An urban heat island is a metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak. The main cause of the urban heat island is modification of the land surface by urban development which uses materials which effectively retain heat. Waste heat generated by energy usage is a secondary contributor. As a population center grows, it tends to expand its area, and increase in its average temperature. How far away does this effect persist? Does it significantly contribute to global warming?
An urban heat island is a metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak. The main cause of the urban heat island is modification of the land surface by urban development which uses materials which effectively retain heat. Waste heat generated by energy usage is a secondary contributor. As a population center grows, it tends to expand its area, and increase in its average temperature. How far away does this effect persist? Does it significantly contribute to global warming?
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The heat generated by everyday activities in metropolitan areas has a significant enough warming effect to influence both the local area and the character of the jet stream and other major atmospheric systems during winter months, according to a trio of climate researchers.
Other researchers have also found an impact on local weather from a warm urban area. Aside from the effect on temperature, urban areas can produce secondary effects on local meteorology, including the altering of local wind patterns, the development of clouds and fog, the humidity, and the rates of precipitation. The extra heat provided by the urban area leads to greater upward motion, which can induce additional shower and thunderstorm activity. In addition, the urban area creates during the day a local low pressure area where relatively moist air from its rural surroundings converges, possibly leading to more favorable conditions for cloud formation. Rainfall rates downwind of cities are increased between 48% and 116%. Partly as a result of this warming, monthly rainfall is about 28% greater between 20 miles to 40 miles downwind of cities, compared with upwind.
Led by Guang Zhang, a research meteorologist at Scripps Institution of Oceanography, UC San Diego, the scientists report in the journal Nature Climate Change that the extra heat given off by Northern Hemisphere urban areas causes as much as 1 degree C (1.8 degrees F) of warming in winter. They added that this effect helps explain the disparity between actual observed warming in the last half-century and the amount of warming that computer models have been able to account for.
"What we found is that energy use from multiple urban areas collectively can warm the atmosphere remotely, thousands of miles away from the energy consumption regions," said Zhang. "This is accomplished through atmospheric circulation change."
There are several causes of an urban heat island. The principal reason for the nighttime warming is that buildings block surface heat from radiating into the relatively cold night sky. Two other reasons are changes in the thermal properties of surface materials and lack of evapotranspiration (for example through lack of vegetation) in urban areas.
Other causes are due to geometric effects. The tall buildings within many urban areas provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. Waste heat from automobiles, air conditioning, industry, and other sources also contribute.
The release of waste heat is different from energy that is naturally distributed in the atmosphere, the researchers noted. The largest source of heat, solar energy, warms Earth's surface and atmospheric circulations distribute that energy from one region to another. Human energy consumption distributes energy that had lain dormant and sequestered for millions of years, mostly in the form of oil or coal. Though the amount of human-generated energy is a small portion of that transported by nature, it is highly concentrated in urban areas. In the Northern Hemisphere, many of those urban areas lie directly under major atmospheric troughs and jet streams.
Zhang said the effect his team studied is distinct from the so-called urban heat island effect, an increase in the warmth of cities compared to unpopulated areas caused by human activities.
The authors report that the influence of urban heat can widen the jet stream and strengthens atmospheric flows at mid-latitudes. They add that the warming is not uniform. Partially counterbalancing it, the changes in major atmospheric systems cool areas of Europe by as much as 1 degree C, with much of the temperature decrease occurring in the fall.
Overall, these changes have a noticeable but slight effect on global temperatures, increasing them worldwide by an average of about 0.1 degree C.
The study does not address whether the urban heating effect disrupts atmospheric weather patterns or plays a role in accelerating global warming, though Zhang said drawing power from renewable sources such as solar or wind provides a societal benefit in that it does not add net energy into the atmosphere.
The authors also contend that the urban heat effect accounts for the discrepancy between observed warming and winter warming simulated in the models used by the climate science community for analysis and prediction of climate. They suggest that the influence of energy consumption accompany heat-trapping gases and aerosols as necessary variables in computer models.
For further information see City Heat.
Atlanta Thermals image via Wikipedia.
