ITHACA, N.Y. -- During last week's enervating hot spell in the Northeast, the discomfort was not entirely due to the heat or the relative humidity. The real culprit, say Cornell University climatologists, was the high dew point.

The dew point is the day-to-day measure of humidity in the atmosphere. Another critical measure is the "design dew point" -- the maximum humidity level at which air-conditioning systems can operate efficiently in different regions.

According to the Northeast Regional Climate Center (NRCC) at Cornell, in 13 major Northeast cities last week, high dew points exceeded the design dew points -- meaning that the humidity exceeded the engineering specifications of air-conditioning systems, causing them to operate inefficiently, use more energy and lessen their cooling effect.

Boston's design dew point is 72 degrees, but the dew point there on July 18 was 76 degrees. The high temperature that day was a seasonal 86 degrees, but when combined with the high dew point, the heat index (what it felt like) rose to 96 degrees. 

Meanwhile, the design dew point in Wilmington, Del., is 75 degrees, but the dew point on July 17 was 78 degrees. When that high dew point mixed with the day's high temperature (87 degrees), the result was a heat index temperature of 99 degrees. 

On July 18, Syracuse, N.Y., tied its all-time record high dew point temperature of 77 degrees, set July 4, 1999. That dew point boosted the 91 degrees high temperature to a heat index of 104 degrees. 

Not only has the Northeast seen high dew points, but they have persisted for weeks. So far this season, Albany, N.Y., 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. 

Energy researchers use the design dew point as a convenient and meaningful threshold, explained Dan Graybeal, a research climatologist at the NRCC. Design dew points for U.S. and world cities are published by the American Society of Heating, Refrigeration and Air-Conditioning Engineers. "The Northeast has had high energy demand," he said. "Exceeding the design dew point is an important factor in that high energy demand. The result is that, along with the high temperatures, air conditioning systems -- which are also dehumidifiers -- are being asked to carry quite a heavy load, even beyond their efficiency breakpoint," he said.

"Generally speaking, this is the Northeast's first hot summer in three years, marked by the return of the dominance of the Bermuda High," said Graybeal. The warm humid air flowing into the Northeast from the south has been exacerbated by muggy air created by moisture from decaying tropical storms -- Arlene and Dennis -- as they diminished over the Mississippi and Ohio river valleys. The storm remnants were carried over the region, bringing thunderstorms and high humidity. "These high dew points do not stem from local moisture sources, as much of the region has been drier than normal in rainfall this season," Graybeal said.

Relative humidity expresses the drying power of the air and is a percentage -- the ratio of the air's moisture content to how much moisture the air could sustain at its temperature. Because of this dependence on temperature, the relative humidity varies throughout the day, from high readings in the morning to low readings at midafternoon. 

The dew point expresses the day-to-day moisture content of the air in terms of a temperature -- that is the temperature at which condensation occurs. "Dew point can be visualized by considering a beverage container in a muggy room," said Graybeal. "If the beverage is cooler than the dew point of the air in the room, condensation occurs on the container. It's a familiar summertime condition."