Urban Heat Islands Basics

Most cities have higher temperatures than surrounding rural areas, a condition that scientists refer to as the "urban heat island effect." These higher temperatures are caused by how we've built our cities - buildings, paved surfaces and other infrastructure absorb solar heat. Waste heat from cars, trucks, industrial facilities, air conditioning and other human activities is also added. In addition, cities have lost some of the natural cooling effect provided by trees and plants. Plants provide shade and retain less heat than many man-made structures. They cool the air through a process called evapotranspiration in which water is taken up by roots and released through leaves. Moisture in the soil has a similar cooling effect.

Heat in urban areas impacts public health, energy use, and environmental quality. Extreme hot weather places some people in danger of heat stroke, organ damage, and even death. There is increased demand for air conditioning and related increases in air pollution from power plants. Higher energy costs, brown-outs, and black-outs can result. Ozone, the primary air pollutant of concern to the Houston region, increases with higher temperatures.

Thermal Image of Houston, TX

Memorial Park on the west side of this thermal image is a "cool island" showing up as dark blue. The light blue lines are concrete paving. The dark blue areas in the Central Business District are shadows of high rise buildings. Buffalo Bayou shows up in dark blue as a curving line from west to east across the upper portion of the image. The red parts of the image are rooftops and hotter paved surfaces. Image from NASA airborne sensor in 1999.

Urban temperatures have been raised by the ways we have built our cities and there are various ways to reduce these temperatures, and reduce the adverse health, excessive energy use, and environmental impacts caused by higher temperatures. The three primary methods are:

• Plant trees and other vegetation: In addition to decreasing temperatures, planting trees in the Houston region can improve comfort levels, reduce cooling and heating energy use and electrical demand, remove air pollutants, improve stormwater control, reduce noise levels, and increase property values.
• Expand use of cool roofing: Reflective roofing products have been used in Houston for many years, but expanding their use can reduce energy costs, peak electricity demand, and air pollution, while improving indoor building comfort. Green roofs or garden roofs are another cool option available to building owners.
• Greater use of lighter colored and porous paving: Lighter colored paving surfaces, whether concrete or asphalt, can increase surface durability, enhance night time illumination, and assist stormwater management efforts.