From the Babbage blog at The Economist:
“the Prineville plant is a leading exponent of a new style of data-centre management. It does away with expensive air-conditioning “chillers”. Instead, air is brought in from outside. For this approach to work, however, the desert is key. For much of the year outside air is actually cool enough to keep the servers from overheating. At the lowest temperatures, just the gentlest of breezes needs to be brought inside at all. And, this being the desert, nights are chilly irrespective of the season, so even in the summer additional cooling is only needed during the hottest times of day.This is provided through a “swamp cooler”. A fine mist of water, much like artificial fog, is sprayed in the direction of the air flow. Heat is leached from the air by the water as it evaporates, and the cooled air passes through the servers.”
These swamp coolers sound like a high tech version of those noisy, but divinely comfortable air coolers used in Delhi in the summer.