There is no limit to the varieties of forms in which precipitation can occur at a place on the earth's surface. It can occur in the form of rain, drizzle, shower, snow, sleet and so many diverse forms. The total amount of rain registered at a given place may consist of contributions from one or more of these various forms and may result from diverse atmospheric situations. There are also extensive areas on the earth's surface where little or no precipitation occurs at any time of the year, even if some clouds may appear at times. These latter areas are mostly the desert areas. In the tropics, rainfall varies greatly with season and between continents and oceans.
In January, climatological mean rainfall maxima are generally to be found in the southern hemisphere, usually centered over the continents of South America, Southeastern Africa, and the northeastern part of Australia, with little rainfall over the
(8)| JAN Precip Rate (mm/day) Xie Arkin CMAP (1979-2005)
(t>) JUL Precip Rate (mm/day) Xie Arkin CMAP (1979-2005)
0 60E 120e 180 i20w 60w 0
0 60E 120e 180 i20w 60w 0
Fig. 5.6 Climatological (1979-2005) rainfall (mmday-1) distribution over the globe during (a) January, and (b) July (Xie and Arkin, 1996) [Courtesy:NCEP/NWS]
oceans except the extreme southwestern parts of the major oceans, while the fields are reversed in the northern hemisphere where the continents are usually dry and, perhaps, more rain occurs over oceans than over land. Whatever rain falls over the continent and oceans is usually brought about by the passage of baroclinic waves.
In July, there is greater rainfall over the continents than oceans in the northern hemisphere, while the fields are reversed in the southern hemisphere. Over the globe as a whole, the belt of rainfall maxima appears to follow the seasonal movement of the Intertropical Convergence Zone (ITCZ). These aspects of the seasonal distribution of mean rainfall over different parts of the globe are brought out by maps of climatological rainfall (Fig. 5.6 a, b).
However, a proper interpretation of the observed distribution of rainfall at the earth's surface is not easy, as it requires consideration of several physical and dynamical factors, characteristic of the region. Qualitatively, evaporation from the underlying surface, net moisture convergence into the overhead vertical column and its lifting by the atmospheric circulation and orography are amongst the more important parameters to determine the amount of precipitation likely to occur at a place.These factors are related by the well-known water balance equation where P denotes precipitation, E is evaporation, q is specific humidity of the air, V is three-dimensional wind vector and p is pressure with po at the surface. The integration is from surface to top of the atmosphere.
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