- Study Confirms That Urban Heat Islands Cause Downwind Rain
Residents of flooded rural areas near San Antonio, Texas, have one
more reason to complain about their city-dwelling neighbors: a new
study shows that the heat generated by cities helps produce
rainstorms downwind from cities. The study, led by the U.S. National
Aeronautics and Space Administration's Goddard Space Flight Center,
used a rain-measuring satellite to verify similar results obtained
in previous ground-based studies. The satellite data demonstrated
that major cities -- including Nashville, Atlanta, Dallas, and, yes,
San Antonio -- increased downwind rainfall by about 28 percent
within 18 to 36 miles from the cities. In some cities, the downwind
area rainfall increased as much as 51 percent. On average, maximum
rainfall rates in downwind regions exceeded the maximum values in
upwind regions by up to 116 percent.
Cities basically act as giant solar collectors: during summer
months, the dark roofs, concrete, asphalt, and other surfaces in the
cities absorb a large amount of heat. The resulting "heat island"
effect can boost urban temperatures by 5 to 9 degrees Fahrenheit.
During the warmer months, the added heat creates wind circulations
and rising air that can produce clouds or enhance existing ones.
Under the right conditions, these clouds can evolve into rain-
producers or storms. See the Goddard Flight Center press release at:
Urban heat islands have long been of concern, since they drive up
the use of air conditioners, increase ozone levels, and exacerbate
health problems. In fact, the city of Toronto hosted the North
American Urban Heat Island Summit in early May. The presentations
from that summit are now posted on the City of Toronto Web site at:
25840 IH-10 West #1
Boerne, Texas 78006