13.2: The Origin of Deserts
- Page ID
- 28302
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Atmospheric Circulation
The engine that drives circulation in the atmosphere and oceans is solar energy which is determined by the average position of the sun over the earth’s surface. Direct light provides uneven heating depending on latitude and angle of incidence, with high solar energy in the tropics, and little or no energy at the poles. Atmospheric circulation and geographic location are the primary causal agents of deserts.
The figure shows the generalized circulation of the atmosphere. There are three generalized circulating cells of rising and sinking air called the Hadley Cell, the Ferrel or Mid-latitude Cell, and the Polar Cell. Solar energy falling on the equatorial belt heats the air and causes it to rise. The rising air cools and its contained moisture falls back on the tropics as rain. The drier air then continues to spread toward the north and south where it collides with the Ferrel Cell and they sink back at about 30 degrees north and south latitudes. This sinking drier air creates belts of predominant high pressure along which desert conditions prevail in what are called the “horse latitudes.” These belts of predominantly high pressure have air that descends along these belts and flows either north to become the westerlies or south to become the trade winds (which is why deserts created here are called trade-wind deserts). These circulation cells in the atmosphere rising in the tropics and polar regions and sinking in the horse latitudes produce the desert belts along the horse latitudes at approximately 30 degrees north and south of the equator [2]. Note the arrows indicating general directions of winds in the latitude zones. The trade winds are predominant in the tropics and the westerlies in the mid-latitudes.
Other deserts have other atmospheric phenomena to owe (at least part of) their origin, like the desert of Utah, Nevada, and the surrounding areas called the Great Basin Desert [3]. This desert, while having some sinking air effects due to global circulation, is also a rain shadow desert produced as moist air from the Pacific rises by orthographic lifting over the Sierra Nevada (and other) Mountains and loses moisture from previous condensation and precipitation on the rainy side of the range(s).
One of the driest places on earth is the Atacama Desert of northern Chile [4]. This is a strip along the west coast of South America, west of the Andes, lying north of 30 degrees south latitude, at the southern edge of the trade wind belt. Warm moist air moves west across the Amazon basin and rises over the Andes where it loses moisture, its precipitation falling on the rain forest side of the mountains. Once over the mountains, it descends onto the Atacama where it meets air cooled by the cold Peru (Humboldt) ocean current flowing north along the coast. This is considered to be the driest (non-polar) place on earth with locations in the Atacama having not received any precipitation for periods of years [5].
Referring again to the figure above, note that the polar regions are also predominantly high-pressure areas of descending cold and dry air. Another circulation cell occurs there known as the Polar Cell [6]. Here, air not only descends convectively because it is cold, but cold air can hold much less moisture than warm air, and thus the driest and coldest places on Earth are the polar deserts. Antarctica is not only currently the driest land on Earth today, but any land that occupies the poles in Earth history should always be dry.