The speed of molecules in the air depends on the temperature and the mass of the molecules. At room temperature (25°C), the average speed of air molecules is about 480 meters per second (1,070 miles per hour). Lighter molecules, such as hydrogen, move faster, while heavier molecules, such as carbon dioxide, move slower. The speed of molecules also increases with temperature. For example, at 100°C, the average speed of air molecules is about 640 meters per second (1,430 miles per hour).