Minimizing a Heat Island Effect

We live in the Sonoran Desert.  Because of that we are accustomed to hot summer temperatures.  Have you ever noticed that the temperatures in Phoenix always seem to be several degrees higher than in the smaller communities around?  This is often referred to as the heat island effect.

The heat island effect is a natural phenomenon that causes daytime temperatures within cities to increase by up to 6 degrees F. over temperatures in rural areas at the same time of the day.  Nighttime temperature differences can be as much as 9 degrees.

Are you finding yourself a little skeptical?  Let’s look at an example.  If the daytime temperature in rural areas hits 106 degrees, which we would say is a pretty nice day for late June and July, the addition of another 6 degrees turns a tolerable day into a 112 degree scorcher.  If the rural temperatures are higher, as we know that they will be, ... well, you can do the math.

Heat islands are caused, pure and simple, by urbanized development and growth.  Roads, buildings and other hard surfaces, especially dark-colored surfaces, absorb and retain heat.  Surface temperatures fifty to seventy degrees hotter than normal soil surfaces are not uncommon.  It is the accumulation of this heat, and its subsequent release back into the atmosphere, that leads to higher ambient temperatures in urban areas.

In a 2002 article in the scientific journal Urban Ecosystems, 6: 183-203 named “Urbanization and Warming of Phoenix (Arizona, USA): Impacts, Feedbacks and Mitigation”, the authors found that at Sky Harbor Airport, urbanization has increased the nighttime minimum temperature by 5 degrees C., or 9 degrees F., and the average daily temperatures by 3.1 degrees C, or 5.6 degrees F.  Additional studies have recorded temperature increases in other cities.

High temperatures affect us in many ways.  In addition to simple heat stress to you and I, other effects, proven or implied, include increased crime rates, higher cooling bills during the summer, additional stress on landscape plants and higher insect pest activity. 

Crime rate increases in warmer weather versus cooler weather have been seen and documented in many places.  A mathematical formula has even been developed.  Plugging in the 3.1 degree C. increase in Phoenix into this formula tells us that the higher temperatures could mean an increase of an additional 25 violent crimes per 100,000 annually.  There is some controversy over whether there is a downturn in the temperature-crime relationship at very high temperatures, so this effect needs more study.  

Higher temperatures bring on a greater need for power to cool our homes, businesses and public buildings to comfortable levels.  Increasing demand generally means higher prices, and in times where the stress is severe, the imposition of a “brown-out” or even a complete short term loss of power is possible.   

In our gardens and landscapes, increased heat can affect the overall health of plants and lead to specific problems, such as water stress, sunburn and leaf tissue damage.  Insect pests, such as whiteflies, whose life cycle is temperature driven, can cause significant problems when their fast growing populations overwhelm a plant’s natural defenses.

The increased heat can also affect local food and fiber production.  Examples include the adverse effect of heat stress on dairy cows and the potential for heat injury to cotton boll production.

So, what can be done?   Houston, Texas has an interesting program called ‘Cool Houston”.  In their program, they advocate the use of cool roofs, cool paving and trees and other vegetation.  You might want to check out their website at http://www.harc.edu/harc/Projects/CoolHouston/. 

Roofs of buildings generally are completely exposed to the harsh sunlight, unless they are protected by the shade of large trees.  The use of highly reflective, lighter colored roofing materials can help reduce temperatures by reflecting sunlight back into the atmosphere instead of allowing it to heat the roof, the interior of the attic, crawl spaces and even the interior of the building.  Most roofing companies recommend this type of covering, especially for flat roofs.

Dark surfaces of streets and parking lots absorb considerable heat during the bright, sunny days and re-radiate the heat back into the atmosphere when the sunlight ends at sunset.  Light-colored pavements tend to reflect sunlight and do not heat as much as darker colored pavements.  There is a need to give these hard structure alternatives some thought.

Trees and shrubs are sure to help minimize heat gain in both public areas and in our home yards.  Not only do they shade surfaces, they also give off water vapor.  This process is known as transpiration and it cools not only the plants but surrounding areas as well.  Most of us have noticed the difference in temperature at night as we enter agricultural zones from bare desert areas.  It is the water vapor given off by plants as part of their essential life processes that cause this cooling effect.  Even most low water use plants transpire water. 

Not only do we need to plant more trees, but we can also take better care of those trees and shrubs that we already have.  Healthy thriving trees give much better shade and transpire more effectively than those that are stressed or dying.  Good management now, including the proper care of trees and shrubs, can give us huge returns later in lower cooling bills and in more comfortable living conditions. 

To finish up, let me boil this discussion down into the basics.  Urban warming affects us all.  The key effects include increased minimum daily temperatures throughout the year, a longer warm period and shorter cool period within a given day and an extended hot season. 

While some warming may be impossible to avoid as urban growth overtakes the communities of Pinal County, there are some steps that can be taken to minimize the effects.  Now is the time for all of us to get involved.

If you have questions, you can visit with one of our Master Gardeners at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande.  The telephone is (520) 836-5221, extension 204.  The author’s email address is gibsonrd@ag.arizona.edu.

The University of Arizona is an equal opportunity, affirmative action institution.  The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.

Rick Gibson
Extension Agent, Agriculture
University of Arizona Cooperative Extension
820 E. Cottonwood Lane, Building C
Casa Grande, Arizona 85122
Voice:    (520) 836-5221
Fax:    (520) 836-1750
email:    gibsonrd@ag.arizona.edu