Trees Can Cool Islands of Heat

Forrest M. Mims III

It's been a record winter for many readers of The Citizen Scientist. But summer is on the way for readers in the Northern Hemisphere, and now is the time to consider ways to cool your environment.

Pavement, sidewalks and roofs soak up sunlight on sunny days and become much hotter than the natural landscape they replaced. This causes the urban heat island effect .

Factories, offices, towns, cities and even your house and driveway are islands of heat. While this can be beneficial during winter, it's usually not during summer.

Figure 1. NASA flew an infrared sensor over Atlanta, Georgia, on 11 May 1997, to obtain data for this enhanced image of the city’s heat island. The only cool spots are trees and shadows of tall buildings. Image courtesy of the Scientific Visualization Studio of the Goddard Space Flight Center.

When she was in high school, my daughter Sarah studied heat islands across Texas and New Mexico during a family vacation. She discovered that even remote highway intersections with only a few buildings were slightly warmer than the surrounding countryside.

Visitors to New York City quickly learn about the urban heat island effect if they visit Central Park on a hot summer day. This huge park is several degrees or more cooler than the city that surrounds it.

People who live in towns and cities need to know that the mean temperature increase caused by the heat island effect has probably warmed their homes more than natural climate cycles or manmade greenhouse gases. This explains why wildflowers in cities bloom earlier than those in the nearby countryside.

The heat islands formed by Atlanta, Houston, San Antonio and other cities have been the subject of various scientific studies. For example, Dr. Daniel Boice of the Southwest Research Institute, studied the warming of San Antonio relative to nearby New Braunfels from 1946 to 1990. He found that the minimum summer temperature in San Antonio warmed 1.7 degrees Celsius (3 degrees Fahrenheit) more than New Braunfels.

Because New Braunfels is also a heat island, San Antonio probably warmed more than 1.7 C (3 F) when compared with the surrounding countryside. For example, according to Houston Advanced Research Center (HARC), “Houston's urbanized areas are hotter than surrounding rural areas by 6 [to 8 degrees Fahrenheit].”

By reducing their heat island effect, towns and cities can reduce air pollution and energy consumption while also increasing their outdoor comfort level. So what can be done?

Trees do much to clean and cool the air. Yet, according to HARC, “From 1972 to 1999, the Houston region lost about 400 square miles of tree canopy or 25 acres per day, causing Houston's urban heat islands to grow larger and hotter.”

Houston's lost trees were replaced by paved roads, parking lots houses and shopping centers, all of which become considerably hotter than trees on summer days. What has happened to Houston is well underway in other large urban areas around the world as large tracts of land are cleared for new housing and roads.

Retaining existing trees and planting new ones is a viable and esthetically pleasing way to reduce the heat island effect.

Shopping centers can warm entire neighborhoods. Replacing dark pavement and roofing materials with those that better reflect sunlight can reduce the heat islands formed by shopping centers and individual homes.

Much more about reducing the heat island effect can be found on the web at “Cool Houston!” and at “Heat Island Effect.”

Forrest M. Mims III and his science are featured online at www.forrestmims.org and www.sunandsky.org. "World of Science" columns are selected and sometimes revised from columns published in the San Antonio Express-News or the Seguin Gazette-Enterprise. The columns are intended for a general audience.