Rethinking the Cooling Degree Day

Researchers at the University of Hawaiʻi at Mānoa say the standard tool used to estimate air-conditioning demand, called cooling degree days, has been getting things wrong for decades. That measure only tracks temperature, but real cooling systems also have to work harder in humid air, since they must wring moisture out along with lowering heat. The team built a new version, called effective cooling degree days, that factors in both temperature and humidity, based on how actual refrigeration systems behave.

Applying it to 50 years of weather data, they found cooling efficiency across North America has dropped 2% to 4% per decade since 1971. The effects vary widely by climate: in dry regions like the desert Southwest, easier cooling can offset the penalty of higher heat, so efficiency holds steady or even improves. In humid regions, heat and moisture compound each other, making the true energy burden worse than temperature alone would suggest. Looking ahead, using climate projections through mid-century, the researchers found the Northwest, Great Lakes, and Mid-Atlantic face the steepest increases in cooling demand, with some power grid regions potentially seeing cooling-related electricity demand more than double under a high-emissions scenario.

For homeowners, this doesn't change anything about your own equipment or bills today, but it matters for how utilities and grid planners prepare for future summers, especially if you live somewhere humid or in one of the regions flagged for rising demand. It's a sign that the assumptions behind future electricity planning, and possibly future strain on the grid during heat waves, may need updating as the climate changes.