Facts of the Matter


More urban areas
mean more ‘heat islands’

The news this week that Tokyo's heat island is changing local weather patterns shouldn't come as a surprise. For nearly two centuries it has been known that cities become warmer than the surrounding countryside.

There are many examples of such urban heat islands worldwide. In fact, just about any city becomes a heat island when pavement replaces soil and vegetation; streets and sidewalks absorb the sun's energy, increasing daytime temperatures and releasing heat at night to raise nighttime temperatures as well.

The effects of the heat island are more than increased temperature, although that is the most apparent effect to the casual observer.

Weather statistics are even more telling. On the average there are nearly one-third more summer thunderstorms in cities compared with surrounding rural areas. Other weather elements that are changed include rainfall, relative humidity, frequency of cloudiness, fog and wind. The amount of change at any one of these elements at any time depends on many different factors, including the extent of the urban complex, heights of buildings, the types of industry, topography, proximity to bodies of water, time of day, season and current weather conditions, including wind.

There are several factors that contribute to the creation of an urban heat island in addition to the effects of heated pavement.

One is the quick removal of precipitation that would normally soak into the ground or evaporate to cool the surface. Heat generated from artificial sources such as industry, power generation, motor vehicles and domestic heating and cooling also play a significant role.

Increased atmospheric pollution inhibits the loss of upward-directed radiation from the surface, and tall buildings alter the flow of air and interfere with the exchange of heat between the atmosphere and surface.

A steady, strong wind can mediate the effects, but the speed of wind necessary to counteract the effects of the city is greater the larger the urban complex.

In many cases the magnitude of human-made energy released in metropolitan areas can be a significant fraction of the energy received from the sun. In Manhattan one study showed that during winter months the amount of heat produced from combustion alone was 2.5 times greater than the energy received from the sun. In the summer the figure was only about 16 percent, but in other cities it has been measured at up to one-third.

The problem is not due to population growth alone. As population has grown there has been a concomitant shift from rural to urban settings. In 1800 only about 2 percent of the world's population lived in cities. In 2000 nearly 3 billion people were city dwellers, representing about 40 percent of the world's people, and the urban migration is predicted to continue. The effects of urban heat islands will increase as population grows and cities get larger. Furthermore, as temperatures increase there tends to be a greater use of artificial cooling, which generates even more heat, which adds to the effects, makes the city warmer, and so on.

The measurable effects of heat islands are a warning and should serve as a model for the potential of global warning. As much disagreement as there is among scientists, politicians and environmentalists about whether global warming is a reality or merely a scientific theory, the potential for mankind's activities to induce climate change cannot be disputed.

It is a fact that average temperatures are increasing worldwide, regardless of cause. It is difficult to measure the heating effects on the earth as a whole since there are no rural planets surrounding the earth to serve as baselines the way that we compare the heat islands of cities to adjacent rural areas.

At present our human activities produce only a small fraction of the total heat received from the sun, but no one knows how much it takes to be significant, or how much it takes to alter the earth's climate overall. However much it is, the laws of thermodynamics cannot be amended. We speak of energy "consumption," but all of the energy we use ultimately winds up as heat, and that heat has to go somewhere.

It doesn't just disappear, but accumulates somewhere until it can be radiated away. The "somewhere" is the problem. There is no way to predict where it will go, how long it will take to dissipate, what weather phenomena it will precipitate or what the long-term effects will be.

The only thing certain is that any significant alteration of the heat budget will have some effect, somewhere.

We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at

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