Heat warning, advisory, and watch data from the National Weather Service (NWS) were available as shapefiles from NWS. View metadata for this dataset.

NWS defines heat advisories, watches, and warnings as follows:

"Excessive Heat Warning—Take Action! An Excessive Heat Warning is issued within 12 hours of the onset of extremely dangerous heat conditions. The general rule of thumb for this Warning is when the maximum heat index temperature is expected to be 105° or higher for at least 2 days and night time air temperatures will not drop below 75°; however, these criteria vary across the country, especially for areas not used to extreme heat conditions. If you don't take precautions immediately when conditions are extreme, you may become seriously ill or even die.

Excessive Heat Watches—Be Prepared! Heat watches are issued when conditions are favorable for an excessive heat event in the next 24 to 72 hours. A Watch is used when the risk of a heat wave has increased but its occurrence and timing is still uncertain.

Heat Advisory—Take Action! A Heat Advisory is issued within 12 hours of the onset of extremely dangerous heat conditions. The general rule of thumb for this Advisory is when the maximum heat index temperature is expected to be 100° or higher for at least 2 days, and night time air temperatures will not drop below 75°; however, these criteria vary across the country, especially for areas that are not used to dangerous heat conditions. Take precautions to avoid heat illness. If you don't take precautions, you may become seriously ill or even die."

AirNow is a one-stop source for air quality data. AirNow reports air quality using the official U.S. Air Quality Index (AQI), a color-coded index designed to communicate whether air quality is healthy or unhealthy.

The AQI is calculated for four major air pollutants regulated by the Clean Air Act: ground level ozone, particle pollution, carbon monoxide, and sulfur dioxide. For each of these pollutants, EPA has established national air quality standards to protect public health. Of these, ground-level ozone and airborne particles are the two pollutants that pose the greatest threat to human health.

AirNow is a partnership of the EPA, NOAA, National Park Service, NASA, Centers for Disease Control, and tribal, state, and local air quality agencies.

Data from the National Weather Service Weather Prediction Center's (WPC) Day 3–7 U.S. Hazards Outlook were available as shapefiles from the CPC Hazards Web Service. This map displays Hazardous Heat risk areas.

Below are the criteria used for issuing Hazardous Heat risk areas. Because prior conditions play a role in impacts of these hazards, these hazard definitions are only guidelines. Often, actual considerations for the hazard outlook are based on subjectivity such as time of year, deviation from normal, or expected impacts.

Hazardous Heat: The Hazardous Heat hazard risk is typically issued when there is a 40% or greater chance for of exceeding  NWS Heat Advisory criteria (based on the Heat Index for your location within the day 3–7 forecast period) or widespread major (level 3) or scattered extreme (level 4) HeatRisk. For Alaska, criteria include high temperature thresholds ranging between 70–90 degrees during the summer months.