The Data Behind Heat Factor®

Heat Factor uses the First Street Extreme Heat Model to find heat risks across the United States.

  • 16.1M
    Properties with Extreme Risk
  • 107M
    Population with yearly days above 125°F in 30 years
  • 158%
    Avg. increase in local hot days
heat map

Heat Factor distribution of properties at risk* (143.5M analyzed)

  1. Minimal - 6.6M
  2. Minor - 15.9M
  3. Moderate - 39.9M
  4. Major - 36.9M
  5. Severe - 28.1M
  6. Extreme - 16.1M

* Risk is calculated as the average monthly max temperature over 30 years.

Providing first-of-its-kind heat analysis

The First Street Extreme Heat Model is a first-of-its-kind, nationwide spatial temperature model that shows a specific location’s exposure to extreme heat events based on the surface temperature, topography, land cover, and humidity in the surrounding area. It builds off of decades of peer-reviewed research and forecasts how heat effects will change over time due to changes in the environment.

Calculating and mapping heat risks

The First Street Extreme Heat Model is a high-resolution spatial temperature model, which means it considers how a specific location’s temperature is affected by its immediate surroundings and uses that information to assess the effects on a property’s exposure to extreme heat.

The process starts by asking “What does it mean to be hot at this property?” The First Street Extreme Heat Model answers this by building a high-resolution forecast of the average high “feels like” temperature during the hottest month of the summer using multiple datasets from the US Geological Service (USGS) and the National Oceanic and Atmospheric Association (NOAA) that include both temperature and humidity. Local variations in temperature are driven by the vegetation and building materials – it’s cooler under a tree and hotter on an asphalt parking lot. Because of this, the data includes information about tree canopy and land cover. By averaging ten years’ worth of data, the model is able to generate an accurate map that compares how temperatures vary from property to property within a community.

    Land temperature
    Impervious surfaces

Determining future heat exposure

As with any First Street Foundation risk model, the inclusion of environmental changes that impact future heat risk is an essential part of the First Street Foundation Extreme Heat Model. The model analyzes multiple possibilities under the RCP 4.5 carbon emissions scenario to forecast how temperatures will change 30 years into the future and applies the outcomes of that forecast to the high-resolution spatial temperature model. This allows the First Street Foundation Extreme Heat Model to predict temperatures 30 years from now in a way that meets the rigorous standard of scientific peer review.

IPCC Representative Concentration Pathways

Scoring system for properties

Once the average daily high “feels like” temperature has been calculated at a property-level for both this year and 30 years, the average of that 30-year time period is taken in order to arrive at a single temperature value that represents both how hot a specific location is and how much the temperature is expected to grow at that property. A property’s Heat Factor is based on that 30-year average, which means that a property with a higher Heat Factor currently experiences higher temperatures during the hottest month of the year or will in the future.

Simplifying heat risks

Temperature is not something that is often thought of on a property-by-property basis and is instead communicated through comparisons to local experiences and norms. This means that any analysis of heat has to go a step further and ask “What is a hot day in the area around this property?”

To answer that question, the First Street Foundation Extreme Heat Model uses data from the Climate Model Intercomparison Project to identify the temperature on the hottest seven days in a given area and uses that to define what someone in the area would reasonably call a “hot day.” Once the benchmark temperature for these “hot days” has been identified, the model uses the high-resolution property-to-property heat data to adjust the number of hot days that are experienced to an individual property level and then uses the temperature predictions associated with the same RCP4.5 carbon emissions scenarios to predict the number of hot days 30 years into the future, showing how the felt experience of heat will change over time based on a property’s surroundings.

Identifying heat safety issues

While the experience of heat may vary from community to community, there are certain health effects from heat that cannot be ignored. Extreme heat is the deadliest of all natural events, and as such an important goal of the First Street Foundation Extreme Heat Model is identifying heat events that pose a threat to human safety. To do this, the model uses the same Climate Model Intercomparison Project data used to identify local hot days to measure the number of days both this year and in 30 years that a community will experience exceeding safety thresholds informed by the National Weather Service.

Measuring the effect of heat on energy costs

Unlike flooding and wildfire, heat events aren’t overtly destructive to property. That does not mean, however, that heat comes without a cost to homeowners. The most obvious way that heat impacts homeowners is through the energy cost associated with cooling a home.

In order to give individuals a better understanding of this impact, the First Street Foundation Extreme Heat Model combines open-source data on building square footage and construction year with data from the U.S. Energy Information Association on energy costs to include a calculation of the approximate cost of cooling a home. This projection is based not just on the number of days cooling is recommended in the home, but also on the degrees of cooling required to maintain a consistent indoor temperature. As with all of the data included in the model, this is not only calculated for the present but also for 30 years in the future, allowing homeowners and potential home buyers to see how their cooling costs will be impacted by rising temperatures and a changing climate.

Ensuring scientific accuracy

The creation of the First Street Foundation Extreme Heat Model required an unprecedented partnership of top climate scientists, modelers, technologists, and analysts.

  • Where possible, data has been validated against historic temperature records and government records.
  • All methods used by the First Street Foundation Extreme Heat Model have been submitted to scientific peer-review journals

Continuously improving over time

First Street Foundation has made its extreme heat model’s full technical methodology available to the public because it supports scientific collaboration and data transparency. The First Street Foundation Extreme Heat Model will continue to incorporate feedback and expand its model over time, including an annual data update.

Why are heat risks increasing?

A changing environment means higher temperatures and changing humidity, creating conditions which exacerbate the effects of extreme heat.

What can be done to address heat risk?

Although heat risk can never be completely eliminated, there are a number of steps homeowners and communities can take to reduce risk.

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