The methodology behind the model used by Heat Factor

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First Street Foundation’s Extreme Heat Model is a first-of-its-kind model that relies on public data to determine property-specific heat risk. 

Data on Heat Factor comes from the First Street Foundation Extreme Heat Model (FSF-EHM). The model is a spatial temperature model that determines a specific location’s exposure to extreme heat. Exposure to extreme heat is determined by measuring how the temperature of a specific location is affected by its immediate surroundings.

A property's Heat Factor is an indicator of its risk of extreme heat exposure over the next thirty years.  The model assigns each property a Heat Factor, ranging from 1 (minimal risk) to 10 (extreme risk). Heat Factors are based on the current average daily high temperature and humidity of a property’s specific location, and how much the average daily high temperature and humidity in that location is expected to grow over the next 30 years.   

Changing heat risks and local heat variability 

Climate change influences average temperatures and humidity, causing heat risks to change. 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. For example, the urban heat island effect is commonly discussed as an example of how temperatures vary significantly. This effect causes temperatures to typically be warmer in urban areas compared to the more leafy, and less dense, suburban areas of a given city. 

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 are in most areas in July, and in some areas in August. Historic temperature records between 2014 - 2020 are used to create a map of average daily high temperatures across the United States and compare how temperatures vary from property to property within a community. 

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The process used to develop the Extreme Heat Model

The impact of climate on heat 

The expected change in average temperature is influenced by the changing environment. Increased temperatures and increased humidity work together to make heat index more extreme and risky health impact more likely. 

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.

The calculation of property-specific Heat Factors 

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

These temperatures are then compared to the safety thresholds for heat index informed by the National Weather Service. The National Weather Service considers “Caution” days to be temperatures above 80°F and associated with relatively minor consequences, such as fatigue. Days above 125°Fare considered “Extreme Danger” as they’re associated with life-threatening effects such as heat stroke.


NWS Heat Caution Index

Properties with higher Heat Factors have higher current and/or future daily “feels like” temperatures during the summer months.

Putting it in local context  

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.

For this reason, the Extreme Heat Model determines what someone would reasonably consider a “hot day” based on the specific location of a property. A “hot day” refers to the temperature on the hottest days 7 days for that specific location. The term “hot day” is used to represent local heat index temperatures. 


Local hot day threshold temperatures in 2023 (°F)

The probability of heat waves

The probability of a heat wave for a given area is based on daily high “feels like” temperature or heat index temperature. Based on current environmental conditions and future environmental conditions, the number of days in a row the temperature is above the “feels like” temperature is used to determine the length of a heat wave for a given location.

A heat wave consists of 3 or more days in a row where the temperature is warmer than the local definition of a “hot day”. The probability of a heat wave refers to the likelihood that any 3+ day stretch of temperatures is above the local “hot day” temperature.

Impact on health and safety

The human body uses sweat to cool down, which is important because the body may overheat when in hot temperatures. When there’s more moisture in the air or higher humidity, the body is unable to properly cool down because sweat does not effectively evaporate in humid environments. That’s why dry heat is typically thought of as more comfortable than humid heat. Increases in extreme heat conditions create health concerns. Heat may threaten human safety by leading to dehydration, fatigue, heat stroke, heat exhaustion, heat cramps, hospitalization, and even death. Because extreme heat is the deadliest of all natural events year over year, an important goal of Heat Factor is to communicate where heat risks can pose a threat to human safety. 

Calculating the cost of heat

When temperatures reach such extremes that can cause these types of serious health impacts, people may take respite in air conditioning. However, not everyone has access to homes or buildings with air conditioning. Understanding how extremely hot days are projected to increase in the future can help communities better prepare for seasonal energy use changes and protect those who might be more susceptible to hospitalization at higher temperatures. 

As temperatures increase into summer months each year, most properties spend more on electricity during these months due to the increased use of air conditioning. However, the cost to cool a home depends on the temperature outdoors and specific property characteristics. The square footage of a home and the year it was built affect its expected AC usage and efficiency. The latitude, number of cooling degree days, and the degrees of cooling required to maintain a consistent indoor temperature provide information on the location of the home and its annual temperature ranges. A property’s annual electricity consumption is estimated as the price that each household could pay for electricity. However, the average cost of electricity in a particular area is dependent on how the local grid is supplied, the companies that supply an area, and the number of homes. Using historic data on energy rates over the past five years (2016-2021), energy costs from June to September are calculated for each state.

It's important to note that the costs associated with heat only consider air conditioning costs and do not account for reductions in heating costs or other electricity usage. 

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