Under the name of each gas its molecular weight is given as a measure of corresponding specific weight. The quantity of water vapour was not included when the percentages of the other gases were calculated, because it changes considerably with locality and time. The number given in the table for water is the mean for the entire globe--it corresponds to 11.4 grammes per cubic metre (.31 oz. per cu. yd.)--or the amount present in air saturated with moisture at 16.5° C. (61.7° F.). The bulk of the water vapour forms a layer strongly concentrated toward the surface of the earth. Carbon dioxide also tapers rapidly with increasing height because its density is 1.5 times greater than that of air. This is apparent from the molecular weight 44 stated under carbon dioxide, while the average molecular weight of air is 29. Faster yet do krypton, with a molecular weight of 83, and xenon, with a molecular weight of 131, decrease as we ascend in the atmosphere. These gases, like neon, whose percentage first increases slightly with height, and argon, which decreases upward as shown in the table, do not perceptibly influence the processes of nature. The reverse is true about water vapour and carbon dioxide, which nourish the plants and also protect the Earth against a too rapid heat radiation into space. We well remember how abruptly the temperature changes in the course of the day in the dry desert climate, while corresponding variation is comparatively slight in humid climates (compare page 86). This is the result of the ability of water vapour to arrest the radiation from the Earth. Carbon dioxide is about evenly distributed over the globe--although somewhat sparser over highlands--and its heat-conserving and equalizing influence is, therefore, not so manifest as that of moisture. Only by the most accurate investigations has this influence been demonstrated.