Spend time in a city in the summer and you can feel the urban heat rising from the sidewalk and radiating from buildings. Cities are generally warmer than surrounding rural areas, but even within cities, some neighborhoods become dangerously warmer than others just a few miles away.
Within these “micro-urban heat islands,” communities can experience heatwave conditions long before officials declare a heat-emergency.
I use Earth observation satellites and population data to map these hotspots, often on projects with NASA. Satellites like the Landsat program have become crucial in identifying urban risks so cities can prepare for and respond to extreme heat, a top weather-related killer.
Among the many things we were able to detect with increasingly detailed satellite data is that the warmest neighborhoods are typically low-income and often have predominantly black or Spanish residents.
Two types of urban heat, both dangerous
The urban heat island effect was first described in 1818, more than 200 years ago, in “The Climate of London” by Luke Howard, an early pioneer of meteorology.
There are two different types of urban heat island: the atmospheric urban heat island and the surface urban heat island. They are measured in different ways.
The atmospheric urban heat island, the phenomenon Howard describes, is simply the warmer air in urban areas relative to cooler air in remote locations.
The surface urban heat island is the result of surfaces made up of heat-absorbing materials, such as asphalt, concrete and metal. Such materials are highly effective heat energy absorbers of the Sun, and their surfaces heat up rapidly and in turn emit the absorbed energy. You can feel the heat when you touch them.
The surface urban heat island contributes directly to the atmospheric urban heat island and is usually the most intense on sunny days. Urbanization also contributes to the heat island effect through deforestation and the removal of other vegetation that will provide some cooling.
Where communities face the highest heat
With rising global temperatures increasing the likelihood of dangerous heat waves, cities need to know which neighborhoods are at high risk. Excessive heat can lead to dehydration, heat exhaustion, heat stroke and even death with prolonged exposure, and most at-risk residents often do not have the financial resources to adapt.
Satellite instruments can identify communities that are vulnerable to extreme heat because they can measure and map the surface urban heat island in high detail.
Industrial and commercial areas, for example, are often one of the hottest areas in cities. They typically have fewer trees to cool the air and more paving and buildings to retain and radiate heat.
Certain residential layouts are also more prone to higher surface temperatures than others. These neighborhoods usually have minimal vegetation, and houses are built close together, with more roads and sidewalks and little green space. Often, especially in northern climates, houses in these neighborhoods are built with materials such as brick that retain heat to keep residents warmer in winter. Communities with many apartment buildings and shops surrounded by parking lots are also at risk.
My research has found that low-income communities of color on hot summer days can experience extreme heat conditions that are often more than 10 degrees Fahrenheit (5.5 C) warmer than surrounding areas. Other research has found similar differences between neighborhoods and sharp racial and economic inequalities when it comes to heat exposure.
One recent study found that the poorest areas were significantly warmer than the richest in 76% of urban U.S. provinces. It also found that neighborhoods with large black, Hispanic, and Asian populations were in significantly warmer areas in 71% of the provinces, and that that difference remained even when adjusted for income. These areas tend to have less vegetation and a higher density of houses.
Another study looked at communities that were once relegated, a discriminatory practice that banks used in the early to mid-20th century to deny loans to racial and ethnic minority communities. Nationally, these previously red-line neighborhoods were 4.6 F (2.6 C) warmer than non-red-line areas.
50 years of Landsat
Several satellite systems can now measure the surface urban heat island, but the Landsat program, which celebrates its 50th anniversary in 2022, provides decades of continuous, comparable data in the details needed to explore variations within a city. That continuity helps scientists measure the impact of change and track how development patterns change a neighborhood’s heat profile.
The first Landsat satellite was launched on 23 July 1972 with a sensor that collected data in green, red and near-infrared wavelengths that made it useful for mapping vegetation. Launched with Landsat 4, launched in July 1982, scientists were able to map and measure the thermal properties of the earth’s surface. Today, Landsat 8 and Landsat 9 work, and a tenth is being developed.
How cities can use this data to help
There are numerous ways cities can use this data to help residents combat extreme heat.
In Indianapolis, local government and faith-based organizations have used extreme heat vulnerability indices, which use indicators of heat health risk and past heat waves to highlight high-risk communities. Knowing which communities are most likely to be at risk enables them to reach out to the most vulnerable people, both before and during periods of rising temperatures.
New York’s “Cool Neighborhoods NYC” program includes strategic planting of trees and vegetation to increase shade and evapotranspiration, which cools the surrounding area. It also discusses the painting of roofs and sidewalk light colors to reflect solar energy and to put communities at risk over heat risk and ways they can get help.
As the climate continues to warm and affect urban health, the Landsat satellites’ sensors are one of our best tools to monitor the thermal variations of the urban heat island. Such work also serves as one of the best examples of the use of satellite-based measurements to monitor and implement response to public health threats.