Water vapor in the atmosphere

The clouds in our atmosphere are a condensed form of water (water droplets and ice particles) suspended in air. Such a system is called an aerosol. Naturally, the necessary constituent in air for forming clouds is water vapor. Thus it is important for us to understand the general situation of water vapor in our atmosphere.

Water vapor is so pervasive in our daily life that many do not know that its concentration is actually quite small. Table 1.1 lists the five major gaseous constituents and their volume concentrations in the Earth’s atmosphere. Water vapor ranks fourth after N2, O2, and Ar.

Water vapor is also different from the other four gases in another important aspect: whereas the concentrations of N2, O2, Ar, and CO2 remain fairly constant from place to place, the concentration of H2O is highly variable. The layer immediately above the warm tropical ocean surface is literally steaming with water vapor; the highest value is ~4%(tropical Indian Ocean), whereas the surface layer over the Sahara Desert in North Africa or the Taklimakan Desert in western China is close to 0%. Thus, even though the water vapor concentration in the whole atmosphere is more than that of CO2, as shown in Table 1.1, there are places in the world where its concentration is less than that of CO2.