Spend time in a city in the summer and you can feel the urban heat rising from the sidewalks and emanating from the buildings. Cities are generally hotter than surrounding rural areas, but even within cities, some residential neighborhoods become dangerously hot more than others just a few miles away.
Within these “micro-urban heat islands,” communities may experience heat wave conditions before officials declare a heat emergency.
I often use Earth-observation satellites and population data to map these hot spots on projects with NASA. Satellites such as the Landsat program have become important for pinpointing urban risks so that cities can prepare for and respond to extreme heat, a top killer of weather-related issues.
Among the many things we are able to track, with increasingly detailed satellite data, is that the hottest neighborhoods are typically low-income and often have predominantly black or Hispanic residents.
Two types of urban heat, both dangerous
The urban heat island effect was first described in 1818, 200 years earlier, in “Climate of London”, by Luke Howard, an early pioneer of meteorology.
There are two different types of urban heat islands: atmospheric urban heat islands and surface urban heat islands. They are measured in different ways.
The atmospheric urban heat island, described by Howard, is simply warm air in urban areas relative to cooler air in outdoor spaces.
Surface urban heat islands are the result of surfaces made of heat-absorbing materials, such as asphalt, concrete and metal. Such materials are highly effective absorbers of heat energy from 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 directly contributes to the atmospheric urban heat island and is usually most intense on sunny days. Urbanization also contributes to the heat island effect through deforestation and the removal of other vegetation that would provide some cooling.
Where communities face the most heat
With rising global temperatures increasing the potential for dangerous heat waves, cities need to know which neighborhoods are at high risk. Extreme heat can lead to dehydration, heat exhaustion, heat stroke and even death from prolonged exposure, and residents most at risk often lack the financial resources to adapt.
Satellite instruments can identify communities vulnerable to extreme heat because they can measure and map the surface urban heat island in high detail.
For example, industrial and commercial areas are often among the hottest areas in cities. They usually have fewer trees to cool the air and more sidewalks and buildings to retain and radiate heat.
Some residential layouts are also more prone to high surface temperatures than others. These neighborhoods usually have minimal vegetation, and houses are built closer together, with more streets and sidewalks and little green space. Often, especially in northern climates, houses in these neighborhoods were built with materials such as bricks that retain heat to keep occupants warm in winter. Communities with multiple apartment buildings and stores surrounded by parking lots are also at higher risk.
My research has found that on hot summer days, low-income communities of color can experience extreme heat conditions that are often more than 10 degrees Fahrenheit (5.5 C.) Other research has found similar differences between neighborhoods and racial and economic disparities when it comes to heat exposure.
A recent study found that the poorest areas in 76 percent of urban US counties had significantly more heat than the richest areas. It also found that neighborhoods with large Black, Hispanic and Asian populations were in significantly warmer areas in 71% of the county, and this gap persisted even after adjusting for income. These areas have less vegetation and higher density of houses.
Another study looked at communities that were once again underserved, a discriminatory practice banks used in the mid-20th century to deny loans to racial and ethnic minority communities. Nationally, these formerly redlined neighborhoods were 4.6 F (2.6 C) warmer than non-lined areas.
50 years of Landsat
Many 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 detail needed to examine variations within a city. This continuum helps scientists measure the impact of changes and track how growth patterns change the heat profile of neighborhoods.
The first Landsat satellite was launched on July 23, 1972, with a sensor that collected data in green, red and near-infrared wavelengths, which made it useful for mapping vegetation. Beginning with Landsat 4, launched in July 1982, scientists could map and measure the thermal characteristics of Earth’s surface. Today, Landsat 8 and Landsat 9 are in operation, and a 10th one is being developed.
How cities can use this data to help
There are many ways cities can use this data to help residents cope with extreme heat.
In Indianapolis, local government and faith-based organizations use extreme heat vulnerability indices, which use indicators of heat-health exposure and past heat waves, to highlight communities at high risk. Knowing which communities are most likely to be at risk allows them to access those most vulnerable before and during periods of elevated temperatures.
New York’s “Cool Neighborhood NYC” program involves strategically planting trees and vegetation to increase shade and evaporation, which cools the surrounding area. It also discusses painting roofs and sidewalks lighter colors to reflect solar energy and educating at-risk communities about heat exposure and ways to get help.
As the climate continues to warm and affect urban health, sensors from the Landsat satellites are among our best tools for monitoring the thermal variations of an urban heat island. This kind of work also serves as one of the best examples of employing satellite-based measurements to monitor and implement response to public health threats.