Nights are getting toastier more quickly in cities than afield

In another illustration of the urban heat island at work, U.S. climate data over the last four decades show that unusually warm summer nights are increasing in cities at a rate three times faster than in the country. A similar skewness appears in the rate of unusually cold nights, which are disappearing almost three times as quickly at urban as at rural stations.

The study by Arthur DeGaetano and Robert Allen (both of the Northeast Regional Climate Center, based at Cornell University) appeared last autumn in the Journal of Climate. Much previous work had shown that the overall U.S. warming over the last few decades was especially pronounced in cities. However, the findings on extreme temperature events—the ones that affect residents most dramatically—have been less conclusive, according to the authors. Despite keen interest, “the unavailability of high-quality daily data has led to inconsistencies in describing time-dependent changes in both warm and cold temperature extremes.”

Using a newly developed climatology tailored for extreme highs and lows, the authors examined data from 361 U.S. weather stations. They focused on the 5th and 95th percentiles of temperature—the warmest and coldest 5% of high and low temperatures in the long-term record. For example, in the New York City area the warmest 5% of nights (about 18 in a typical year) historically stay above 20°C (68°F), while the coldest 5% get down to about –11°C (12°F). The comparable values in the Dallas–Fort Worth region are 23°C (73°F) and –2°C (28°F).

In this study, supported by NASA and NOAA, DeGaetano and Allen found that urban areas across the United States now average roughly 10 more nights in the historic 95th-percentile range of warmth than they did in the 1960s. In rural areas the jump averaged 3 or 4 nights per year. Warming trends also showed up in extreme highs, although the urban-rural discrepancies are less marked for highs than for lows.

The authors also concluded that a rise of 0.5°C (0.9°F) in average summertime readings by 2050 would result in about six more highs and six more lows per year falling in the 95th percentile ranges. “If such a relationship proves to be robust,” they add, “it could facilitate the analysis of global extreme trends based on seasonal means.”

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