The mass of the earth's atmosphere per unit area is 1 x 104 kg / m2, equivalent to a liquid water column approximately 10 m high. The half of the atmospheric mass resides between the sea surface and the height approximately 5 km above it, and the half of the rest is within the next 5 km layer. Thus three quarters of the atmosphere is within 10 km of the surface. Though atmospheric temperature varies with latitudes and seasons as well, a standard vertical profile, effectively averaging out those variations, is shown in Fig. 4.1-1. The atmosphere is divided into troposphere, stratosphere, mesosphere and thermosphere principally based on whether the temperature increases or decreases with respect to height.
The amount of water (H2O, including solid, liquid and gas phases) on the earth is shown in Table 4.1-1. Though the values are shown as the equivalent volume of liquid water (unit: km3), they are better regarded as mass (unit: 1012 kg). 97 % of water on the earth is sea water, covering approximately 70 % of the surface area of the earth. The next largest storage is ice sheets in Antarctica and Greenland.
Water on the earth continues the cycle consisting of precipitation, motion of liquid water (river discharge or ocean currents), evaporation, and motion of water vapor in the atmosphere. Both precipitation and evaporation is estimated to be approximately 1000 mm (i.e. 1000 kg / m2) when globally and annually averaged. The amount of water vapor in the atmosphere is approximately equivalent to a 25 mm layer of liquid water. Therefore, the mean residence time of water vapor in the atmosphere is approximately 9 days. (This is just the average residence time. The residence time of a certain molecule may be longer or shorter.)
Besides water plays an important role in climate as will be discussed in this section, it is indispensable matter for the maintenance of life, and it is also an agent of movement of rocky materials through erosion, transportation and sedimentation. A technical term `climate system' is also used to mean the atmosphere and the hydrosphere (i.e. water on the earth) together. The meaning of this term is explained in Section 4.2.1 .