Heat rising from cities appears to be changing regional weather patterns
©2000 San Francisco Chronicle
March 6, 2000
Jim Doyle
For millennia, humans have thought of the weather as an immutable force of nature, impervious to human influences. Increasingly, however, scientists are finding evidence that growing human populations may be altering the climate.
The best example is the controversial theory that burning fossil fuels is warming the atmosphere through the so-called greenhouse effect. Many scientists believe this warming could touch off widespread disruptions in global climate and cause sea levels to rise.
But humans also are influencing the weather in other ways and on a more local and regional scale, say researchers from the National Aeronautics and Space Administration who are studying how suburban sprawl has created ``urban heat islands.''
New satellite observations and computer models show that urban areas have more effect on local climates than geographers and meteorologists had realized. In heavy populated areas along the Eastern seaboard, the heat island effect can alter weather on a regional scale.
A NASA research team presented its latest findings on urban heat islands last month at the American Association for the Advancement of Science annual meeting in Washington, D.C. The group's working assumption: If you change land use and land cover, you will affect weather and air quality.
Dale Quattrochi of NASA's Marshall Space Flight Center in Huntsville, Ala., said that large cities have air temperatures from 3 to 12 degrees warmer than adjacent areas. In the summer, these ``domes of heated air'' often create their own thunderstorms.
The urban heat island is a well-documented phenomenon. In the early 1800s, British researcher Luke Howard found London temperatures to be significantly higher than those of the surrounding countryside.
But without the aid of sophisticated tools, scientists had no clue as to how large or long-lasting these effects were, or whether they occurred during winter.
Scientists have found that the sun's energy heats up the nonreflecting surfaces of an urban area throughout the day. Heat from pavements, buildings and rooftops is then radiated back into the atmosphere until a few hours after sunset.
In sprawling cities such as Phoenix, Atlanta and Chicago, the heat island effect can put additional stress on the human body, exacerbate air pollution and contribute to poor health.
Heat is a major element in the production of ozone from automobile exhaust and other gases, for example. Ozone is the major component of smog, and high ozone levels can cause asthma attacks and other respiratory ailments.
For that reason, air quality regulators are looking for ways to mitigate the heat island effect through the planting of trees, which not only provide shade for a city's surfaces but also give off water vapor, which creates more humidity that cools the air.
They are also studying whether East Coast rooftops and streets, which tend to be black, can be made to be more reflective.
The NASA team is currently focusing on Atlanta. The U.S. Environmental Protection Agency has also financed similar studies in Baton Rouge, Salt Lake City and Sacramento. EPA scientists, who hope that cooling cities will reduce air pollution, are also studying satellite data from Los Angeles, Phoenix, Tucson, Chicago, Nashville and Houston -- the nation's most heavily polluted city.
In 1996, the National Weather Service made a fascinating discovery when it installed meteorological and air quality sensors in and around Atlanta to predict weather for the Summer Olympics: The heat island effect helps to create thundershowers downwind of the city.
It happens because the heat island causes a low-pressure system to form over Atlanta. This system sucks in cooler air, which grows warmer and rises, forming rain-bearing clouds. The result: thundershowers at strange hours, from 2 a.m. to 6 a.m.
NASA scientists have also found that large storms approaching Atlanta are often broken apart by the rough textures of the city's buildings. The storm clouds skirt the central city before regrouping and dumping their rainfall on the suburbs.
Last summer, Atlanta -- which has the longest average daily commute of any urban area -- had 62 straight days of ``high ozone'' events. NASA computer models show that if Atlanta's temperatures are lowered by 2 to 4 degrees Fahrenheit, its production of ozone will be reduced by 10 to 14 percent.
One of the biggest mysteries being studied by the NASA team is whether urban heat islands actually create puffy cumulus clouds.
``There are more clouds inside the city than outside the city,'' said NASA team member Stan Kidder, a cloud modeler based at Colorado State University in Fort Collins. He is using computer models to study how smaller, nonstorm clouds are formed and the effect these clouds have on urban heat islands.
Such clouds may have beneficial effects: They shade the Earth's surface, preventing temperatures from increasing as much as they would and blocking the solar radiation that helps to make smog.
``If you live in Atlanta, clouds are your friends. They tend to decrease the ozone and decrease the temperature,'' he said. ``My hypothesis: If you change land use, you change cloud formation.''
Analyzing remote sensing data from satellites and airplanes, the NASA team also has studied the expansion of Atlanta and other cities. Between 1973 and 1992, Atlanta's growth devoured 380,000 acres of trees -- an average of 55 acres a day.
Scientists believe that Salt Lake City may have created the opposite effect -- an ``urban cool island'' with reduced temperatures -- by planting vast amounts of water-guzzling East Coast trees.
The space agency and other scientists are working with tree planting organizations in Atlanta and other cities, including Sacramento, which has begun a project to install rooftop gardens on its major buildings as a cooling influence.
Quattrochi, the NASA team's principal investigator, said the urban heat island effect is ``a potential contributor'' to global warming.
He points out that urban areas cover about 3 percent of the land mass in the United States, and that that percentage is rising. By the year 2025, approximately 80 percent of the world's population is expected to live in megacities.
``The larger question is whether urban heat
islands have a cumulative impact on global warming,'' he said.
``We cannot say for sure, but it is obvious they do have an
effect on weather in local and regional areas.''
--There are far fewer trees and other plants to cool the air and provide shade.
--Buildings and dark pavements that absorb the sun's rays increase the temperature of the surrounding air.
-- Waste heat from buildings cars, and trains can contribute as much as a third of the heat received from sunlight.
-- The canyons created by tall buildings trap solar energy, raising air temperature.
Many cities have become "heat islands," soaking up radiant energy and trapping heat like a sponge holds water. NASA researchers are studying the effect on Atlanta, one of the fastest growing U.S. cities.
Data was collected May 11 and 12, 1997. While daytime air temperatures on that date were only about 80 degrees, surface temperatures reached as much as 118 (F)
In the past 17 years, urban growth in Atlanta has spread and blossomed, but not without dramatic changes to the surrounding area. In this visualization, red and orange points indicate areas of highest urban growth. Researchers assembled data from Landsat satellites in the early 70's to the late 90's.
Source: NASA and Chronicle research
Chronicle Graphic
-- Smog
The mixture of airborne pollutants that cause smog is likely to intensify at higher temperatures.
-- Increased Energy Consumption
Higher temperatures lead to the increased use of air conditioning, driving up energy consumption.
-- Air Pollution
Increased energy consumption means more greenhouse gases and pollutants are produced at electric power plants.
-- Increased Vegetation
Aggressive urban planting programs can ultimately reduce cooling costs by an average of 10-20%
-- Cooler Roofing Materials
Buildings with light-colored roofs reflect more sunlight. A new rating system called the solar reflective index (SRI) is being developed for roofing materials.
-- Cooler Pavements
Now there are materials available for roads and parking lots that reflect more sunlight and stay cooler, lasting longer.
Trees cool through evaporation of water on leaf surfaces
The transformation of Los Angeles from an area predominantly covered with orchards into a built-up metropolis illustrates the evolution of a modern-day urban heat island. Using computer simulation, scientists at the Berkeley National Laboratory added trees and reflective surfaces in about 15% of the possible areas in the L.A. basin. Results showed a 6 degree drop in summer temperatures at 3:00 p.m. and an overall 10% reduction in smog, the equivalent of three to five million less cars on the road.
Chronicle Graphic Sources: Lawrence Berkeley National Laboratory, University of Wisconsin Cooperative Institute For Meteorological Satellite Studies