m mv + md where mv is the mass of water vapor in kg, m is the mass of the atmosphere in kg, and md is the mass of dry air in kg, for a specific volume. Specific humidity as the ratio of masses is dimensionless, but the convention is to express it in units of g of water vapor per kg of moist air.
Figure 7.6 shows the annual mean distribution of specific humidity at the Earth's surface. Other commonly used specific humidity expressions are the value for a layer of air between two pressure levels or the vertical-mean specific humidity for the entire atmospheric column. The quantities are different in each representation, but the general distribution patterns are similar. Several distinctive characteristics are evident in the global pattern of annual mean specific humidity at the Earth's surface. Two prominent features are the gradient of decreasing quantities from the equator to the poles and the ocean-land contrast at the same latitude.
High humidity in equatorial regions is expected due to the coincidence of abundant energy and moisture at these latitudes. High saturation vapor pressures in equatorial regions support the capacity of the atmosphere to retain large water vapor volumes. At high latitudes, the atmosphere is cool, saturation vapor pressures are lower, and the volume of atmospheric water vapor is low. A general pattern of zonal symmetry is evident in both hemispheres, but the
pattern is more regular in the Southern Hemisphere where the large ocean expanse presents a consistent surface that promotes expression of atmospheric moisture limited only by available energy to drive the upward flux. The presence of large land areas in the Northern Hemisphere disrupts the zonal pattern and produces a more complex scheme.
The most apparent influence superimposed on the pattern of decreasing specific humidity with increasing latitude is the contrasting effect of ocean and continental surfaces. As a general rule, specific humidity is greater over oceans than over continents at the same latitude. The high specific humidity values for the Amazon Basin in South America and the Congo Basin in Africa are major exceptions. The zonal Southern Hemisphere pattern due to ocean-dominated surfaces contrasts markedly with the Northern Hemisphere complex specific humidity pattern shaped by a combination of oceans and continents. Specific humidity isopleths are deflected near the western and eastern coasts of continents by topography and the presence of warm and cold ocean currents. Deserts in North Africa and Central Australia are evident by their low specific humidity relative to their respective zonal averages. The high-latitude expanse of North America and Eurasia contributes to a prominent reduction in specific humidity for the core regions of these continents (Peixoto and Oort, 1992).
Surface air generally contains more moisture during the high-sun season when available energy is most abundant, and expected seasonal changes in specific humidity driven by the energy balance are magnified by land-ocean differences. Specific humidity in January (Fig. 7.7) for a specific latitude is greater in the Southern Hemisphere than in the Northern Hemisphere. The relationship is reversed in July (Fig. 7.8). These patterns are most evident over the oceans where seasonal changes are smallest. January and July continental specific humidity changes are relatively small for areas near the equator, but seasonal differences are large at higher latitudes. Consequently, July specific humidity is 50% greater than January values over continental interior areas of the Northern Hemisphere. The Southern Hemisphere displays smaller seasonal changes over its continental area poleward of 20° S because these land areas are less expansive. Seasonal hemispheric water vapor exchanges related to seasonal atmospheric pressure changes are addressed in Section 7.6.
The vertical structure of specific humidity reveals a rapidly decreasing pattern with increasing altitude. More than 50% of water vapor is concentrated below the 850 hPa surface, and more than 90% is confined to the layer below 500 hPa (Peixoto and Oort, 1992). This pattern is expected due to the Earth's surface serving as the source of atmospheric moisture and the atmospheric drying with increasing height due to reduction in the saturation vapor pressure.
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