Local characteristics surrounding a property can exacerbate the effects of heat, creating heat islands.
Average local temperatures are influenced by latitude, exposure to sunlight, elevation, and climate. Yet, within a given area, there can be pockets of warmer temperatures due to distance to water, vegetation, and neighborhood construction materials. This phenomena, known as the heat island effect, are urban or metropolitan areas that are warmer than the surrounding area due to human activities.
Temperature differences between the heat island and the surrounding area are usually more prominent at night than during the day, which increases the cost of cooling for homes and businesses within a heat island. Nevertheless, daytime maximum temperatures within a heat island can vary by as much as 7 degrees from the surrounding neighborhood or area.
Compared to surrounding rural and suburban areas, temperatures in a city can be several degrees warmer on a hot summer afternoon (Source).
How are heat islands formed?
Heat islands form as a result of reduced natural landscapes. Some of the common causes of heat islands include:
Distance to water and vegetation
Areas far from water and vegetation are more likely to experience heat island effects. Trees, plants, and other vegetation use the sun's heat for energy, which is then used to evaporate water and reduce the surrounding temperature. Water bodies help maintain low daytime and nighttime temperatures by trapping and distributing heat.
Neighborhood construction materials
Asphalt, concrete, and glass trap and reflect heat. Dense concentrations of these man-made materials cause heat to radiate. Hard surfaces also trap a greater amount of heat and release it slower than natural materials, causing high temperatures to last beyond sunset.
City planning and layout
How buildings are arranged and spaced across an area also significantly affects how heat from sunlight is trapped in urban and suburban environments and how air flows between buildings. Heavily developed areas such as cities and industrial parks can create pockets of insulation that trap heat and prevent airflow that would release it.
Heat is released as a by-product from operating vehicles, air-conditioning, and industrial activities. Meaning the most severe heat island effects will be felt in areas where these activities occur.
How Heat Factor considers local differences in temperature
The urban heat island effect explains why temperatures are warmer in urban areas than in leafy, less dense, suburban areas. Urban heat islands are just one example of how air temperatures vary significantly.
Local characteristics explain why some places will experience significant changes in heat risk while others will experience milder changes. For this reason, the model looks at temperatures and humidity during the hottest months of the year, typically July and, in some areas, August. Historic temperature records between 2014 - 2020 are used to map average high temperatures across the United States and compare how temperatures vary from property to property within a community.
Temperature is typically communicated through comparisons to local experiences and norms. For example, if you live in Texas, a daily high of 100°F may not seem extreme, but if you live in Michigan, a daily high of 100°F is likely alarming and much higher than what's typically considered “hot” for your area. For this reason, the Extreme Heat Model determines what someone would reasonably consider a “hot day” based on the specific location of a property. A “hot day” refers to the temperature on the hottest days 7 days for that specific location. The term “hot day” is used to represent local heat index temperatures.