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An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surroundings. As population centres grow in size from village to town to city, they tend to have a corresponding increase in average temperature, which is more often welcome in winter months than in summertime. The EPA says: "On hot summer days, urban air can be 2-10°F [2-6°C] hotter than the surrounding countryside. Not to be confused with global warming, scientists call this phenomenon the 'urban heat island effect'" [1].

There is no controversy about cities generally tending to be warmer than their surroundings. What is controversial about these heat islands is whether, and if so how much, this additional warmth affects trends in (global) temperature record. The current state of the science is that the effect on the global temperature trend is small to negligible—see below.

Scientists compiling the historical temperature record are aware of the UHI effect, but they vary as to how significant they think it is. Some scientists (see Peterson, below) have published peer reviewed papers indicating that the effect of the UHI has been overestimated, and that it does not affect the record at all. Other scientists have used various methods to compensate for it. Some advocates charge that temperature data from heat islands has been mistakenly used as evidence for global warming.

As a result of the urban heat island effect, monthly rainfall is about 28% greater between 20-40 miles downwind of cities, compared with upwind.

Urban heat island
An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surroundings. As population centres grow in size from village to town to city, they tend to have a corresponding increase in average temperature, which is more often welcome in winter months than in summertime. The EPA says: "On hot summer days, urban air can be 2-10°F [2-6°C] hotter than the surrounding countryside. Not to be confused with global warming, scientists call this phenomenon the 'urban heat island effect'".

There is no controversy about cities generally tending to be warmer than their surroundings. What is controversial about these heat islands is whether, and if so how much, this additional warmth affects trends in (global) temperature record. The current state of the science is that the effect on the global temperature trend is small to negligible—see below.

Scientists compiling the historical temperature record are aware of the UHI effect, but they vary as to how significant they think it is. Some scientists (see Peterson, below) have published peer reviewed papers indicating that the effect of the UHI has been overestimated, and that it does not affect the record at all. Other scientists have used various methods to compensate for it. Some advocates charge that temperature data from heat islands has been mistakenly used as evidence for global warming.

As a result of the urban heat island effect, monthly rainfall is about 28% greater between 20-40 miles downwind of cities, compared with upwind.