Why do cities stay warmer

An urban heat island UHI is a metropolitan area which is significantly warmer than its surroundings. According to the EPA, many U. This temperature difference usually is larger at night than during the day and larger in winter than in summer, and is most apparent when winds are weak. The main causes are changes in the land surface by urban development along with waste heat generated by energy use.

As population centers grow, they tend to change greater areas of land which then undergo a corresponding increase in average temperature. Heat islands form as vegetation is replaced by asphalt and concrete for roads, buildings, and other structures necessary to accommodate growing populations. These surfaces absorb—rather than reflect—the sun's heat, causing surface temperatures and overall ambient temperatures to rise.

Displacing trees and vegetation minimizes the natural cooling effects of shading and evaporation of water from soil and leaves evapotranspiration. Tall buildings and narrow streets can heat air trapped between them and reduce air flow.

Waste heat from vehicles, factories, and air conditioners may add warmth to their surroundings, further exacerbating the heat island effect. Heat islands can occur year-round during the day or night.

Dark surfaces--whether a black t-shirt or an asphalt street--absorb the sun's heat, while lighter colored surfaces reflect heat from the sun. This is done by covering black asphalt streets, parking lots, and dark roofs with a more reflective gray coating. These changes can drop urban air temperatures dramatically, especially during the heat of summer. Planting gardens on urban rooftops can also help to cool down the city, too! Urban building materials are another reason that urban areas trap heat.

Many modern building materials are impervious surfaces. Without a cycle of flowing and evaporating water, these surfaces have nothing to cool them down. To help cool the heat island, builders can use materials that will allow water to flow through. These building materials—called permeable materials—promote the capture and flow of water, which cools urban regions.

Urban heat islands are one of the easiest ways to see how human impact can change our planet. And although these structures are essential to city living, the heat islands they create can be dangerous for humans. These two factors are the conspiring menaces of extreme heat: Our bodies respond to high temperatures by perspiring, which is more fancily known as evaporative cooling.

But humidity makes this process less efficient, because the more moist the air is, the less readily it accepts evaporating sweat from our bodies. By Katie M. Palmer and Matt Simon. Heat and humidity are not only uncomfortable; they can be dangerous. Mora has identified 27 ways heat can kill a person. In extreme heat, this can spiral out of control, resulting in ischemia, or the critically low flow of blood to the organs. This can damage crucial organs like the brain or heart.

In addition, a high body temperature can cause cell death, known as heat cytotoxity. Extreme heat can harm healthy people, and those with heart or respiratory conditions like asthma are particularly vulnerable. Gas Prices. Bluff City Life. About Us. Meet The Team. Editorial Board. Latest Newscasts. Investigate TV. Gray DC Bureau. Breakdown: Why cities are often warmer than rural areas. By Erin Thomas. Published: Sep. Share on Facebook. Email This Link.

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