Air-conditioning demand forecasts across North America may be built on a yardstick that ignores one of the biggest things making hot weather miserable: moisture. Two atmospheric scientists at the University of Hawaiʻi at Mānoa have developed a replacement that folds humidity and refrigeration physics into the calculation, finding that cooling efficiency has declined 2% to 4% per decade since 1971.
The new metric, called effective cooling degree days, is meant to show how much work cooling equipment actually has to do under different temperature and humidity conditions. UH News reports that Jake Casselman and Christina Karamperidou built it by combining climate science with refrigeration engineering, after finding that the older method can overstate cooling needs in some regions and understate them in others.
Why The Old Cooling Yardstick Falls Short
Cooling degree days are usually calculated from how far a day's average temperature rises above 65°F, with the total used as an indicator of air-conditioning energy requirements. The U.S. Energy Information Administration describes the measure as a standard energy-analysis tool, but the calculation does not directly account for the extra work needed to remove moisture from indoor air.
In the new approach, a simplified refrigeration-cycle model estimates how equipment efficiency changes as outdoor conditions shift. The study applied the metric to 50 years of high-resolution weather data from 1971 through 2020, then tested future projections from 19 climate models under a high-emissions scenario, according to Nature Communications.
Dry Heat And Humid Heat Pull The Map Apart
The regional result is less intuitive than a simple hotter-is-worse map. UH News says the desert Southwest could see steady or improving cooling efficiency in some locations as air becomes drier, while humid regions face a larger burden because air conditioners must lower temperatures and wring out moisture. Its high-emissions projections also put the Northwest, Great Lakes and Mid-Atlantic among the regions facing the steepest increases, with some grid areas potentially seeing cooling-related electricity demand more than double by mid-century.
The paper projects effective cooling degree days rising 10% to 80% across North America, while hot extremes in humid climates amplify efficiency losses and dry heat can partly offset them, Nature Communications reports. That split is why the researchers say temperature-only analysis can miss where the grid will actually feel the greatest strain.
Why Utilities May Care About A Better Forecast
Cooling degree days are not just a weather statistic: they feed estimates of air-conditioning demand used in energy analysis, and unusual summer heat can move electricity consumption and costs. The U.S. Energy Information Administration uses the measure in its energy outlook work, underscoring why a biased estimate can ripple into forecasts for generation, transmission and peak-load planning.
The practical takeaway is blunt: planners may need to estimate not only how hot a place gets, but how hot and humid conditions change the machines doing the cooling. For Honolulu researchers, the next generation of cooling forecasts is less about counting hot days and more about measuring the full workload behind every one.
