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The Latitudinal Profiles
of Precipitation and Evaporation

A world map of climates reveals to two bands of deserts. In the northern hemisphere this band is at the latitude of the Sahara Desert and continues into the Arabian Peninsula and on throught Persian Gulf area and into northtern Pakistan and northwest India. On the other side of the Pacific Ocean it continues across northern Mexico and the U.S. states of Arizona and New Mexico.

In the southern hemisphere the belt of desert includes the Namib and Kalihari Deserts in southern Africa and most of Australia. Across the Pacific it spans northern Chile.

Other factors bysides latitude such as topography and altitude are important in determining the existence of deserts but clearly latitude is a factor.

The biological state of land is determined by the level of its moisture. This in turn is determined by the net flow of water. Two prime determinants of this net flow is precipitation and evaporation. The purpose of this material is to put together information on the latitudinal profiles of precipitation and evaporation and correlate these with the latitudinal profile of temperature so some estimation may be made of the shifts in desert areas as a result of global warming.

Precipitation

The graphs shown below give estimates of the precipitation profiles for land. One is for the northern winter-southern summer, December, January and February.

Several features are notable. Precipitation is at a peak near the equator, probably at the Intropical Convergence Zone (ITCZ). Away from the equator precipitation drops off to a minimum in the region of the 20 to 30 degree band. From there precipitation rises and then falls off to near zero at the poles. It is notablel that the profile is not symmetrical with respect to the equator and that the southern hemisphere has a greater peak in precipitation.

The information for the southern hemispheric winter (June, July and August) is similar but with the ITCZ shifted.

The precipitation on the land is affected by topography. It is better to look at the precipitation over the ocean to find the effect of latitude on precipitation. The diagrams for the ocean only precipitation are shown below.

The ocean-only precipitation profile is more symmetric than the land-only profile but it is still asymmetric and there is still a more prominent peak in the southern hemisphere.

Evaporation

There are three processes involving the transfer of water from Earth's surface into the atmosphere. There is evaporation per se which is the conversion of liquid water into water vapor. There is also sublimation the conversion of solid water as ice and snow into water vapor. And finally there is transpiration the transmission of water from plant surfaces into water vapor. In the material below all three will be referred to simply as evaporation.

In understanding evaporation it is the potential evaporation which is crucial. The actual evaporation may be limited by the surface availability of water. Two processes could be at work in evaporation. In the layer of air closest to the surface, the layer known as the laminar viscous layer, the transfer process is by molecular diffusion. In this layer Fick's Law applies; i.e., the rate of evaporation per unit area E is proportional to the vertical gradient of specific humidity q. This is usually expressed as


E = −ραW(∂q/∂z)
 

where ρ is the density of water and αW is a dimensionless coefficient. This diffusion process is less significant than the transfer by turbulence. This transfer by turbulence could be proportional to the product of the fluctuations in specific humidity and the vertical wind velocity. This would be eddy diffusion and processes of this type are often assumed to follow a law identical in form to the one for diffusion but with the coefficient, called the eddy diffusivity coefficient, being greater in magnitude. This approach to eddy diffusion was popularized by the aerodynamicist Prandtl. However the empirical relationship that has been more useful is:


E = −ρCW|v(zs|[q(zs)−q(z0)]
 

where zs is a standard height above the surface, such as 10 meters, and |v(zs)| is the wind speed at that height. The coefficient CW is called the bulk transfer coefficient for water vapor. The value of CW is not well established but CW=0.0013 has been used in empirical work.

Peixoto and Oort used the above formula and value for CW to compute the latitudinal profile of potential evaporation shown be below. In effect, their estimate is for evaporation over the ocean and large lakes.

(To be continued.)

For more on the mechanism of evaporation see Evaporation.


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