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What is a heat wave: A survey and literature synthesis of heat wave definitions across the United States [1]
['Erin L. Bunting', 'Department Of Geography', 'Environment', 'Spatial Sciences', 'Michigan State University', 'East Lansing', 'Michigan', 'United States Of America', 'Remote Sensing', 'Gis Research']
Date: 2024-10
The professional profile of those that completed the survey is diverse and does not skew to one group of individuals or those with certain specialties. Eighty-seven (53.7%) of the completed surveys came from those that identified forecasting as their occupation. Of those 62 worked across multiple counties in a single state, 11 at the state level, 7 at the regional (multiple states or parts of multiple states), 3 at the county level, 2 at an “other level” (city to local level), 1 globally, and 1 at the community level. After forecasting the next highest group of responses came from emergency managers. Overall, 46 (28.2%) emergency managers from across the US completed the survey. Of those emergency managers 32 worked at the county level, 2 worked across multiple counties, 1 worked at the regional scale (across multiple states), 1 worked at the national level, 1 worked at the community level, and the remaining 9 worked at other scales (e.g., tribal lands, city, ecoregion, etc.). Lastly, 29 individuals identified their career as “other”, including: researchers, retired state climatologists, climatologists, land managers, and those in academia. From this group the majority worked at the state to global scale.
Dots represents those that work at the county level. Colored polygons represent those that work at the state level and the number of people that responded to the survey that work at the state level. Numbers represent those that responded that worked at another scale of geometry (e.g., city, community, multiple counties, townships, etc.). Not represented are those that worked globally (n = 8), nationwide (n = 4), and those that work sub-county level (city or community). Map created in ArcGIS using survey data and a states shapefile from the US Census Bureau (
https://www.census.gov/geographies/mapping-files/time-series/geo/carto-boundary-file.html ).
We see large diversity in the response pool, both spatially and across profession. Overall,162 individuals took part in this project with 137 fully completing the survey. This corresponds to an 84.55% completion rate. The 162 responses came from across the US and across a wide variety of subfields related to climatology, emergency management, and meteorology. Spatially, responses were collected from 43 of the 50 states with Hawaii, South Carolina, Massachusetts, Rhode Islands, Connecticut, Virginia, and West Virginia being the exceptions. While there are no direct responses from those states, they are partially represented by those that work regionally, nationally, or at the global scale. While there are survey responses across the country there is a slight skew to the southern portion of the United States. For those respondents that work county to statewide we see the most responses from Alabama, Mississippi, Florida, Arizona, and Texas ( Fig 2 ). However, there are responses at this scale spread evenly across the U.S.
Lastly, respondents were asked “What other atmospheric variables are part of your definition of a heat wave?” This question was asked so respondents have free response, instead of choosing from a bank of options as with the previous questions, the climatological variables that they use in management, forecasting, and planning. It is important to note most respondents did not list any additional atmospheric variables as part of their heat wave temperature. Of those that did the most common response was wet bulb globe temperature (WBGT), wind, cloud cover, and insolation. A few respondents listed, repetitively, humidity and heat index, in response to this question. Overall, this word usage pattern highlights the strong pattern of traditional climatological variables in the definition of heat waves, rather than characteristics of land, demographics, human health, or exposure.
When asked, “What atmospheric variables are part of your definition of a heat wave”, the most selected term was maximum temperature (n = 129) followed by heat index (n = 85), minimum temperature (n = 75), and humidity (n = 74). These four terms were far more common than the next terms of the list, average temperature (n = 45) and other (n = 25). Respondents were given a list of possible terms for this question and asked to select all that apply to their definition of heat waves. The terms included include maximum temperature, minimum temperature, average temperature, humidity, heat index, and other. This word usage pattern highlights that extreme terms are central to the definition of heat waves.
Response trend word clouds and frequency counts for the questions: (A) “What is your definition of the climatological term heat wave?”, and (B) “What atmospheric variables are part of your definition of a heat wave?”, and (C) “What other atmospheric variables are part of your definition of a heat wave”.
Text mining of survey responses showed interesting keyword usage patterns ( Fig 3 ). For the question “What is your definition of the climatological term ‘heat wave’”, heat was not the most used word, instead days was mentioned 71 times by respondents. Answers related to this term included definitions like: “Several days of 95+ degrees”, “A period of multiple days beyond normal temperatures”, and “At least 3 consecutive days of high maximum temperatures”. After days, the terms heat (n = 68), period (n = 65), normal (n = 47), and high (n = 42) were the other common terms ( Fig 3 ). Overall, this word usage pattern highlights a highly important and common thought pattern in defining heat waves, such extreme events have a duration aspect that needs to be considered and defined. Heat waves are multi-day extreme events.
3.c Beyond climatological terms, other factors to considered in defining heat waves
Beyond traditional climatological terms we asked respondent “Does your definition of a heat wave include non-atmospheric variables?” Only 27.5% of respondents replied in the affirmative that they did include non-atmospheric variables in their definition of a heatwave (Fig 4). The common non-atmospheric variables included in the heat wave definition were grouped into categories of: (1) impact on humans, (2) seasonal variation, (3) physical variables, and (4) other. The highest percentage of responses were within the impact to humans categories with 35.3%. Common non-atmospheric variables listed by respondents included: soil moisture, land type, duration, percent impervious surfaces, and crop stress. Most respondents that included non-atmospheric variables in their definition of heat waves were forecasters (n = 25), with only 5 emergency managers and those identified as “other” in their career including non-atmospheric variables. Geographically, those that included non-atmospheric variables mostly worked at the state to multi county scale (n = 22). Others that included non-atmospheric variables worked at varying geographic extents including county scale (n = 7), regional scale (n = 3), and global scale (n = 2). Spatially, that 27.5%, where not clusters in one portion of the US. Those that included non-atmospheric variables spanned from Arizona to Vermont.
Next, respondents were asked if their heat wave definition included a threshold that must be surpassed to be considered a heat wave and what threshold measurement they used. More than 56% of respondents said yes, their definition included a threshold that must be crossed. Of these 18 were emergency managers, 33 were forecasters, and 16 listed other as their career. As such, of the respondents that completed the survey approximately 38% of forecasters, 39% of emergency managers, and 55% of others included a threshold in their heat wave definition. Spatially, those that included a threshold worked across all geographic extents from community / local to global, though slightly more worked at the statewide or multicounty scale.
It was thought that the main threshold both emergency managers and forecasters would use in their definition would be related to temperature. Many of the provided definitions stated something like:
“Temperature above 95 degrees Fahrenheit for an extended period of time”
Or
“A persistent anomaly in daily surface temperature usually many days about the 98th percentile.”
Instead, survey results show that only 45.7% of respondents reported using temperature thresholds in their heat wave definitions, followed by heat index (27.9%), heat duration (23.5%), and heat risk (Fig 5). Spatially, if we look at the common temperature threshold mentioned by survey respondents there is an interesting dynamic playing out (Table 1). Those surveyed from the Northeast, Southwest, and Central regions of the United States, as defined by the NOAA climate regions, all listed a temperature threshold of 90°F with little variability. The lowest temperature threshold mentioned was 80°F and it was from a respondent in the south region. Whereas the highest threshold reported was 105°F, occurring in both the East North Central and South regions. Overall, a north to south temperature threshold gradient was expected, as the more southerly regions are subject to climatically higher mean temperatures, but this was not seen in the survey responses.
Fig 5B shows that numerous respondents suggest a threshold in their heat wave definition based on duration. During the survey respondents were specifically asked “Does your definition of a heat wave include a minimum duration”. This question was asked, in tandem with the aforementioned in case a respondent did not feel duration was a threshold measure. Over 74% of respondents said yes that event duration was a part of their heat wave definition. Of these 25 were emergency managers (54.3% of EM total population), 56 where forecasters (64.4% of F total population, and 19 listed their career as other. Importantly, the most noted durations for events were 2 and 3 days (Fig 6). Overall, 22.2% or respondents mentioned 2 days as the minimum duration for a heat wave to be defined and 56.6% listed 3 days as the minimum duration. The response pattern of did not vary by geographic extent of work or the region respondents work in.
At this point, respondents had solely been asked about common aspects of heat wave definitions such as duration, intensity, etc. The next question of the survey asked, “Is size of area affected part of your definition of a heat wave?” Only 21.8% of respondents considered event size in their heat wave definition, equating to just 29 respondents. Of these there was an even split between emergency managers and forecasters, the geographic extent of their work was mostly single or multiple counties, and almost all included duration in their heat wave definition.
The majority of those that responded in the affirmative said that a heat wave had to have a duration of 2 or 3 days to be defined as such an event. The range of durations suggested by those surveyed was as short as 1 day (3 respondents) and as long as 10 days (1 respondent) (Fig 7).
If a heat wave only impacts a small area, does it matter in emergency management and forecasting as much as large events? That question was the impetus for the next survey question, “is size of area affected part of your definition of a heat wave.” The vast majority (78.2%) of people said no, size does not matter. Those that answered yes to this question were almost all forecasters (except 2 out of 14). Those that answered in the affirmative had a clear theme, larger events impact more people compared to isolated events and that is why event size matters. With this theme several people mentioned that an event needs to be state-wide or span multiple counties (44.8% of the previous questions affirmative answers).
The last two questions of the survey look to see how respondents think about defining heat waves across space and time. Respondents were asked “Is your definition of a heat wave dependent on time of year?” This question was almost a 50/50 split in terms of percentages with 53% saying the definition is not time of year dependent and 47% saying it is (Fig 8). For those that answered in the affirmative they were asked how their definition varies through the year. There were a wide range of responses to this follow up question from those mentioning seasonal threshold values to others saying only summer matters as it’s the warmest season. Overall, the consensus of those surveyed is well represented by one respondent’s remakes, “I generally only call things "heat waves" when it is “hot” outside. I wouldn’t call a winter stretch of warmer than normal weather a "heat wave" unless it was drastically warmer than normal.” There is a consistent trend in these types of answers with others saying outright “heat waves have only occurred climatologically in our late spring to early fall months, when it’s hottest”. However, there were a small group (labeled statistical in Fig 8) that again link back to thresholds and say, in their opinion, that heat waves can happen if the temperatures exceed the 95th percentile for that given time frames normal.
Lastly, respondents were asked if their definition of a heat wave varies across space. This question was asked to see if one core definition of heat waves cannot be developed and instead a regional definition would be needed. Overall, only 36.3% responded in the affirmative whereas 63.7% said their heat wave definition did not vary across space (Fig 9). Those answering in the affirmative, that their definition varied across space, were then asked how with the options of: (A) regional differences, (B) type of territory, (C) statistically, (D) varies by the gridded data, and (E) other. Overall, 42.9% of respondents that thought heat wave definitions should vary across space thought there were regional differences. When asked to explain their answer respondents had a variety of answers and mentioned topic such as: “definitions can be different across climate regions because citizens are acclimated to different levels” and “In my work the western high plains should have a higher threshold for defining heat waves than the eastern portion of the Southern Plains”, and “Since humans can become acclimated to "normal" conditions, I would vary the definition based upon a certain amount above climatology”. Next, 22.5% of respondent said heat wave definitions should vary because of type of territory. Those that responded in this manner mentioned: “amount of vegetation vs bare ground is important” and “population density and impervious surfaces need to be considered to account for urban heat island effects”. Third, 22.5% said statistically heat wave definitions should vary spatially. These respondents provided further understanding of their response with comments such as: “Heat wave might be the top 0.1% of high temperatures of all time for each location” and “temperature percentile varies by location”. Lastly, only 6.1% of respondents said their definition varies over space because of gridded data. Specifically, those that answered in this manner added comments like: “heat risk is calculated on a spatial grid by entire forecast area” and “gridded temperature data is 2.5 km resolution—so the values for defining the heat wave vary spatially even if the definition remains the same.” Overall, these responses highlight a need to better understand duration, thresholds, and the underlaying population exposure.
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[1] Url:
https://journals.plos.org/climate/article?id=10.1371/journal.pclm.0000468
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