What type of air movement is primarily responsible for the rotation in a cyclone?

The rotation in a cyclone is primarily caused by horizontal movement of air. In a cyclone, air flows inward toward the low-pressure center, and as it does so, the Coriolis effect comes into play due to the Earth's rotation. This effect causes the air to be deflected, creating a spin around the center of the cyclone.

As warm, moist air rises from the surface, it creates lower pressure near the surface, while cooler air aloft sinks, further enhancing this rotation. The combination of this inflow of air and the conservation of angular momentum leads to the characteristic spiral shape of a cyclone.

Vertical movement, while important in terms of updrafts and downdrafts within the storm, is primarily the result of the horizontal inflow of air. Thermal movement relates to the temperature and density differences that contribute to convection but does not directly account for the rotational aspect. Inertial movement is not a standard term used in meteorology when discussing cyclonic activity. Therefore, horizontal movement is the correct answer, as it fundamentally describes how the rotating motion in cyclones is generated.