SYRACUSE, N.Y. (AP) – Imagine. It’s even too hot for the air conditioners.

Climatologists at the Northeast Regional Climate Center at Cornell University reported Tuesday that dew points – the day-to-day measure of humidity in the air – were so high last week that in 13 major Northeast cities they exceeded the “design dew point,” or maximum humidity level at which air conditioning systems can operate efficiently.

“Air conditioners have design capacities for temperature and humidity. If one is exceeded but not the other, it’s not a problem,” said Larry Spielvogel, who runs a consulting engineering business in suburban Philadelphia that advises building owners on air conditioning systems.

“It’s when both exceed the design capacity, that’s when you have a problem,” said Spielvogel, who also helped develop national energy conservation standards for the American Society of Heating, Refrigeration and Air Conditioning Engineers.

The society publishes design dew points for U.S. and world cities. When both temperature and humidity barriers are breached, air conditioners use more energy to run less efficiently with reduced cooling effect, Spielvogel said.

Temperatures soared past 100 again in several cities Monday and Tuesday, and the National Weather Service posted excessive heat warnings and advisories from Illinois to Louisiana and from Nebraska to the District of Columbia.

The blistering heat has caused numerous deaths this summer. In the Phoenix area alone, 24 people, most of them homeless, have died.

During July’s extended hot spell, design dew points were exceeded in Albany, N.Y.; Boston; Buffalo; Burlington, Vt.; Charleston, W.Va.; New York City; Newark, N.J.; Philadelphia; Portland, Maine; Providence, R.I.; Syracuse; Washington, D.C.; and Wilmington, Del., said Dan Graybeal, a research climatologist at the regional climate center.

The dew point expresses the day-to-day moisture content of the air in terms of a temperature at which condensation occurs, Graybeal said.

Not only has the Northeast seen high dew points, but they have persisted for weeks. So far this season, Albany and Pittsburgh have set new records for the number of days with 70-degree or higher dew points: Albany with 20 days, and Pittsburgh with 26 days.

“The Northeast has had high energy demand,” Graybeal said. “Exceeding the design dew point is an important factor in that high energy demand. “

Graybeal said this has been the Northeast’s first unusually hot summer in three years. The high humidity has been caused by the return of the dominance of the Bermuda High. The warm humid air flowing into the Northeast from the South has been exacerbated by muggy air created by moisture from decaying tropical storms as they diminished over the Mississippi and Ohio river valleys.

There are two types of air conditioning systems – one that uses chemically refrigerated air and another that uses water, said Johnny Johnson, an engineer for Carrier Corp., the world’s largest makers of air conditioners. Both can be affected by the combination of high temperatures and excessive humidity, but more so water-cooled systems.

Spielvogel said most homes use air-cooled systems while larger buildings – factories, malls and libraries, for instance – use water-cooled systems and large industrial chillers.

“Water-cooled systems work by rejecting heat by evaporating water. Obviously, when the air is saturated with moisture, the water is more difficult to evaporate and the machine must work harder,” said Spielvogel, who said homeowners can lessen the stress on their home units through simple steps, such as by keeping curtains drawn to reduce the greenhouse effect of the sun shining through windows.

Johnson recommended air conditioner owners also make sure their units have adequate levels of refrigerant and that they have the proper capacity for the size of area to be cooled.

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On the Net:

Northeast Regional Climate Center: http://www.news.cornell.edu/stories/July05/NRCC-dewpoints.bpf.html

American Society of Heating, Refrigeration and Air-Conditioning Engineers: http://www.ashrae.org

AP-ES-07-26-05 1629EDT