Evaluating Urban Canopy's Role in Alleviating Extreme Heat: An i-Tree Cool Air Analysis

This report details findings from i-Tree Cool Air simulations for the hottest day of 2021 at the census block group level. The report is split into the following sections:

  • Temperature: maximum temperature reached
  • Heat Index: maximum heat index reached
  • Risk Scale: normalized risk scale (0-1) for extreme heat exposure
  • Canopy: maximum temperatures reached with adjusted canopy cover
  • Impervious: maximum temperatures reached with adjusted impervious cover
  • Urban areas: analysis of heat in urban areas only

If you are looking for regional reports, select your area of interest below:

Image Map

Air Temperature

Using i-Tree Cool Air, the hottest day across the United States was simulated at a 300m raster resolution. These runs were combined to create a national map representing the hottest air temperature's reached throughout the country.

Attribute Degrees (°F)
Average Maximum Temperature 92.47
90th Percentile Maximum Temperature 99.45

Heat Index

The average maximum heat index in 2021 is represented at the census block group level.

Attribute Degrees (°F)
Average Maximum Heat Index 93.6
90th Percentile Maximum Heat Index 103.2

Heat Index is defined as the apparent temperature in shaded areas based on the air temperature and relative humidity. Thresholds of heat index safety set by the National Weather Service (NWS) are as follows:

Heat Index Classification Effect on the body with prolonged exposure/activity
80-90°F Caution Fatigue possible
90-103°F Extreme Caution Heat stroke, heat cramps, or heat exhaustion possible
103-124°F Danger Heat cramps or heat exhaustion likely, and heat stroke possible
125°F+ Extreme Danger Heat stroke highly likely

Normalized Risk Scale

The normalized risk scale is between 0 and 1, based on the following formula:

    Risk = place value of air temperature / value of maximum US place value


Effects of Canopy on Extreme Heat

Canopy cover was adjusted for i-Tree Cool Air simulations to compare the Heat Index at -50% canopy cover to +50% canopy cover.

Attribute Temperature (°F)
-50% Canopy 94.91
+50% Canopy 90.99
Average temperature increase per 10% loss of canopy1 0.49
Average temperature decrease per 10% gain of canopy1 -0.294

1Where each pixel is a surface area of 90000m2, 1% is equal to 900m2 of canopy. For more information, see the Methodology section.


canopy

Effects of Impervious Cover on Extreme Heat

Impervious cover was adjusted for i-Tree Cool Air simulations to compare the Heat Index at -50% impervious cover to +50% impervious cover.

Attribute Temperature (°F)
+50% Impervious Cover 97.44
Average temperature increase per 10% gain of impervious cover1 0.99

1Where each pixel is a surface area of 90000m2, 1% is equal to 900m2 of impervious area. For more information, see the Methodology section.


impervious

Heat in Urban Areas

Census block groups within urban areas as defined by the United States census were isolated for a separate analysis. For a more in-depth look at urban heat, see the subregional reports.

Attribute Value Difference from National Averages
Average Temperature (°F) 93.64 +1.17
Average Heat Index(°F) 95.14 +1.54
Average temperature increase per 10% gain of impervious cover(°F) +1.152 +0.162
Average temperature increase per 10% loss of canopy(°F) 0.63 +0.14
Average temperature decrease per 10% gain of canopy(°F) -0.318 -0.024
90th percentile of temperature(°F) 106.643 +7.193

Average canopy cover for +90th percentile of temperature (%) 3.037