Climate Matters•May 24, 2023
Burning Hot: 50 Years of Fire Weather Across the United States
Climate Central examined historical trends in fire weather—a combination of low humidity, high heat, and strong winds—across the U.S.
This analysis uses data from 476 weather stations to assess trends in 245 climate divisions spanning all 48 contiguous U.S. states over a 50-year period (1973-2022).
Wildfire seasons are lengthening and intensifying, particularly in the West. Many parts of the East have seen smaller but impactful increases in fire weather.
Even small increases in fire weather in the East, which has nearly 28 million homes located in zones prone to burn, puts more people at risk.
Climate Central’s new report, Wildfire Weather: Analyzing the 50-year shift across America, expands on wildfire risks and adaptation across the country.
Climate change is affecting weather conditions in ways that increase wildfire risks. Warming temperatures and increasingly dry air, vegetation, and soils make it easier for fires to spread, and more difficult to fight or prevent.
Fire weather primes the landscape for wildfires to start and spread.
Fire weather refers to meteorological conditions that promote the spread of wildfires. Climate Central’s analysis focuses on three conditions fundamental to fire weather: relative humidity, temperature, and wind.
When relative humidity levels are very low, the air pulls moisture from the land, leaving vegetation dry and prone to burning. This analysis shows decreasing relative humidity has been a major factor in boosting annual fire weather days for many locations across the U.S.
Hotter temperatures have a direct influence on fire behavior, heating the fuels and making them more likely to ignite. Fire weather conditions also restrict firefighters’ ability to put out wildfires. Climate change is causing warmer nighttime temperatures, which decreases the overnight relative humidity that once helped firefighters gain control over wildfires.
Wind supplies oxygen to a fire, causing it to burn more rapidly. Wind increases evaporation, drying out the land. Wind also carries embers, which helps fires spread.
The full report Wildfire Weather: Analyzing the 50-year shift across America discusses other key factors that influence wildfire, including fuels (like dried vegetation or downed trees), other weather conditions, and human activity.
Climate differences across the country influence fire weather trends
To investigate trends in fire weather, a key factor of wildfire risk, Climate Central analyzed data from 476 weather stations to assess trends in 245 climate divisions spanning the 48 contiguous U.S. states during the last 50 years (1973-2022).
The contiguous U.S. is 2.5°F warmer today than it was in 1970. Warming that influences fire weather has been observed across the country. But key regional climate differences spanning from the West to the East result in distinct fire weather trends.
As this analysis demonstrates, fire weather isn’t limited to the western U.S.—all regions experience conditions that increase the likelihood of more intense wildfires. Although the findings are divided into broad geographic regions (West and East) these two groups are comprised of multiple climate regions and climate divisions, as defined by NOAA’s National Centers for Environmental Information. Note: the NCEI South region was split into the western South (Texas, Oklahoma, and Kansas) and eastern South (Arkansas, Louisiana, and Mississippi) and grouped as indicated in the tables below.
|NCEI Climate Region (West)||Aggregated NCEI Climate Divisons (#)||Average annual fire weather days, 1973-2022||Average change in annual fire weather days, 1974-2022||Households units in the wildland urban interface (WUI), 2020|
|Northern Rockies & Plains||29||26||5||797,184|
|NCEI Climate Region (East)||Aggregated NCEI Climate Divisons (#)||Average annual fire weather days, 1973-2022||Average change in annual fire weather days, 1973-2022||Households units in the wildland urban interface (WUI), 2020|
Fifty Years of Fire Weather in the West
Southern California, Texas, and New Mexico have experienced some of the greatest increases in fire weather days each year, with some areas now seeing around two more months of fire weather compared to a half century ago.
Some places, including parts of Texas, California, Oregon, and Washington, are experiencing fire weather more than twice as often now than in the early 1970s.
Western interior regions saw greater increases in fire weather compared to western coastal regions, where higher humidity levels mean these areas are less likely to reach the low relative humidity threshold used in the analysis.
By contrast, parts of Idaho, North Dakota, and South Dakota have experienced a decline in the frequency of fire weather days. The Dakotas are part of a region where springs have been cooling slightly—likely a temporary trend influenced by the cooling effects of agricultural development.
Generally, the West has seen the greatest jump in fire weather days in the summer (particularly in the Southwest).
Fifty Years of Fire Weather in the East
A band stretching from coastal Maine through the Northeast and Piedmont Valley and into northern Florida experiences the highest average number of annual fire weather days in the East.
Parts of the Southeast experience several weeks of fire weather on average each year. Northern Florida experiences around a month.
The analysis shows relatively modest changes to the average number of fire weather days in the East since the 1970s. The humid heat that’s typical in the East makes extremely dry days less common, and likely dampens fire weather trends.
Parts of the Southeast and Northeast have experienced some of the greatest increases in annual fire weather days across the East.
Most eastern areas included in this analysis saw, at most, a few more days of fire weather since 1973—and around 20% of eastern climate divisions saw either no change or a decrease.
New England has seen a decrease in annual fire weather days, driven largely by fewer days where the wind speed variable hit the analytical threshold. Slowing wind speeds could be partially attributed to a decrease in the temperature gradient between land and sea along the Gulf of Maine, as sea surface temperatures have warmed drastically due to climate change.
Across much of the region, the greatest increase in fire weather days is happening in the spring months, which aligns with spring warming trends.
More people are at risk from fire weather
Three of the top states with the greatest number of households in the WUI also saw an increase in fire weather days during the period of Climate Central’s analysis: California, Texas, and North Carolina.
California contains the most households in the WUI (5.1 million) of the 48 contiguous U.S. states. But most of the WUI is concentrated in the East—nearly 80% of WUI lands as well as more than 60% of WUI housing is in eastern states. The Southeast alone has around 10.6 million households in the WUI.
Other consequences of more frequent fire weather
In addition to increasing the risk of fire, more frequent fire weather conditions are causing problems even when fires don’t ignite.
The same weather variables that influence fire weather are factors in determining the safe application of prescribed fire—an important fuel-reducing forest management activity. More fire weather days mean fewer windows to conduct prescribed burning, leaving fuels like dried leaves, grasses, and downed trees to build up on the landscape.
In response to high-risk fire weather conditions, and in the hopes of avoiding equipment-related ignitions, power companies in the West are shutting off electricity to millions of people during public safety power shutoffs.
Climate Central’s report Wildfire Weather: Analyzing the 50-year shift across America details how climate change is amplifying risks from increasing fire weather and impacting communities across the country.
LOCAL STORY ANGLES
Does your local area face immediate fire risk?
NOAA’s Storm Prediction Center produces daily reports of elevated fire weather conditions. The Incident Information System identifies where wildfires have been reported and provides detailed information on conditions. You can stay updated about risk of fire with the North American Seasonal Fire Assessment and Outlook, produced by the National Interagency Fire Center (NIFC) in Boise, Idaho at the beginning of each month.
Are wildfires affecting air quality in your area?
Wildfire smoke can travel thousands of miles, carried along the jet stream, and affect air quality far beyond the fire. Find wildfire smoke tracking maps through the NOAA Office of Satellite and Product Operations and AirNow.
How many people live in areas prone to wildfire in your state?
The University of Wisconsin-Madison’s SILVIS Lab provides maps and data on the wildland urban interface (WUI) from 1990-2020.
What can communities do to adapt to wildfire risks?
The U.S. Fire Administration outlines steps communities can take toward being wildfire adapted. A number of fire adaptation strategies include increased use of land management techniques that eliminate excessive fuels, such as prescribed burns, or creating defensible space around homes and other structures. There are toolkits to prepare homes and create evacuation plans.
John Abatzoglou, PhD
Professor of Climatology and Graduate Program Chair (MIST)
University of California, Merced
Relevant expertise: western wildfire, climate change
Daniel L. Swain, PhD
Climate Scientist, Institute of the Environment & Sustainability, UCLA California Climate Fellow, The Nature Conservancy
Relevant expertise: wildfire and climate change
Colleen Reid, PhD
Assistant Professor of Geography
University of Colorado, Boulder
Relevant expertise: health impacts of wildfire smoke
Find local experts through the Fire Science Exchange Network, which includes 15 fire science exchanges across the US.
Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts.
Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.
This analysis defines a “fire weather day” as one where the following three conditions co-occur in at least two hourly measurements:
Relative humidity within 5% of regional thresholds defined by NOAA’s Storm Prediction Center (based on predominant fuel type and local climate)
Temperature of at least 45-55°F, depending on the season (winter: 45°F; summer: 55°F; spring and fall: 50°F)
Sustained wind speeds of 15 mph or greater (10 mph or greater for Florida locations)
Hourly observations for 476 stations across the contiguous United States were obtained from 1973-2022 using NOAA/NCEI’s Local Climatological Data. Stations needed to pass several data completeness checks in order to be included in this analysis. Using the thresholds detailed above, Climate Central calculated the number of annual and seasonal fire weather days at each station over the past 50 years. Thresholds were not available for Alaska, Hawaii, or Puerto Rico, and therefore these locations were not included in this analysis. Results were mapped to each station’s climate division. For climate divisions with multiple stations, data were averaged across all stations. Additionally, we calculated the average annual and seasonal fire weather days and used linear regression to calculate the total change. Data were rounded to the nearest whole day. Some climate divisions only had one weather station inside its boundary. While confidence in our results increase with additional stations, our results are consistent with recent findings (for example, here and here).
The complete methodology is available in the report, Wildfire Weather: Analyzing the 50-year shift across America.