makes a circle that big once per day, moving at a velocity of about 1700 kph (1000 mph). But someone standing at the pole isn't making a circle at all; they would just spin in place once a day. At an intermediate point, say a latitude of 45°, someone would be moving around in a circle at a velocity of about 1200 kph (700 mph).
Now imagine you're standing on the Equator, moving east at 1700 kph. I suddenly magically transport you to 45° north latitude. What happens? Well, you're still moving east at 1700 kph, but the ground is only moving east at 1200 kph. That means relative to the ground you're moving faster to the east by 500 kph! To someone standing there, you'd pop out of thin air screaming eastward nearly as fast as an airplane. Better pack a parachute.
The opposite is true if you're at the north pole, not moving at all, and I whisk you south to 45°: now the ground is moving east of you at 1200 kph. To someone already there, you'd appear moving west at that speed. It's all relative.
Weird, isn't it? But it's a natural consequence of living on a rotating ball.
Now imagine you have a low pressure system over the Great Lakes (or simply look at the picture above). Air is rushing in, toward the center, from all directions, including north and south. But those masses of air are moving east at different velocities due to the rotating Earth's speed. Air from the south is moving east more quickly than the center of the weather system, so (in the picture) it moves to the east (right) as it heads north. Air moving south is moving slower than the ground, so it bears west (left). This sets up a counter-clockwise rotation, just like you see in the picture.
That's why hurricanes rotate counter-clockwise in the northern hemisphere. In the southern hemisphere, this is reversed: air moving south is traveling faster than the ground beneath it, and slews east; air moving north slews west, and you get clockwise rotation.
This almost seems like a force, doesn't it, something pushing the air around? In many ways it does act like a force, though it depends on whether you're looking at it from the ground, rotating with the Earth, or from space, watching the Earth spin beneath you. This whole thing was first figured out in detail by Gaspard Gustave de Coriolis in the 1800s, and we name it after him: the Coriolis effect (or, sometimes, the Coriolis force).
This only works on large scales, since the effect is caused by the change in rotation speed with latitude on the Earth. It has nothing to do with your toilet; the water would flush the same way if you're in Australia or Austria (the water going into the toilet boil spins due to the way the flow is directed by the bowl itself, and not by the rotation of our planet).
But on large scale it drives megatons of air one way or another. So if you're in the eastern US and feeling glum due to gray weather, remember: there are vast forces at work making it that way, and there's not much you can do about it. And if that feels like spin, well, that's because it is.
Image credit: NASA/Aqua