Methodology used to determine community Heat Factors 

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Understand the methodological approach used to calculate a community’s heat risk. 

The First Street Foundation Extreme Heat Model (FSF-EHM) was developed to inform, arm, and help individuals and communities understand and prepare for their climate risks, and how they can protect themselves.  The peer-reviewed model is a first-of-its-kind temperature model that determines how the frequency, duration, and intensity of extremely hot days will change over the next 30 years due to a changing climate. 

A changing environment means higher average temperatures and increased humidity, which makes risky health impacts more likely. It’s important to learn about the risk of extreme heat in your area in order for communities and individuals to take preventative steps to reduce heat-related illnesses and deaths. Use Risk Factor to find heat risk assessments for any U.S. address, neighborhood, zip code, city, and county.

Community risk

Heat islands are urban or metropolitan areas that are significantly warmer than the surrounding area due to human activities. The temperature difference is usually larger at night than during the day, increasing the cost and amount of energy required to homes and businesses located in a heat island. Nevertheless, daytime maximum temperatures within a heat island can vary by as much as 7 degrees from the surrounding neighborhood or city. 

Temperature is usually explained with comparisons to local norms. For example, if you live in Texas, a daily high of 100°F may not seem extreme, but if you live in Michigan, a daily high of 100°F is likely alarming and much higher than what's typically considered “hot” for your area.

Risk Factor allows you to search by various community levels to understand how heat can uniquely impact neighborhoods, zip codes, municipalities/cities, and counties. Communities levels available and how they are defined:

  • Neighborhoods: Encompasses macro neighborhoods, neighborhoods, sub-neighborhoods, and residential districts (e.g. subdivisions and apartment complexes) are not an official designation and vary in size depending on local usage
  • Zipcodes: Zip code tabulation areas as provided by the U.S Census Bureau. 
  • Cities: Place as provided by the US Census Bureau. Refer to a village, town, or city typically governed by a mayor and council.
  • Counties: Territorial divisions of a state are typically government units that sit below the state level. County or county equivalents as provided by the US Census Bureau.

Heat Risk 

Local variations in extreme temperatures

The amount of change in heat risk is influenced by local factors such as an area's landscape, vegetation, elevation, urbanization, and distance to water bodies and coastlines. These factors explain why some places will experience large changes in heat risk while others will experience more mild changes. 


Local characteristics that affect heat risk

Neighboring towns and cities can have very different temperatures due to local characteristics. It's easiest to identify local differences when temperatures are the warmest. For this reason, the model looks at temperatures and humidity during the hottest months of the year, which is typically July and in some areas August. Historic temperature records between 2014 - 2020 are used to create a map of average high temperatures across the United States and compare how temperatures vary from property to property within a community.

Determining the impact of climate on heat 

The Extreme Heat Model uses the RCP 4.5 carbon emissions scenario to forecast how temperatures will change 30 years from now. This allows the model to predict temperatures 30 years from now using scientific and peer-reviewed best practices.

Community Heat Risk 

Heat risk for properties within a community

Temperatures during the hottest month of the year are reviewed for each property within a community. For the majority of areas, the hottest month of the year is July, for some areas it's August. The daily “high” temperatures during this month are then averaged to determine the average daily high temperature for a property’s specific location this year, and 30 years into the future. 

These temperatures are then compared to the safety thresholds for heat index informed by the National Weather Service. Heat index, also known as a “feels like” temperature, is a measure used to indicate the level of discomfort the average person is thought to experience as a result of the combined effects of the temperature and humidity in the air. “Health Caution” days are considered to be temperatures around 80 degrees and associated with relatively minor consequences, such as fatigue. Days above 125 degrees are considered “Extreme Danger” as they’re associated with life-threatening effects such as heat stroke.

Calculation of community heat risk 

Community risk for heat does not include specific ratings for different infrastructure categories within a community (i.e. infrastructure, social, roads, etc). Unlike floods and wildfires, heat generally impacts a community and building type fairly equally. Heat does not pose a unique threat to specific building types, daily temperatures of 95°F will impact a hospital and a police station similarly. As a result, community risk is calculated the same as individual properties, based on the average high temperature in the hottest month for all properties in a locality.

Each property’s high “feels like” temperature is determined by averaging their current and future high “feels like” temperature together. A community’s Heat Factor is the overall average high “feels like” temperature, based on the average high “feels like” temperature for all properties located within that community. The overall average temperature for all properties is used to determine the level of heat risk a community faces. If the average high temperature in the hottest month for all properties in a locality is:

  • Less than 80°F: a community will have Minimal heat risk 
  • 80°F  - 83°F: a community will have Minor heat risk 
  • 83°F  - 89°F: a community will have Moderate heat risk 
  • 89°F  - 95°F: a community will have Major heat risk
  • 95°F  - 104°F: a community will have Severe heat risk
  • Greater than 104°F: a community will have Extreme heat risk

Due to the lack of local variation in extreme temperatures, community risk scores are typically similar to the average Heat Factor score of properties in the geography. Geographies with higher individual property scores can expect higher community scores. Additionally, you can still find a particular facility’s heat risk by searching its address on, as each community facility still has its own Heat Factor.

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