The Philosophical Society of Washington

Minutes of the 2,195th Meeting

President Robert Hershey called the 2,195th meeting to order at 8:18 pm September 23, 2005. The minutes of the 2,194th meeting were read and approved.

Mr. Hershey introduced the speaker of the evening, Mr. J. Marshall Shephard of the National Aeronautics and Space Administration. Mr. Shephard is deputy project scientist of NASA's Global Precipitation Measurement Mission.


Mr. Shephard began with a picture of the earth at night. The big bright spots were the United States east of the Mississippi, Europe, and Japan. Smaller bright spots were the West Coast, Rio de Janeiro, and there are some smaller spots on the eastern rim of Asia and in western Asia, notably India. Most of Africa and China are black. There are small bright spots in various places in the world, mostly on the coasts. The population of the world is growing and those are the places where it is growing. By far most of the growth is in urban areas.

He showed a graph of the temperatures recorded since 1820 at West Point, New York and Central Park in Manhattan. The diversion of the lines is obvious. The challenge is the question of whether that represents global warming, the development of an urban heat island, or both.

Speaking of global warming, he said, what about global wetting? Global wetting is even more impressive, as he showed with a graph of precipitation in the United States showing an increase in precipitation from an average of about 2 mm a day in 1900 to almost 2.2 mm today. There has also been a definite increase in flood damage. Note, however, the precipitation increase has actually been since the 1950's. Until then, it actually decreased slightly.

Urban heat islands are as much as 3.5 C above surrounding rural areas. The heat rise is caused by several factors. One is urban surfaces which absorb more heat than they radiate. They also shed water, which results in less evaporative cooling. Black rooftops can be as much as 70 F higher than lighter surfaces. There is less vegetation in cities. Then, there are human activities, car exhausts, industrial processes, and so on.

A Landsat map of the Washington area showed that even conditions within a city count. The shape of the urban heat island of Washington is affected by the Agriculture Department's plant at Beltsville and perhaps even to a small extent by Rock Creek Park.

Urban heat islands have a number of effects. They often reduce the number of freezing rain events. There is an increase in smog, measured as ozone. There are disasters, for example a heat wave in 1995, exacerbated by Chicago's urban heat island, killed approximately 525 people. Heat, incidentally, kills far more people than cold or storms of any kind.

He mentioned two possible ways to reduce urban heat islands. One is strategic landscaping to shade buildings and for evapotranspiration. Another is albedo modification, primarily reflective surfaces on roofs and pavements.

Cities can produce their own rain and thunderstorms. This happens when urban heat drives moisture laden air masses upward and cools them. There are also increased levels of particles in the air which facilitate condensation. There are also times, though, when the urban heat island splits approaching systems and precipitation goes on both sides of the city. Satellite data have been particularly helpful in studying these effects.

A particularly interesting example of the effects was the Houston area, where the heat island seems to be modifying the sea breeze from the Galveston area. The Houston heat seems to draw the air farther inland where it rises over Galveston.

It has not been established whether the urban heat island effects are localized or if they affect weather more generally. This is a question that is driving planned research. It is known that there is more convection and mixing near cities, just as there is more convection and mixing over land than over the ocean. Increased aerosol and particulate levels also have effects. In general, there is a slight increase in storm frequency and intensity near cities.

Mr. Shephard looks forward to research that may answer questions about:

He also looks forward to developing partnerships with other organizations to produce a more powerful research model and improve the utilization of research knowledge.


Mr. Hershey encouraged guests to consider joining the Society. He made the usual parking announcement. Finally, he invited everyone to participate in the social hour, and he adjourned the 2,195th meeting at 9:44 pm.

Weather: Mild
Temperature: 17°C
Attendance: 42

Respectfully submitted,

Ronald O. Hietala,
Recording secretary


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