Here's why:

* Temperature is a measure of the average kinetic energy of the molecules. Kinetic energy is the energy of motion. The faster the molecules move, the higher their kinetic energy, and the higher the temperature.

* Molecular motion is random. Air molecules are constantly bouncing around, colliding with each other and the walls of their container.

* Higher temperature means faster movement. As the temperature increases, the molecules move faster on average.

While temperature doesn't directly measure the speed of *every* individual molecule, it provides a good indication of the overall speed of the molecules in a given sample of air.