Question: What causes the eye of a storm to form?

Susan Cartwright answered on 20 Jun 2015:

Although the details are complicated (they always are with weather, which is why it’s so hard to predict!), the root cause is that the Earth is rotating. Because the Earth is a solid body, in order for everything to get back to where it was after one day, the land at the equator has to be moving faster than the land near the poles – at the equator, the land has to cover 40000 km in 24 hours, whereas in Sheffield (latitude 55 degrees or so) it only has to travel about 23000 km. But in any case, the land is moving towards the east – the Sun appears to move east to west because the Earth spins west to east.

This means that if air tries to flow due north in the northern hemisphere, because the Earth is spinning underneath it, its path as seen relative to the Earth’s surface actually curves towards the east, because it started out with the faster eastward motion caused by being closer to the equator. The result of this is that when air tries to flow in towards a region of low pressure, instead of coming in directly towards the centre, it spirals in anticlockwise. The effect of this spiral pattern is to form a calm area in the middle of the storm – the eye.

In the southern hemisphere, the effect is reversed, and storms spin clockwise instead of anticlockwise.

There is a nice YouTube video of this at https://www.youtube.com/watch?v=i2mec3vgeaI