Build Your Own Desert

The Mojave Desert, at a latitude of 35^o North

Copyright: Alice Bryant 2010

Consult a map of the globe and you’ll find two bands of desert sandwiching the rainforests that straddle the equator.  Their symmetry is no accident; nor is that of the temperate regions, half-way to each pole, where Britain (amongst others) enjoys its changeable weather.  The deserts and their neighbours are a manifestation of the temperature difference between the equator and the poles.  That such a difference exists in the first place is no surprise: The land at the equator faces the sun head on, whilst the icy tundra can spend months in perpetual darkness.  Warm air at the equator tends to rise, whilst cold air sinks at the poles; and in an idea put forward by George Hadley (an Eighteenth century barrister, and gentleman scientist), these were the two ends of a giant circulation, transporting polar air along the surface to the equator, and taking equatorial air to the poles in the upper atmosphere.  But there was a problem:  It didn’t explain the deserts, or the other patterns of weather found at different latitudes.

Though Hadley hadn’t quite cracked the global circulation, he was almost there when he described the trade winds, which blow across the seas close to the equator, as a manifestation of the Earth’s spin.  A century later, and his ideas had been developed to give a view that remains relatively unchanged to this day:

The three cells of the global circulation - Diagram from Met Ed.

The spin of the Earth complicates matters, creating three conveyors, or ‘cells’, in each hemisphere.  The Hadley cell starts at the equator, where warm air rises, creating clouds and rain as it expands and cools on the way up; that rain creates the rainforests that straddle the equator.  At around 30^o latitude, the air begins to sink, warming on the way down, dissolving any hint of clouds to create clear dry skies, and conjuring desert from the parched land beneath.

Above the deserts, the next of our three cells starts, with the descending air of the Ferrel cell turning Northward along the surface, until it starts to climb at 60^o latitude, creating more cloud and rain.  Here it meets the polar cell, where the constant battle between the two creates the turbulent weather of Northern Europe.  As its name suggests, the Polar Cell ends at the poles, where its cold air descends, creating more clear skies.

Embedded in these enormous currents are many subtleties, creating seasonal, regional, and daily changes in our weather; but when it comes to building a desert, it’s the global picture that counts.

Text Copyright: Christopher Lee, 2012
Photo Copyright: Alice Bryant, 2010