Rey one of these out:

http://www.everestbag.com/cat_messenger.html

I too live in a relatively hot and dry environment, and I use a messenger bag everyday. It doesn't make me sweat, and it is more comfortable and convenient than a shoulder bag.
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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)