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 eventsthe
ones that affect residents most dramaticallyhave 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 temperaturethe 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 DallasFort 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.