## Chemistry 12th Edition

We know that under the same conditions of temperature and pressure equal volumes of all gases contain an equal number of molecules.Dry air has $N_2$ with molar mass 28 and $O_2$ having a molar mass 32. When water with molar mass 18 is mixed with it, it replaces some of $O_2$ and $N_2$ which have a higher mass. Thus, for the same volume of air, moist air will weigh less than that of dry air. Since the moist air comes down from a rain cloud, dry air will rise creating low pressure and hence a low pressure shows an imminent rainfall.