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Methodology and models behind ClimRR future projections
Each climate model generates 8 temperature readings per day. The maximum daily temperature refers to the highest of these 8 readings, which often occurs in the middle of the daytime and is comparable to the 'high temperature' in a daily weather forecast. Similarly, the minimum daily temperature refers to the lowest of these 8 readings, which often occurs overnight and is comparable to the 'low temperature' in a daily weather forecast. The mean daily temperature computes the average of all 8 temperature readings for a given day.
Argonne calculated minimum, mean, and maximum daily temperatures at both seasonal and annual timesteps. Each of these calculations involves extracting the min/mean/max for each day of a particular timestep (e.g., winter) for all years within a scenario (e.g., 2045-2054) using data from all three downscaled GCMs.
Each climate model estimates an amount of precipitation (whether rain, snow, sleet, or ice) that occurs every 3 hours across the entire modeled period. These 3-hour precipitation estimates were used to calculate the average precipitation over a designated amount of time, ranging from daily to annually. For example, Argonne calculated average annual precipitation by adding all 3-hour precipitation estimates for each year within a scenario (e.g., 2045-2054) using data from all three downscaled GCMs.
Each climate model estimates an amount of precipitation (whether rain, snow, sleet, or ice) that occurs every 3 hours across the entire modeled time period. These 3-hour precipitation estimates were used to identify days in which no precipitation occurred (i.e., the total daily precipitation quantity equaled zero). Using this information, Argonne calculated the greatest number of consecutive days in which no precipitation occurred for each year in a scenario. This process was repeated for each year in a scenario in order to identify the maximum value (e.g., the decadal maximum) for each climate model. This resulted in 3 values (one for each downscaled GCM) were averaged to estimate the decade's highest number of consecutive days without precipitation in a single year.
Each climate model estimates average wind speed magnitudes every 3 hours, or 8 readings per day, across the entire modeled period. These 3-hour wind speed estimates were used to calculate average daily wind speeds; these daily estimates were then used to calculate average wind speeds over the course of a year. For example, Argonne calculated annual wind speeds by taking the average of each day's average wind speed estimate for each year within a scenario (e.g., 2045-2054) using data from all three downscaled GCMs.
Degree days measure how cold or warm a location is by comparing the daily average temperature to a reference temperature, usually 65°F (18.33°C). Heating or cooling degree days roughly correlate with building heating or cooling needs, providing a simple useful estimate in energy planning. Heating degree days (HDD) measure how cold the temperature was on a given day or period of days, which shapes the need for a certain amount of building heating. For example, a day with an average temperature of 40°F has 25 HDD (65 - 40 = 25 HDD). Similarly, cooling degree days (CDD) measure how hot the temperature was on a given day; a day with an average temperature of 90°F has 25 CDD (90 – 65 = 25 CDD).
For ClimRR, we used each day's maximum temperature (the daily high) and minimum temperature (the daily low) to estimate the daily average temperature. This daily average was used to calculate that day's HDD (if the average temperature was below 65°F) or CDD (if the average temperature was above 65°F). Daily HDD or CDD were aggregated to produce seasonal averages or annual averages, using ensemble means for each emissions scenario.
The Fire Weather Index (FWI) estimates wildfire danger using weather conditions that influence the spread of wildfires. The FWI is comprised of multiple components that are developed using daily readings of temperature, relative humidity, wind speed, and 24-hour precipitation. This weather information is used to estimate the total fuel available for combustion (dry organic material) and the rate of fire spread. The FWI is useful for evaluating weather-based conditions that heighten the danger of wildfire spread once ignition has occurred; it does not account for sources of ignition, which can have both natural and human causes.
The FWI ranges from zero to infinity, with higher numbers corresponding to greater fire danger. The level of wildfire danger, as represented by FWI, varies based on regional characteristics, such as a region's typical level of fire danger and its land cover. For example, areas in the U.S. Southwest, which are often exceptionally dry, will have higher average daily FWI values than areas in the Northeast. A relatively high FWI that suggests a heightened level of fire danger in the Northeast might correspond to average fire danger in the Southwest. Despite this variation, FWI values in excess of 20 typically represent high levels of fire danger, with levels above 30 representing very high to potentially extreme levels of fire danger. A representative example of fire danger classes used by the European Forest Fire Information System can be found here.
The FWI is comprised of six components that are developed using daily readings of temperature, relative humidity, wind speed, and 24-hour precipitation. These readings are taken at solar noon and reflect fire danger at early- to mid-afternoon, typically the time of day in which fire weather conditions are more conducive to fire spread.
Three of the FWI's components estimate the amount of moisture on and beneath the forest floor. These include measures of the moisture content of:
These moisture content components feed into the two primary subindices that generate the FWI. The first of these is the Initial Spread Index, which measures the expected rate of fire spread; it is based on wind speeds and the moisture content of fine fuels (#1 above). The second is the Buildup Index, which represents the total amount of forest fuel available for consumption, as measured via moisture in intermediate and deep organic layers (#2 and #3 above). The Initial Spread Index and the Buildup Index combine to generate the Fire Weather Index, or the index depicted in ClimRR.
The FWI, also known as the Canadian Forest Fire Weather Index, was developed by the Canadian Forest Service. For more information on the index and its methodology, please visit the National Wildfire Coordinating Group or the Canadian Forestry Service's report detailing FWI's development and structure.
Heat index combines temperature and relative humidity to reflect how hot conditions actually feel to people outdoors.
ClimRR uses an "extended heat index" formula designed to better capture extreme heat, which matters because future conditions may exceed historical norms.
Each climate model generates data "every 3 hours, or 8 readings per day," from which the daily peak value is selected.
Warmer-weather seasons examined:
Daily max heat index – averages each day's peak reading across ~92 days per season, then across the decade.
Seasonal max heat index – identifies the single highest daily peak per season annually, averaged over the decade.
Threshold exceedance days – counts days surpassing "95°F, 105°F, 115°F, and 125°F," thresholds representing a spectrum of heat-related human health risk, averaged annually across the decade.
Wet Bulb Globe Temperature, or WBGT, is a measure of heat stress on the human body, accounting for four major atmospheric factors: temperature, humidity, wind speed, and solar irradiance. WBGT is a measure of apparent temperature, better relating how hot it feels to the human body than temperature alone. Whereas heat index uses only temperature and humidity and is calculated for shady areas, WBGT accounts for heat effects from sunlight as well as cooling effects from wind.
WBGT is widely used to evaluate heat-related risks for both outdoor and indoor activities. While guidelines vary by organization, 78°F represents a typical threshold for moderate heat risk, 82°F for high heat risk, and 86°F for very high or extreme heat risk. Those undertaking activities in these conditions should increase hydration, take more frequent breaks, and monitor for signs of heat illness.
Each climate model used as inputs in ClimRR generates temperature, relative humidity, wind speed, and solar irradiance from readings every 3 hours, or 8 readings per day. This information was used to calculate WBGT for all 8 time points per day. We then used these readings to calculate the following:
Freeze-free days are calculated by comparing daily minimum temperatures to 32°F, then tallying days above that threshold.
Season Length: Defined as "the longest period of consecutive days without freezing temperatures each year," with median values selected across each decade.
Start/End Date Changes: Negative values indicate earlier future starts; positive values reflect later ones.
Near-Continuous Seasons: Where freeze-free days exceed 360 annually, the data is nullified and regions display as a "Near Continuous Growing Season."
