While local officials dispense advice about how the elderly and poor can try to keep cool and some districts open refuges in empty schools, the air-conditioned middle classes suddenly find themselves vulnerable, too.
Power companies are reducing the voltage, pleading with consumers to turn down coolers, and warning that the grid is becoming dangerously overloaded. As a result, the temperature is rising fast in the political arena as well and "energy deregulation", so long a background issue, is flaring up again.
The first harbinger of trouble was an 18-hour power cut three weeks ago in Upper Manhattan. Rich and poor alike found themselves without cool or light. Columbia University lost priceless laboratory experiments. Although he was not directly at fault, the outage was seen as a rare black mark against Mayor Rudolph Giuliani for administrative efficiency.
The Manhattan power cut has not been repeated, but neighbouring New Jersey has been subject to "rolling" power cuts. And the more that is known about these north-eastern power outages, the less they seem to derive from abnormal temperatures.
The preliminary inquest on the Manhattan cut showed that power cables in that part of the grid have been the least reliable in the Consolidated Edison company's system for years. It also showed that the surplus capacity the company is supposed to maintain for just such eventualities had been whittled down far below the required level.
From across the country came similar reports of repairs not made, of old, coal-burning power stations being put back into service - additional air pollution notwithstanding - of a generating capacity that has actually declined in the last five years in the face of growing demand. The reason? The impending deregulation of the energy industry across the US.
Across America the big political divide is not whether energy should be deregulated, but how. Until the "rules of the road" are finalised and legislated, says Dan Lashof of the watchdog Natural Resources Defence Council, power companies will be unwilling to invest in new plant. Investors want to know where the best tax breaks will be before committing their money; and existing power companies want to shape the new structures. Meanwhile the bills wind their tortuous way through Congressional committees.
So long as White House air conditioners continue to whirr, President Clinton - who would like deregulation to be an accomplishment of his second term - can afford to sit back and let the political temperature rise. His adviser on deregulation, Richard Glick, said this week almost cheerfully: "We're going to see a couple of tough summers." The hope seems to be that the worse things get, the more pressure will mount on Congress to legislate.