America alone has more than 7,000 data centres, according to IDC, a market-research firm. And each is housing ever more servers, the powerful computers that crunch and dish up data. In America the number of servers is expected to grow to 15.8m by 2010—three times as many as a decade earlier.

As space gets tight and energy costs climb, many firms have begun consolidating and simplifying their computing infrastructure. Hewlett-Packard, the world's biggest computer-maker, for instance, is replacing its 85 data centres across the world with just six in America.

Internet firms, meanwhile, need ever larger amounts of computing power. Google is said to operate a global network of about three dozen data centres with, according to some estimates, more than 1m servers. To catch up, Microsoft is investing billions of dollars and adding up to 20,000 servers a month.

Microsoft's $500m new facility near Chicago, for instance, will need three electrical substations with a total capacity of 198 megawatts. As a result, finding a site for a large data centre is now, above all, about securing a cheap and reliable source of power, says Rich Miller of Data Center Knowledge, a website that chronicles the boom in data-centre construction.

Microsoft needs a lot of choice: if a new service suddenly becomes popular, it needs to be able to increase computing capacity quickly. That is also why it uses shipping containers pre-loaded with up to 2,000 servers, which can be up and running within hours.

Yet it will not just be market economics that determines the shape of the clouds. Local governments give tax breaks in the hope that the presence of big data centres will attract other businesses (the computing plants themselves usually employ only a few dozen people). Regulation is a factor, too. SWIFT, a bank-transfer consortium, has announced plans to build a data centre in neutral Switzerland, so that data collected in Europe will not be stored in an American facility, where it could be subpoenaed by the United States government.