Air flows towards a low-pressure centre, so why do tropical cyclones spin instead of just filling in? The answer is that we live on a rotating planet.

Earth turns once in about 24 hours, so Ω = 7.292 × 10⁻⁵ rad/s. When you describe motion from the rotating Earth's point of view, a moving object appears to be pushed sideways by an apparent force called the Coriolis effect. Watching from outside, nothing is pushing it; the ground is turning underneath it. In the rotating frame the acceleration is a = −2 Ω × v, which is always at right angles to the motion.