“Heat Island Index” Fact Sheet

by | Aug 29, 2019 | Blog

What does the “Heat Index or “Apparent Temperature” Mean?

In winter, meteorologists talk about wind chill factors – which combines the temperature and gusting winds to determine how cold a person might feel. The “Heat Index” or “Apparent Temperature” is the measure of how hot it feels when relative to temperature and humidity. For example, when the temperature hits 100°F and carries a 65% humidity factor, temperature to the human body feels like 136°F.

What is “Heat Island Index”?

The Heat Index is exacerbated with the concentration of buildings in cities. Rising urban temperatures are measured through a heat island index, which calculates the temperature disparity between urban and rural areas.

The difference in temperatures, as calculated for the Heat Island Index, can be significant. According to the Environmental Protection Agency (EPA), daytime temperatures average one to six degrees Fahrenheit hotter in urban areas vs. rural ones, and up to 22 degrees hotter at night.

What causes “Heat Island Index”?

Heat Island occurs when cities experience much warmer temperatures than that of the nearby rural areas. According to Dr. Glenn Horner of Bambu Global, Heat Island occurs when the sun hits a roof or other flat surface, much of that heat is absorbed and eventually emitted into the surrounding environment.

Other contributors to heat islands include closely situated buildings, which trap heat, asphalt roads and concrete sidewalks that absorb the heat from the sun as well as exhaust from buses, cars and factories.

The difference between the temperature between urban and less-developed areas has to do with how well the surfaces in each environment absorb and hold heat. And within these highly developed areas, there is limited reflective components to repel heat away. While white objects reflect all wavelengths of light, black, brown and grey colors that sit atop materials like asphalt, steel and brick, absorb wavelengths of light energy and convert them into heat, much of what we see as the colors in developed structures.

In terms of buildings, much of their heat comes directly from the sun’s rays on the roof. While some of this heat gets reflected by the roof, a good portion of it gets absorbed and transferred to the building below, significantly increasing internal temperatures and, in turn, driving up the cost to maintain a comfortable temperature for people. Black or dark roofs retain more heat, absorbing both visible and nonvisible light that can reach temperatures of 150-degree Fahrenheit or more in the summer according to the Department of Energy.

Why is the “Heat Island Index” a problem?

In the case of a roof, heat from the sun can be transferred to the building below, increasing internal temperatures and driving up cooling costs. At least half of the costs comes from HVAC systems. The hotter the internal temperatures, the higher the demand for air conditioning; unfortunately, air conditioning units generate heat – which can distribute heat to other buildings, further aggravating the heat island index problem in densely populated areas.

Heat islands don’t just impact buildings and energy costs. They also have a major impact on the environment and human health such as:

  • Reduced air quality because of increased pollutants released into the air from exhausts from vehicles, factories and other sources don’t have an effective way to dissipate around densely packed buildings.
  • Worse water qualityvia warmer streams and other bodies, which negatively affect aquatic life.
  • Increase in heat stroke and heat exhaustion, respiratory problems, and other health issues – all of which can lead to illness or fatalities. As people try to escape hot temperatures, air conditioning usage increases, which in turn can cause brownouts and blackouts that exacerbate the health risks.

These environmental and health issues associated with heat islands are compounded during heat waves when temperatures soar. The health problems hit the most vulnerable populations the hardest, such as the elderly, pregnant women and children. The CDC estimated that in the period from 1979-2003 alone, 8,000 deaths were caused by excessive heat exposure, which was more than the number of fatalities from hurricanes, lightning, tornadoes, floods and earthquakes during that period combined.

What can be done to address “Heat Island Index”?

A recent trend to drastically reduce or eliminate this problem, cities have begun “lightening” streets by applying reflective gray coatings. This has led to a drop in urban air temperatures dramatically, especially in summer months. The rapid increase in high temps pose major challenges to facility, equipment and asset managers when it comes to the high cost of energy – especially as it relates to heating and cooling.

Unfortunately, in some cases, using white asphalt causes an additional set of problems, including producing glare that can be distracting to residents, drivers and, especially around airports, pilots.

Here are some of the ways cities, towns and building owners can take to combat heat islands and help the environment:

  1. Use materials that reflect solar energy on streets, highways, parking lots and other areas to create cool pavements.
  2. Plan for smart growth by creating more open spaces, promoting compact buildings, and pushing for more conservation efforts through better transportation options and other initiatives.
  3. Plant trees and other vegetation to provide shade as well as reduce air and surface temperatures.
  4. Plant vegetation on green roofs to reduce heat.
  5. Use cool paints that reflect solar energy and reduce building temperature on cool roofs. A study conducted in California found that cool roofs can reduce annual energy costs by 50 cents per square foot.

Many of these efforts are well underway in cities and towns and can have a big impact. A study by Dr. Brian Stone, Jr. of Georgia Tech’s School of City and Regional Planning estimated that by planting more vegetation and using reflective building materials, the cities of Philadelphia, Atlanta and Phoenix could significantly prevent many of the increased fatalities expected from urban heat islands and climate change in 2050.

Best practices for building owners

Reducing heat on roofs is one of the better ways to reduce building heat. While creating green roofs is an option to reduce heat, it is more expensive and requires more maintenance than painting or coating the roof. Heat-reflecting cool paints are an effective solution for keeping roofs cool and are being required in many municipalities. Typically, cool paints are white or light in color, because white tends to better reflect the sun’s rays. White cool paint can come in many different forms – some are simple white acrylic paints, while others have added polymers for increased durability.

But cool paints now also come in colors other than white. Advanced polymer coating technology for cool paints available today enables black or dark colored roof coatings to reflect the sun’s infrared rays and significantly reduce heat. Regardless of the color, it’s key to look for highly durable paints that won’t crack.

With climate change, temperatures are expected to increase and exacerbate heat islands, which will continue to boost building cooling costs. With new approaches and advanced technologies, such as polymer cool paints, building owners and facility managers can significantly reduce their expenses and have a positive effect on the environment. After all, a cool roof is one of the simplest and most cost-effective building upgrades to improve energy efficiency. And a solution that saves money, is easy to use and helps environment is a win-win all the way around.

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James Florentino is the Director of Sales & Business Development at Bambu Global, a technology company providing breakthrough color and light solutions for a variety of markets. He is responsible for the growth and revenue surrounding Bambu Global subsidiaries Nygra Coatings and Bambu Way.

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