The Houston Metropolitan Area is home to about 7 million people and covers an area of over 10,000 square miles. Approximately 30% of the land surface, in developed regions, is roadways and parking lots, and another 20% is rooftops. In the summer, rooftops reach temperatures of up to 160 degrees Fahrenheit (F). Pavement heat is a problem also. The sun, beating down on roadways, sidewalks, and parking lots, heats them. When pavement and rooftops heat up, they radiate their heat back into the atmosphere. This total heat effect then raises urban temperatures by 6 to 8 degrees F over the normal regional temperature.
This whole process is referred to as the urban island heat effect. The physics of this effect is simple – color. Albedo is a measure of the portion of solar radiation that a surface reflects. The higher the albedo, then the more radiation reflected. A surface with an albedo of 1 reflects all incident solar radiation. However, a surface with an albedo of 0 absorbs all radiation. Light colors have high albedo (more reflective), and dark colors have a lower albedo. So, a perfectly black surface has an albedo of 0. Thus, black asphalt roadways and rooftops busily absorb heat on a summer day and then radiate that heat back into the atmosphere.
Cooling the city
Cooling the pavement heat in urban areas is a significant part of reducing the urban heat island effect. Cool pavement strategies involve a variety of technologies, but the underlying physics is still color for many of these technologies.
The solar reflectance of newly poured concrete is between 35% and 40%. However, as the concrete ages, the reflectance drops by about 10%. Now, concrete is composed of cement, water, and rock aggregate. The cement contains a portion of clay, and the iron oxides in that clay darken the color of the cement, thus decreasing its albedo. But white concrete uses cement with kaolin instead of ordinary clay to produce a surface with an albedo higher than normal concrete.
Another cooling technology is porous cement. This type of cement has two interesting characteristics. Firstly, the concrete produced is less dense than regular concrete. Thus there is less mass to absorb heat. Secondly, water can drain through the pavement and provide a cooling effect. An additional environmental benefit from porous cement is water retention. Since the pavement is porous, it allows for water storage in the pavement substrate. This effect then reduces the negative environmental impact of urban stormwater runoff.
Increasing the albedo of asphalt is another cooling strategy. This approach again involves color change. Interestingly, age has the opposite effect on asphalt than it does on concrete. The albedo of concrete decreases with age, and the albedo of asphalt increases with age since asphalt becomes lighter in color as it ages. Lightening the color of asphalt is also achieved by using a lighter aggregate or by mixing lighter colors into the binding material.
Lowering the urban temperature
Concrete and pavement heat are important components of the urban island heat effect, but there are other factors that also help with cooling. More green growth is an easy win. Increasing the tree cover and green spaces in an urban setting also decreases the amount of radiant heat. Where flat-topped roofs are used, then adding a rooftop garden reduces radiant heat and provides new space for relaxation.
Concrete is essential to urban development, but it carries a negative environmental impact. Carbon dioxide emissions from concrete production account for 8 percent of global greenhouse emissions. Also, concrete makes a significant contribution to the urban island heat effect. A third major issue faced in urban settings is pollution related to stormwater runoff from the large tracts of paved land surfaces. Solutions to these issues often involve a series of smaller incremental actions like lighter colored surfaces and porous concrete.
Sources:
Cool Houston! Cool Houston! A Plan for Cooling the Region A Plan for Cooling the Region (Source HARC) – https://www.harcresearch.org/sites/default/files/documents/projects/CoolHoustonPlan_0.pdf Also:
Feature Image: Three cat’s eyes on a road (Modified) – By The photograph was taken by w:User: Shoecream, and uploaded to the English Wikipedia with the tag w:Template: PD-self. – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=137618
