Operator ∇ inner products on both sides of Combination of Boyles’ law and Chares law (“B-C law” in short), we got the “Wind Speed Equation of a Point in Air” (“Wind Speed Equation” in short). It suits for describing straight-line motion, and It states that mu ̇ is in proportion to ∇•T. Operator ∇ outer products on both sides of “Wind Speed Equation” (where T is replaced by T), we get the “Rotation Equation of a Point in Air” (“Rotation Equation” in short). It is a vector partial differential equation (PDE), suits for describing circular motion. It states that (mu ̇ ) is in proportion to T. Its solution is found by the method of separating variables. The existence of vector T is proved by the existence of rotation in the atmosphere and the solution of the “Rotation Equation”. It reveals that the vector form of B-C law holds in rotating air. Examples of up-side-down vertical rotation and horizontal rotation are given.