What is an Urban Heat Island and How Does It Affect the Global Water Cycle?

It is estimated by the United Nations that by the year 2025, 60% of the world's population will live in cities. As cities continue to grow, urban sprawl creates unique problems related to housing, traffic, pollution, and land use. The growth of cities can also impact the weather, climate, and the global water cycle. A close look at the local weather maps on television everyday shows that cities tend to be 1-10ƒ F warmer than the surrounding suburbs and rural areas. The heat-retaining properties of these buildings, roads, and other surfaces (e.g. asphalt, concrete) contribute to the formation of this "urban heat island" effect. Less obvious from these maps, however, is that large cities also may "create" their own weather. Historical ground-based studies and recent satellite-based studies suggests that the urban heat island noticeably impacts summer rainfall over and downwind from major cities such as Atlanta, Dallas, San Antonio, Houston, Tokyo, and Nashville. If you live in certain parts of a major urban area, it may be more than your imagination that your part of the city gets more rain than other parts of the city.

The rainfall distribution may stem from the urban heat island effect because the heating creates a wind circulation that promotes rising air over the city. During the warmer months, the rising air can produce clouds or enhance existing ones. Under the right conditions, these clouds can evolve into rain-producers or storms. It is suspected that converging air due to rougher city surfaces (e.g. buildings) also enhances rising air needed to produce rainfall. Others have suggested that increased particles in the atmosphere from cars and smokestacks in cities contribute to more efficient cloud formation. Nevertheless, earlier studies using ground-based instruments around cities like St. Louis, Chicago, Mexico City, and Atlanta have shown that large cities can impact rainfall over and slightly downwind of metropolitan areas

A recent study by Marshall Shepherd and colleagues at NASA is one of the first published attempts (possibly the first) to identify rainfall modification by cities using space-based rain measurements. We utilize the world's first space-based rain radar aboard NASA's Tropical Rainfall Measuring Mission (TRMM) satellite to overcome the many limitations of ground-based observations and allow for investigation of urban rainfall in numerous cities simultaneously around the world.

By demonstrating the capability of space-borne platforms to identify rainfall changes linked to cities and urban sprawl, the research may allow for design of better drainage systems, planning of land-use, or identification of optimal areas for agricultural activity. Additionally, the results may alert meteorologists that urban surfaces must be considered in the sophisticated computer models that produce weather forecasts. Finally, the results further highlight the potential implications of human-induced change on the Earth System. Further information on Urban Heat Islands can be found at http://rsd.gsfc.nasa.gov/912/urban/. This informational website was produced by a high school student who spent the summer working at NASA. It provides useful links to information on the urban heat island.

Dr. J. Marshall Shepherd is a research meteorologist in the Laboratory for Atmospheres at NASA Goddard Space Flight Center. He specializes in mesoscale weather systems (e.g. rainfall systems, hurricanes, and thunderstorms) and remote sensing meteorology. He also serves as the Deputy Project Scientist for the forthcoming Global Precipitation Measurement (GPM) mission.