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Climate & Health – High Heat Index Days (Absolute)

This indicator reports the absolute threshold heat index days (2020-22 three-year average) of the daily heat metrics as 95, 100, and 105 degrees Fahrenheit. The “heat index” is a single value that takes both temperature and humidity into account. The higher the heat index, the hotter the weather feels, since sweat does not readily evaporate and cool the skin. The heat index is a better measure than air temperature alone for estimating the risk to workers from environmental heat sources. Data were obtained from the North America Land Data Assimilation System (NLDAS) via the CDC National Environmental Public Health Tracking Network, 2024.

Source

Centers for Disease Control and Prevention, CDC – National Environmental Public Health Tracking, 2020-22.

Source Description

Since 2002, the CDC National Environmental Public Health Tracking Network (Tracking Network) brings together health data and environment data from national, state, and city sources and provides supporting information to make the data easier to understand. The Tracking Network has data and information on environments and hazards, health effects, and population health.

Measures of the Historical Temperature & Heat Index include

  1. Number of Extreme Heat Days (as reported in this indicator report)
  2. Dates of Extreme Heat Days
  3. Number of Extreme Heat Events
  4. Dates of Extreme Heat Events
  5. Daily Estimates of Maximum Temperature for Summer Months (May-September)
  6. Daily Estimates of Maximum Heat Index for Summer Months (May–September)
  7. Weekly Average Maximum Temperature

Data for the first six measures are obtained from the Forcing File A of Phase Two of the North American Land Data Assimilation System (NLDAS-2) (1979-ongoing), and are available for all states except Alaska and Hawaii. For the last measure – Weekly Average Maximum Temperature, data are obtained from the Gridded 5km GHCN – Daily Temperature and Precipitation Dataset (nCLIMGRID) – Gridded 5km GHCN-Daily Temperature and Precipitation Dataset, Version 1 (noaa.gov) (2017-ongoing). For more information please check out the Tracking Network’s indicator page for Historical Temperature & Heat Index.

Methodology

Heat index data are obtained from the CDC Environmental Public Health Tracking division. The CDC provides the following information about the underlying data and calculation:

The heat measures are derived from estimates of air temperature (K) at 2 meters above the surface, specific humidity (kg/kg) at 2 meters above the surface, and surface pressure (Pa) from Forcing File A of Phase 2 of the North American Land Data Assimilation System (NLDAS-2). NLDAS-2 is available at the 1/8th-degree grid (approximately 14×14 km) and consists of 103,936 grid cells that cover the entire United States, excluding Alaska and Hawaii.

The gridded raw data were summarized to the U.S. county or census tract level to aid in estimating population exposure to high temperature and heat index conditions and to enable linkage with health-related datasets. To accomplish this, U.S. census block group centroids were attributed to individual NLDAS grid cells based on a containment relationship. Maximum daily temperature and heat index were determined for each block group by identifying the maximum hourly value for each day. Using census block group population as weights, population-weighted averages by U.S. county and census tract were calculated.

Heat index was estimated using a modified version of the Rothfusz regression as implemented by the National Weather Service [1]. Relative humidity (needed for heat index calculation) was calculated from specific humidity data acquired from NLDAS-2 using the Wexler saturated water vapor pressure equation [2].

The 90th, 95th, 98th, and 99th percentile values of the daily heat metrics were determined for each county and census tract for the period between 1979 and 2021 (May – September values only). Extreme heat days are classified according to the following thresholds: (1) absolute (e.g., 90°F, 95°F, 100°F, 105°F) and (2) relative (e.g., 90th, 95th, 98th, and 99th percentile) values.

References:
1. Heat Index Equation. The National Weather Service. Last modified May 2014.