How tropical is the world?

“Rivers are the major conduits for the transfer of water, particulates and dissolved material from the land to the ocean and more than 50% of this flux enters the global ocean in the tropics” (*)

This sentence made me wonder, incidentally, what proportion of the world’s surface area is contained between the two tropics. A friend pointed me to two websites that answer the question.

First, someone who had wondered just the same thing: http://godplaysdice.blogspot.com/2007/12/how-much-land-is-in-tropics.html

And a list of formulae relating to spheres: http://mathforum.org/dr.math/faq/formulas/faq.sphere.html

The answer is surprisingly simple: the surface area of a sphere (radius ‘r’) that lies in between two parallel planes (‘h’ apart) = 2*pi*r*h !!

In the case of the tropics, h = 2*sin(23.5)*r … so the answer to the tropical surface area is approx. 0.398, or 39.8%, or about two fifths.

Next step (for the GIS) is to calculate the proportion of the land surface area that is tropical…

(*) PETER J HEDLEY, MICHAEL I BIRD, RUTH A J ROBINSON. (2010) Evolution of the Irrawaddy delta region since 1850. Geographical Journal 176:2, 138-149

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