In the mind's eye, the desert is a place of sand. The word triggers images of undulating dunes and fractious camels, of Rudolph Valentino with Arabian sheiks, the futile grandeur of Shelley's Ozymandias, king of kings, or the bacchanalian despair so vividly portrayed by Edward Fitzgerald in his Rubaiyat:
Think, in this batter'd Caravanserai
Whose Portals are alternate Night and Day,
How Sultan after Sultan with his pomp
Abode his destined Hour, and went his way.
But there are other kinds of desert - deserts on which no rain falls, like those above, but which are nonetheless, quite paradoxically, awash with limitless supplies of water. There are the deserts of the sea. Over each hemisphere of the Earth, between latitudes 25 and 45 or thereabouts, are two vast semi-permanent belts of high pressure girdling the globe. They are separated by the rainy equatorial zone of low pressure, the doldrums, and they move northwards and southwards with the seasons, being about 5 of latitude closer to the equator in winter than they are in summer.
It is the periodic, and it must be said anomalous, extension of one of these, the Azores High, that gives Ireland a brief taste of summer nearly every year. These zones of high pressure are, in general, areas of light winds and gentle subsidence. The subsiding air is warmed by compression as it sinks, resulting in a very low humidity, so rain-bearing clouds are comparatively rare, the weather normally is fine and sunny, and land areas beneath experience a very arid climate. It is not surprising, therefore, that most of the world's great deserts lie in the latitudes affected by these semi-permanent highs. They include, in the northern hemisphere, the Sahara of north Africa and its extensions into Saudi Arabia and the Middle East, and the deserts of the south-west United States and adjacent parts of Mexico; in the southern hemisphere are the Atacama of Chile, the Kalahari of south-western Africa, and vast tracts of inland and western Australia.
But this aridity does not confine itself to land. There are also vast expanses of ocean under these high-pressure zones where rain of any consequence is almost non-existent. Decades may go by with hardly any precipitation in that part of the South Pacific stretching westwards from Peru and northern Chile, a corresponding area to the west of Angola in southern Africa, and in the northern hemisphere a region stretching westwards from the Sahara out into the Atlantic.
These are the great deserts of the sea.