Urban Heat Islands and its Risk in San Diego, CA

One might think that when the sun goes down that the heat island effect would disappear. However, nighttime is actually when urban heat islands are most prominent. While surrounding areas cool down, impervious surfaces, such as buildings and concrete, release the solar heat they absorbed during the day. High temperatures maintained at night impede the ability for residents to recover from the day's heat causing increased health impacts.

Urban Heat is a public health hazard that impacts all cities regardless of climate, development level, and jurisdiction. Excess urban heat exacerbated by extreme heat events has become the leading cause of weather-related deaths, outpacing both floods and wildfires (Weinberger et al., 2017). Outside of mortality, urban heat can cause heat-related illness including heatstroke, exhaustion, and amplified respiratory and cardiovascular issues as well as driving up energy costs (Wald, 2019). 

The NASA DEVELOP Spring 2021 San Diego Urban Development team was based out of the Tempe, Arizona node. The team consisted of four members whose expertise include, but are not limited to, urban heat islands, environmental engineering, remote sensing, environmental science, and programming.

A region that had high heat exposure includes the communities of Mid Cities, Eastern Area, and College Area. These regions are further away from the ocean and so people living in these regions do not receive the cooling effects that people who live closer to the coast do. These regions also have high impervious surfaces and low tree canopy, one reason being that the regions include Interstate 805 and Interstate 15.

Just like how heat exposure varies throughout the city, so does vulnerability to heat. Older populations that live alone, people with pre-existing health conditions, and less education attainment are more socially vulnerable to heat exposure (Cooley, 2012). Regions that have high heat risk tend to have lower life expectancy rates and more chronic illnesses (Kerlin, 2019). To the right, is the vulnerability map that takes social and health indicators into consideration. Census tracts that are a darker orange are more vulnerable to extreme heat than census tracts that are lighter.

Combining both heat exposure and vulnerability results in heat risk. This map shows the census tracts that are most at risk to urban heat impacts. Values are normalized to a range of 0 to 1. Regions with higher scores have higher risk. The census tracts that are deep red fall in the 80th to 100th percentile of heat risk.

One region with high heat risk are the census tracts in the San Ysidro community. These areas have high populations of people with health conditions such as heart disease and diabetes and high heat exposure compared to other areas.

The Urban Cooling Model, created by Stanford University's Natural Capital Project, models urban heat mitigation by taking into account the physical characteristics of the land, and the cooling effect of green spaces. The model is based on land use land cover classifications and the associated parameters.

The output from the model produces a Heat Mitigation Index that represents the cooling capacity of the land. Daytime Heat Mitigation Index considers shade, albedo, and evapotranspiration, values that are dependent on land use. In addition, the model considers cooling from greenspaces such as parks and natural open spaces. This map illustrates how San Diego's extreme heat is mitigated unevenly across neighborhoods. Dark blue areas are areas of high heat mitigation while light blue areas indicate areas of low heat mitigation.

A low heat mitigation index often occurs in areas of high-medium density development that are far from greenspaces. Low heat mitigation means that it is harder to cool the area. These areas are often bounded or crossed by freeways or adjacent to industrial areas. Four areas in San Diego were identified as having low heat mitigation index: Mira Mesa, Southeastern San Diego, Barrio Logan, and San Ysidro. Explore the map on the right to observe these areas.

A high heat mitigation index means it is easier to cool the area. The Torrey Pines area ranked high in heat mitigation. These census tracts include two nature preserves, trail heads, and parks. As seen by this map, the presence of natural landscapes contributes to heat mitigation.

At nighttime, the primary driver of urban heat island is the release of heat from buildings. Nighttime heat mitigation index is determined by building intensity, a ratio of floor area to overall land area. An area with high building intensity will have a low heat mitigation index since there is more building material to give off heat. In the image on the right, areas of low nighttime heat mitigation are yellow while natural areas with high heat mitigation are blue.

Daytime and nighttime heat mitigation are driven by different factors so areas at risk during the day differ from those that have low heat mitigation at night because areas with high heat during the day may not have high building intensity and thus does not have heat being emitted from buildings during the night. These areas include, but are not limited to University, Pacific Beach, and North Park. Explore the map to the right to learn more. Dark blue areas represent high heat mitigation and low levels of development. Light blue areas represent a low heat mitigation and a high level of development.

While the dry climate of San Diego presents many challenges to increasing tree canopy, there are opportunities in areas with irrigation. These areas include alongside roads, public spaces such as parks and schools, and partnerships with non-profit organizations that facilitate increasing tree canopy. The images on the right shows the difference in heat mitigation when a 5% increase of tree canopy is applied to the categories mentioned above. The left image is the daytime heat mitigation while the image to the right is nighttime heat mitigation. Green represents high heat mitigation, yellow represents moderate heat mitigation, and red represents low heat mitigation.