Here's why:

* Whirls of air: Thermals are columns of rising warm air, creating a swirling motion.

* Move up and down rapidly: As warm air rises, it cools and can become denser, leading to sinking air currents. This creates the rapid up-and-down motion.

Other possibilities:

* Turbulence: This is more chaotic air movement, not necessarily in a swirling pattern.

* Dust devils: These are small whirlwinds that form near the ground in hot, dry conditions. They are usually weaker than thermals.

Let me know if you have more details!