Water aloft: fluid mechanics of the lower atmosphere

Abstract

On account of the short residence time and great mobility of water vapor in air, the lower atmosphere is one of the critical pathways in the global hydrologic cycle; it transports water and energy around the globe without regard to continental boundaries and thus links the continents, the upper atmosphere, and the oceans. The transport and distribution of water vapor in the lower atmosphere, where it is most abundantly present, are among the main factors controlling precipitation and evaporation from the surface; these processes, in turn, determine soil and groundwater storage and the different runoff phenomena. WATER VAPOR IN AIR Global features The global amount of water vapor contained in the air is roughly (see Table 1.3) equivalent with a layer of liquid water covering the earth, with a thickness of around 25 mm on average. The thickness of this layer, which is the total liquid equivalent of water vapor in the atmospheric column at a given location, is also called the precipitable water , W p . However, this quantity of water vapor is not distributed uniformly and it can greatly vary over a wide range of scales in space and in time. For instance, the water vapor content of the atmosphere, just like the temperature, generally tends to decrease with increasing latitude. Available data (Randel et al. , 1996) show that the precipitable water is more likely to be well below 5 mm near the Poles, and close to 50 mm near the Equator.