Gas molecules are too small and numerous to count. However, the mathematical relationships between gas temperature, pressure, volume, and number of molecules allow scientists to determine the exact number of molecules in a gas sample. Knowing the number of water molecules in a particular volume of air helps scientists to predict when precipitation will occur. Air pressure and air temperature greatly affect the amount of water vapor in the air. When moist air cools, the water vapor in the air changes from a gas to a liquid. When extreme conditions of temperature and pressure occur, hurricanes can form.