Strategic Shade: Using NASA Earth Observations to Identify the Hottest Bus Stops in Phoenix

EarthzineDEVELOP Spring 2017 Article Session, DEVELOP Virtual Poster Session

This article is a part of the NASA DEVELOP’s Spring 2017 Article Session. For more articles like these, click here

The NASA DEVELOP Phoenix Health & Air Quality project applies NASA Earth observations to help researchers and practitioners strategically deploy shade in Phoenix.

Bus Stop with no shade structure. Image Credit: Google Earth


Lance Watkins

David Hondula

Tamara Dunbarr

McKenzie Murphree

Understanding and mitigating the impacts of extreme heat is a persistent issue for residents of Phoenix, Arizona. Average summer temperatures reach 106 degrees Fahrenheit (42 Celsius) and the record high temperature is 122 F (50 C) (NCEI). Exposure to extreme heat can lead to heat exhaustion, heat cramps, heat stroke, and death (Maricopa County Department of Public Health), and significant public health impacts associated with heat exposure are observed each year in central Arizona (Petitti et al. 2015). Exposure to extreme heat also has been shown to have a disproportionate impact on the population, with the elderly, infants, minorities, and those with poor health as being more vulnerable to extreme heat (Harlan et al 2006).

Phoenix residents who regularly use public transit in the summer months frequently encounter uncomfortably (if not dangerously) high temperatures as they walk to and wait at bus stops. While public transit is advocated as a desirable transportation mode for public health and urban sustainability, the discomfort and risks associated with heat exposure can be a deterrent to bus ridership for some individuals. This can pose challenges to achieving public health and sustainability goals related to public transportation, as well as challenges to transit agencies relying on transit fares to provide services. For other individuals, transportation mode choice is limited and coping with the heat while using the public transportation is a matter of daily life. As a result, these individuals may experience health risks related to heat exposure, discomfort and inconveniences associated with being hot or trying to stay cool, and lower satisfaction with the transit system.

Average land surface temperature (LST) in degrees Fahrenheit at the unshaded bus stops in Phoenix, Arizona. Image Credit: NASA DEVELOP

The Phoenix Public Transit Department is focused on improving the comfort of public transit users by adding shading structures to bus stops that currently lack shading of any kind. The Public Transit Department has a longstanding goal to provide shade for as many people as possible, and to date, about 85 percent of bus riders in Phoenix use stops that have a shade structure. The city’s aspiration to provide effective shade for all bus riders has become more attainable since a voter-approved Transportation 2050 Plan and associated sales tax. A portion of the revenue from this tax will be used to fund shade structure enhancements at bus stops. The current prioritization criteria for bus stop enhancements focuses primarily on ridership and little is known about the thermal conditions surrounding bus stops.

The spring 2017 Phoenix Health & Air Quality DEVELOP team used NASA Earth observations to estimate the thermal characteristics of the environment in proximity of åÊbus stops throughout the city. The goal of adding NASA Earth observations to the city’s data resources was to help guide strategy about which of the remaining 600 unshaded bus stops should be prioritized for deployment of future shade structures. Building on an ongoing partnership between Phoenix and Arizona State University’s Urban Climate Research Center and Center for Policy Informatics, the DEVELOP team acquired NASA Earth observation data for both land surface temperature and vegetation coverage and iteratively analyzed the data with input from city staff.

The Phoenix Health & Air Quality team used Landsat 5 Thematic Mapper (TM), Landsat 8 Operational Land Imager (OLI), and Terra Advanced Spaceborne Thermal Emission and Reflections Radiometer (ASTER) data to estimate land surface temperature and vegetation prevalence surrounding bus stops. Terra, Landsat 5, and Landsat 8 were used to estimate land surface temperature around our bus stops for the warm season (May‰ÛÒOctober) from 2006‰ÛÒ2016. The ASTER and Landsat imagery were available from approximately 11 a.m. local standard time. After parsing and processing the imagery the team compiled a total of 111 cloud free images of land surface temperature. Landsat 5 and 8 estimates of land surface temperature were calculated using the thermal bands of each, which are provided at a 30-meter spatial resolution. The ASTER land surface temperature product is provided at a 90-meter spatial resolution. These images were aggregated together to produce an image of average land surface temperature at unshaded bus stops. The team also determined which bus stops were consistently the hottest 5 percent of stops in each image, which roughly corresponds to the number of shade structures the city plans to install each year. In addition to being used for land surface temperature, Landsat 8 imagery also was used to estimate the prevalence of vegetation surrounding unshaded bus stops. Vegetation prevalence was estimated by calculating the Normalized Difference Vegetation Index (NDVI), which provides an estimate of vegetation greenness.

A Central Arizona bus stop. Image Credit: McKenzie Murphree/NASA DEVELOP

Finally, the team brought all of the data together in Google Earth to allow city officials to explore the data and examine conditions on the street through Google Street View. The visualization tool was a catalyst for discussion between city staff members and the research team regarding how stops might be prioritized and the different types of challenges associated with providing and maintaining shade structures throughout the city. Through the course of the project, end users asked new questions facilitated by the inclusion of NASA Earth observations and about the NASA data products. This engagement helped staff better understand the thermal environment around bus stops in the context of their institutional knowledge of the bus stops and population they serve. Building upon this dialogue, the DEVELOP team will continue to engage with city of Phoenix staff in future terms to build a more comprehensive decision support tool for strategically deploying shade where it will have the greatest benefits for the health and well-being of transit users. While NASA Earth observations have directly shown interesting patterns in land surface temperature and vegetation across the public transit system through this project, their additional benefit to generate discussion will likely lead to positive decision-making and outcomes beyond the NASA DEVELOP project term.


[1] D. B. Petitti, D. M. Hondula, S. Yang, S. L. Harlan, and G. Chowell, ‰ÛÏMultiple Trigger Points for Quantifying Heat-Health Impacts: New Evidence from a Hot Climate,‰Û Environ. Health Perspect., vol. 124, no. 2, pp. 176‰ÛÒ183, Feb. 2016.

[2] Climate at a Glance. (2017, March 30). [Online]. Available:

[3] Signs of Heat Illness. (2017, March 30). [Online]. Available: 2457/Signs-of-Heat-Illness

Author Biographies

Lance Watkins is a Ph.D. student from Arizona State University working with DEVELOP at Maricopa County Department of Public Health and Arizona State University as an independent research consultant on the Phoenix Health & Air Quality project.

David Hondula is an assistant professor in the School of Geographical Sciences and Urban Planning at Arizona State University and regional location science adviser for NASA DEVELOP.

Tamara Dunbarr is a recent graduate from Arizona State University working with DEVELOP at Maricopa County Department of Public Health and Arizona State University as an independent research consultant on the Phoenix Health & Air Quality project.

McKenzie Murphree is an undergraduate student from Arizona State University working with DEVELOP at Maricopa County Department of Public Health and Arizona State University as an independent research consultant on the Phoenix Health & Air Quality project.