October 2, 2002

Hot City Nights More Frequent

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As a long hot summer comes to a close, Cornell climate researchers are finding evidence that supports many suspicions and complaints about city heat. The number of hot nights, especially in urban areas, is on the rise according to a recent study.

“Over the past 40 years, there has been an average annual increase of three hot nights in rural areas and an increase of ten hot nights in urban areas,” said Arthur DeGaetano, a researcher at Cornell’s Northeast Regional Climate Center. “The urban heating represents a 300-percent increase over the rural heating.”

Differences in land use are partly responsible for the discrepancy between urban and rural nightly extremes. Bricks and concrete hold heat longer than grass and trees, preventing heat from escaping city environments, according to DeGaetano.

Very warm nighttime temperatures are of interest because they pose the biggest human health impact, he said. Heat waves can be especially potent when city residents receive no relief from daytime heat at night.

“We found that the pattern of very warm temperatures followed a ‘V’ shape with lots of extremes in the 1930s and in recent times,” DeGaetano said.

These warm periods are linked with eras of drought that pivot around a turning point in the 1960s. DeGaetano argues that the drought-like conditions of cities, which have less exposed soil, trees and evaporation, might also be a contributing factor to their accelerated warming.

“Since the 1960s, we’ve seen a sharp change in the number of positive temperature trends. While this urban effect could account for some of the heating, global or [carbon dioxide]-induced warming could be responsible for the significant warming trend seen at rural stations. The effect is just exaggerated in cities because of land use,” he said.

In the study, “hot” is defined as being in the 95th percentile of nightly temperatures relative to a long-term average. Urban and rural classifications are made by using satellite estimates of artificial light from the earth’s surface.

“It is really important when you are dealing with trends to know how the environment is also changing,” said Brian Belcher, a research support specialist for the climate center. “One thing that we don’t really know is how the areas around the stations have changed.”

Since satellite data is either limited or unavailable for earlier decades, it is difficult to classify reporting stations as urban or rural for a specific period in the past, Belcher said.

Disentangling the two is important to climatologists who want to make a distinction between regional warming trends caused by urbanization and global trends more characteristic of human-induced changes in atmospheric composition.

Future research will focus on correlating the rate of increase in extremes with the rate of urbanization.

“If you look at a station that has been urban throughout the period, such as New York’s Central Park, you will find the increases to be similar to the flatter trend of a rural station. We find the real increases in those areas that have rapidly developed over the same period,” DeGaetano said.

Changes in the daily range of temperatures will also be examined. Preliminary evidence suggests that daily low temperatures in urban areas are increasing faster than daily highs. This results in a lower temperature range, making cities less variable than rural areas.

The findings of the research, funded by NASA and the National Oceanic and Atmospheric Administration, will be published in the upcoming issue of the Journal of Climate.

Archived article by Philip Lane