Understanding Your City’s Heat Islands:

Overview and Key Considerations
David Sailor, Ph.D.
Director, Urban Climate Research Center
Professor, School of Geographical Sciences and Urban Planning
David.Sailor@asu.edu

ARIZONA STATE UNIVERSITY
September 7, 2017

Defining “the Urban Heat Island (UHI)”
“An area of higher temperatures in an urban setting
compared to the temperatures of the suburban and rural
surroundings. It appears as an ‘island’ in the pattern of
isotherms on a surface map.”
- Glossary of Weather and Climate, Ira Geer, Ed.

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What causes the urban heat island
phenomenon?
Short-wave (solar) radiation
Convection & mixing

Long-wave (thermal) radiation

Anthropogenic waste heat

Evaporation

Thermal storage

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Most common image when searching the
internet for “Urban Heat Island”

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Air Temperature UHI via empirically-based
models

… usually the “shelter” height of 1.25
to 2 m (~4 to 6 ft)
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Another common type of UHI image…

Camilo Pérez Arrau, 2010

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Surface Temperature UHI is by definition highly
variable…

Image from Moody and Sailor

… and consists of horizontal and vertical surfaces
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A Cautionary Note
“Overall, the quality of the UHI literature and
its empirical content is low at best… nearly
half of the evaluated studies provide
estimates of UHI magnitude that are
unacceptable in terms that environmental
science can reasonably expect.”
Stewart, I.D., 2011.

• Recommendations
1.
2.
3.
4.

Representativeness of sites
Standardized site reporting guidelines
Disclose limits of data (uncertainty)
Use terminology with discretion (e.g., “the UHI”)

Further Caveats about Working with UHI
• Do we really care about a UHI magnitude?
– What is important, DTu-r or T ?

• In fact, how much do we care about T itself?
– Think about the end points that we actually do care about…

• Thermal comfort, heat-related mortality and morbidity
– Air temperature, humidity… and surface temperatures
– Daytime highs are important, but so too are nighttime lows.

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Measuring Urban Thermal Environments
Air Temperatures
• Traditional first-order (NWS) weather
stations
– High quality instruments and generally sound
siting
– Long-term hourly records
– Limited spatial coverage

• Networks of “lower-quality” stations
– Lower quality instruments and siting
– Better spatial coverage

• Mobile traverses
– Poor temporal resolution
– Good spatial coverage (on roads/paths)

• Crowd-sourced data
– Quantity and extensive QA/QC protocols help to
make up for poor quality data.
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General Considerations for Air Temperature
Measurements
• Accuracy of temperature
measurements influenced by
– Sensor type (e.g. Thermistor,
Thermocouple, RTD)
– Sensor temporal response characteristics
– Instrumentation/circuitry
– Sensor environment (e.g. radiation
shielding, aspiration)
– Nearby microclimate

• Representativeness is key to
generating useful data
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Traverse Measurement Example
UHI ~ 6 o C

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Measuring Urban Thermal Environments
Surface Temperatures
• Radiometers on satellites and
aircraft
– Often limited to a few overpasses per
day and require cloud-free conditions
– Vertical surfaces not “visible”
– Intervening atmospheric attenuation
and surface emissivity must be
estimated

• Hand-held IR cameras
– Capable of measuring horizontal and
vertical surfaces
– Limited spatial coverage
– Surface emissivity must be estimated

• Relating to air temperatures
– Uncertainties/errors can be high

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In Summary…
You must be careful in defining your measurement objectives, designing your
measurement/analysis system, and conveying your results… or they are useless.

email: dsailor@asu.edu
URL: UrbanClimate.asu.edu
@ASUrbanClimate

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