An Evaluation of Winter Weather Severity in the United States Using the Weather Stress Index

Abstract scale, relative indices permit inter-regional evaluations of weather-related events such as human health and well-being, The objective of this study is to provide an evaluation of the magnitude employee absenteeism, and crime. of apparent temperature and the weather stress index (WSI) in winter The use of relative climatological indices has uncovered across the United States. In addition, two extremely cold winters, certain human-weather relationships that were previously in- 1976-77 and 1981-82, are analyzed in terms of their relative severity with the assistance of the WSI. determinable using an absolute index or raw weather data. For Mean apparent temperatures around the nation for 0300 LST in example, the authors recently determined that in the summer January show an expected latitudinal trend, with the lowest apparent the central and south-central United States experience the great- temperatures found in the north central United States. Although this est frequencies of extremely high apparent temperatures in the distribution roughly approximates that of mean air temperature for nation using the WSI (Kalkstein and Valimont, 1986). This January, there are significant differences. Large disparities between mean 0300 LST apparent temperature and air temperature exist from region was deemed to be significantly more sultry in summer Kansas to Minnesota. Much-smaller disparities are found in the East than other places with perceived uncomfortable summers such and the South, heightening the latitudinal gradient for apparent tem- as Mobile, Alabama; and Phoenix, Arizona. In addition, the perature. The severity of winter conditions in the north-central United index permitted the isolation of causal synoptic situations that States is clearly noted when evaluating the WSI; in parts of North Dakota and Minnesota, the apparent temperature corresponding to the contribute to frequent sultry conditions in the central and south- 99-percent WSI at 0300 LST is below -45°C. central parts of the country. Balling and Brazel (1986a, 1986b) The winter of 1981-82 is credited as having the most-severe indi- determined that the weather in Phoenix is becoming increas- vidual winter day since 1948. On 11 January 1982, WSI values ex- ingly more oppressive using the WSI. They noted that prior to ceeding 99 percent covered over two-thirds of the nation. However, 1968, only about five percent of the days in Phoenix registered based on the duration of stressful weather conditions, the winter of 1976-77 was more stressful than 1981-82. During 1976-77, the most- high nighttime WSI values, while the average for the 1980s stressful conditions were encountered in the populous East, and in a has soared to above 30 percent. Using a similar approach, they sizable area over one-third of the days were described as stressful (WSI noted that a similar trend had not been uncovered in Tucson exceeding 90 percent). These proportions were much lower during the (Balling and Brazel, 1987), and they isolated environmental winter of 1981-82. reasons for the differential response in WSI magnitude in Phoe- nix and Tucson. Harries et al. (Harris, K. D., S. J. Stadler, and L. S. Kalkstein, personal communication) determined that the WSI is useful to evaluate the impact of weather on crime in summer, and they have analyzed crime rates in two large 1. Introduction cities with different climates, Baltimore, Maryland, and Dallas, Texas. Although a number of climatological indices are available to This article is a complement to the authors' summer-WSI measure the response of humans to severe weather, much of analysis (Kalkstein and Valimont, 1986), and the objective here the recent research has turned to the use of "relative," rather is to provide an evaluation of the magnitude of the WSI in than "absolute," indices to determine both intra- and inter- winter across the nation. In addition, two extremely cold win- regional variations in human response. Absolute indices, such ters, 1976-77 and 1981-82, are analyzed in terms of their as the "windchill index," are place specific in utility, as a relative severity with the assistance of the WSI. windchill of — 10°C might have an enormous impact on human activities in Jacksonville, Florida,3 but very little impact in Fargo, North Dakota (Quayle and Doehring, 1981; Court, 1981). However, a relative index, such as the weather-stress index 2. The weather stress index (WSI), compares a particular weather event to normal weather conditions for a given locale and evaluates how unusual that a. Description of the index event might be (US Department of Commerce, 1983). Thus, A detailed discussion of the development of the WSI has been if the impact of weather is analyzed on a large geographical described in previous manuscripts (Kalkstein, 1982, 1983; Kalkstein and Valimont, 1986), and only a brief review will be provided here. 1 Center for Climatic Research, University of Delaware, Newark, The WSI was developed to provide a relative climatological DE 19716. index that could have widespread application and be understood 2 Department of Geography, University of Delaware, Newark, DE by professionals who are interested in weather impacts but are 19716. not well-versed in climatology. The index possesses certain 3 See Appendix. advantages not found in some of the more-commonly used © 1987 American Meteorological Society climatological indices. For example, the same index-value units

Bulletin American Meteorological Society 1535

Unauthenticated | Downloaded 10/05/21 08:45 AM UTC 1536 Vol. 68, No. 12, December 1987

TABLE 1. Results of multiple regression analysis used in the development of the winter apparent-temperature angorithm.

Step Variable R2 R2 Improvement b Std. Error F

1 Air Temperature (T.) 0.940 0.940 0.991 0.005 39968.94** 2 Windspeed (WND) 0.989 0.049 -1.476 -0.011 5293.75** 3 Air Temp x WND 0.998 0.009 0.330 0.001 1593.59** Constant = 0.4533

AT = 0.4533 + (0.991 Ta) - (1.476 WND) + [0.330 (Ta)(WND)] where; AT = apparent temperature (°C); Ta = air temperature (°C); WND = windspeed (m • s"1). ** denotes significance at the .01 level. are utilized in both the summer or winter seasons. Virtually all apparent temperatures found from northeastern Montana to the other indices, such as the "temperature-humidity index" (THI), Lake Superior region and the highest apparent temperatures "humiture," or the windchill index are usable only within a located in Florida and the southern tip of Texas. The distri- single season (Siple, 1945; Lally and Watson, 1960; Weiss, butions for December and February are generally similar. Stan- 1983). In addition, because of its relative nature, the WSI is dard deviations of mean 0300 LST apparent temperature in the only index capable of evaluating differential impacts of January show a less pronounced latitudinal trend (Fig. 2). The identical weather conditions in two widely separated locales. largest standard deviations are found from South Dakota and Thus, the WSI is particularly well-suited for climate-assessment Iowa extending northeastward to the panhandle region of Mich- analysis. igan. The greatest single value is noted in Duluth, Minnesota, The basis of the WSI is derived from research by R. G. with a standard deviation exceeding 11 °C. A steep incline in Steadman, who evaluated human physiological responses to standard deviation occurs to the west rather than the south, and various weather conditions (Steadman, 1971). Steadman de- the smallest values are found along the Pacific Coast. Los veloped the notion of apparent temperature, which is defined Angeles, California (International Airport) posted the smallest as the perceived air temperature for an individual (Quayle and single value of 2.6°C, while southern Florida standard devia- Doehring, 1981). Our goal was to develop a simple algorithm tions were approximately twice this value. The larger standard that consisted of commonly measured weather elements such deviations in southern Florida suggest that intrusions of arctic as temperature, dewpoint, and windspeed, and explained vir- air are more frequent there than in southern California. tually all of the variation in Steadman's published apparent- These maps appear to approximate the mean-temperature temperature tables for winter (Table 1). The WSI has been maps for January, but significant differences do exist (Fig. 3). developed using the algorithm in Table 1 and comparing any The greatest differences between mean 0300 LST apparent real-time apparent temperature to the mean and standard de- temperature and air temperature occur in the Midwest, extend- viation for that particular location, date, and hour. This was ing from Kansas northward to Minnesota. This is probably accomplished by transforming the normally distributed sample attributed to the importance of windspeed in the determination of apparent temperatures to a standard normal population uti- of apparent temperature, as average 0300 LST windspeeds are lizing a Z-transformation procedure (refer to Kalkstein and relatively high across the Midwest in comparison to much of Valimont, 1986 for a more-detailed discussion). Thus the WSI the nation (Department of Commerce, 1982). Small disparities for winter is defined as the proportion of days with apparent are found over much of the East and the South. temperatures higher than the day under review. A WSI of 99 The 99-percent WSI distribution for 0300 LST in January percent would probably prove to be quite stressful, as 99 percent indicates that the north-central United States experiences the of the days exhibit higher apparent temperatures. Of course, most-extreme apparent temperatures in the country (Fig. 4). the apparent temperature corresponding to a 99-percent weather- Over portions of North Dakota and Minnesota, the apparent stress-index value varies dramatically according to time-of-year temperature corresponding to the 99-percent WSI is below and locale, as indicated in Table 2. However, considering hu- -45°C, indicating that 1 out of 100 days in January have man acclimatization, the interregional impact of these various apparent temperatures that low. The highest values are found apparent temperatures at the 99-percent WSI level should be in southern Florida and southern California, where the apparent somewhat similar on a variety of socioeconomic parameters. temperature associated with the 99-percent WSI is generally above 2°C. The impact of nearby water bodies is especially b. Geographical analysis of the index noteworthy in this distribution; Lakes Michigan and Huron A map of mean apparent temperature for 0300 LST in January appear to increase apparent temperatures associated with the shows an expected latitudinal trend (Fig. 1), with the lowest 99-percent WSI, and a strong Pacific Ocean impact is noted.

TABLE 2. 0300 LST apparent temperatures corresponding to 99-percent January WSI for selected cities. 3. A comparative evaluation of two cold winters

Albuquerque — 16°C Los Angeles 1°C The winters of 1976-77 and 1981-82 were among the most Atlanta — 14°C Mobile -11°C Bismarck — 42°C Oklahoma City — 23°C severe in recent memory. The economic impact of the disas- Boston — 25°C Phoenix — 4°C terously cold weather was extreme, especially during the 1976- Denver — 26°C Seattle -10°C 77 winter (Department of Commerce, 1982a). The relative Key West 10°C Topeka — 26°C severity of these two winters can be assessed with the assistance Little Rock — 16°C Tulsa — 21°C of the WSI.

Unauthenticated | Downloaded 10/05/21 08:45 AM UTC Bulletin American Meteorological Society 1537

FIG. 1. Mean apparent temperature for 0300 LST in January. Appar- FIG. 2. Standard deviation in apparent temperature for 0300 LST in ent-temperature values are in degrees Celsius. January. Apparent-temperature values are in degrees Celsius.

FIG. 3. Difference between the mean apparent temperature and mean FIG. 4. Apparent temperatures associated with the 99-percent air temperature at 0300 LST in January. In all cases, air temperature WSI. January, 0300 LST. Apparent-temperature values are in degrees exceeds apparent temperature. Values are in degrees Celsius. Celsius.

FIG. 5. Apparent-temperature values (degrees Celsius) for 10 January 1982 at 0300 LST. FIG. 6. WSI values (percent) for 11 January 1982.

Unauthenticated | Downloaded 10/05/21 08:45 AM UTC 1538 Vol. 68, No. 12, December 1987

TABLE 3. Apparent temperatures (°C) for selected stations during the coldest days of 1976-77 and 1981-82 at 0300 LST.*

1977 1982 January Station 11 Jan 17 Jan 29 Jan 6 Jan 10 Jan 11 Jan 17 Jan Normal

Albuquerque, NM -18 -8 -5 M -5 -9 -4 -4 Atlanta, GA M -20 -19 M -13 -28 -15 2 Cincinnati, OH -26 -36 M M M -35 -31 -5 Missoula, MT -12 -2 -8 -25 -14 M -11 -9 Philadelphia, PA M -26 -25 -3 -16 -21 -28 -5 St. Louis, MO -23 -25 -29 0 -46 -32 -21 -6 Syracuse, NY -22 -27 -32 -6 -21 -31 -37 -8

* M denotes missing data.

Both winters were noteworthy for having specific days that was less than one-fourth of that figure (Department of Com- were uniquely severe. The lowest apparent temperatures were merce, 1982b). Part of this difference might be attributed to recorded on 10 January 1982, with values approaching — 50°C the extensive drought conditions that existed during the 1976- (Fig. 5). Values lower than — 45°C were noted as far south as 77 winter, but the WSI indicates that other factors also were St. Louis, Missouri; and Chicago, Illinois. On 11 January 1982, responsible. the same cold wave brought brutally uncomfortable conditions Probably the most-significant difference between the two deep into the South. Atlanta, Georgia, recorded an apparent winters relates to the duration of stressful weather conditions temperature of — 29°C and Mobile, Alabama, reached - 19°C. {stressful is subjectively defined as a day with the WSI ex- WSI values approached 100 percent throughout much of the ceeding 90 percent). During the week of the most-bitter weather East and South, indicating that 11 January was the most-un- (8-14 January 1977; 8-14 January 1982) many of the days at comfortable day a person could possibly expect in their locale most eastern and southern stations had index values exceeding (Fig. 6). Index values of 99 percent covered over two-thirds 90 percent (Table 4). On the average, only 1 in 10 of the days of the nation, and the areal extent of these extreme WSI values during any given period should exhibit index values in this marks 11 January 1982 as the most-stressful winter day since range, underscoring the severity of both winters. However, if 1948 (the first year for which the WSI was calculated). The the period is extended beyond this one-week interval, the du- lowest apparent temperatures during the winter of 1976-77 ration of stressful weather conditions during the winter of 1976- occurred between 8 January and 29 January, when apparent 77 greatly exceeds that during 1981-82 (Table 5). temperatures approached - 40°C on several days in North Da- Further evidence of this difference is noted when seasonal kota and Minnesota. During this period, Bismarck, North Da- periods are evaluated (Figs. 7 and 8). During the winter of kota, experienced apparent temperatures below - 30°C on six 1976-77, a sizable portion of the eastern and southern United different days. States experienced stressful WSI values for more than one-fifth Several other bitterly cold periods occurred during both win- of the days in the December through February period. A sub- ters, and the brunt of their effects were felt in the eastern half stantial area from Ohio to the Carolinas "experienced" about of the country (Table 3). When apparent temperatures during one-third of their days in this way. Wilmington, North Carolina these episodes are compared to normal temperatures, their in- (35 percent), Roanoke, Virginia (34 percent), and Cincinnati, tensity is dramatically demonstrated. Ohio (33 percent) recorded the highest seasonal proportion of The winter of 1981-82 has been credited as having individual days with lower apparent temperatures than its counterpart of TABLE 4. Number of days with the WSI exceeding 90 percent for 1976-77. The lowest apparent temperatures during the 1976- the weeks of 8-14 January 1977 and 8-14 January 1982. 77 winter were near — 40°C, and did not approach the values Station 1977 1982 of 10 January 1982. However, when the two winters are com- pared, it appears that the 1976-77 winter was more econom- Buffalo, New York 4 3 ically devastating. The National Oceanic and Atmospheric Dallas, Texas 2 4 Administration's (NOAA) Assessment and Information Ser- Lexington, Kentucky 4 3 vices Center estimated that the monetary impact of 1976-77 Roanoke, Virginia 4 4 St. Louis, Missouri 4 3 was about 36-billion 1980 dollars, while the 1981-82 value

TABLE 5. Number of days with the WSI exceeding 90 percent for 8-28 January 1977 and 8-28 January 1982.

1977 1982

Station 8-14 Jan 15-21 Jan 22-28 Jan 8-14 Jan 15-21 Jan 22-28 Jan

Cleveland, Ohio 4 5 3 2 4 1 Indianapolis, Indiana 3 3 2 111 Palm Beach, Florida 1 4 4 2 10 Raleigh, North Carolina 3 5 2 3 3 1 Topeka, Kansas 4 2 14 0 0

Unauthenticated | Downloaded 10/05/21 08:45 AM UTC Bulletin American Meteorological Society 1539

FIG. 7. Proportion of days (percent) with WSI values exceeding 90 FIG. 8. Proportion of days (percent) with WSI values exceeding 90 percent: December 1976 through February 1977. percent: December 1981 through February 1982.

FIG. 9. Proportion of days (percent) with WSI values exceeding 90 FIG. 10. Proportion of days (percent) with WSI values exceeding 90 percent: January 1977. percent: January 1982. days with stressful WSI values. As these proportions were weather conditions for more than half of the days in January considerably lower in the winter of 1981-82, it stands in strong 1977. A sizable proportion of the East and South exceeded the contrast to the winter of 1976-77. The highest seasonal pro- 30-percent proportion during that month, while a much-smaller portions of days in 1981-82 above the 90-percent level occurred area exceeded 30 percent during January 1982. During February at Dallas, Texas (19 percent) and Chicago, Illinois (18 percent). 1977, the most-stressful weather settled into the southeastern Although these proportions are almost twice the norm, they are United States, and the proportion of days with stressful weather much-less extreme than those for the 1976-77 winter. conditons exceeded 20 percent from southern Ohio and Virginia Another notable feature involves the location of these high south to Florida. February 1982 was milder than normal in the proportions. The winter of 1976-77 was harshest in the eastern eastern third of the country, with virtually no days experiencing third of the country, while the winter of 1981-82 had its greatest WSI values exceeding 90 percent. impact on the Midwest and south-central states. Thus a much- greater segment of the population was negatively influenced during the 1976-77 winter. The 15-percent-frequency isoline of stressful days encompasses well over half of the country's 4. Conclusion population in 1976-77 (Fig. 7), while only about one-fifth of the population is within the 15-percent isoline during 1981-82 The objective of this study was to apply a relative climatological (Fig. 8). index, the WSI, to provide an evaluation of winter weather in A monthly breakdown of the proportions adds further detail the United States. In addition, the index was used to compare to the picture. December 1976 was quite stressful in the Great two very-harsh winters in an attempt to determine which winter Lakes region, with over 30 percent of the days characterized was most severe. as stressful. December 1981 was not unusually cold. The month The lowest mean apparent temperatures for January at 0300 of January exhibited the most-severe weather during both winter LST were located in North Dakota and Minnesota and the seasons (Figs. 9 and 10). Parts of Ohio suffered under stressful lowest apparent temperatures associated with the 99-percent

Unauthenticated | Downloaded 10/05/21 08:45 AM UTC 1540 Vol. 68, No. 12, December 1987

The following alphabetic key identifies each place: A) Bis- marck, North Dakota; B) Chicago, Illinois; C) Cincinnati, Ohio, D) Dallas, Texas; E) Duluth, Minnesota; F) Fargo, North Da- kota; G) Florida; H) Iowa; I) Jacksonville, Florida; J) Kansas; K) Lake Superior; L) Los Angeles, California; M) Minnesota; N) Mobile, Alabama; O) Montana; P) North Carolina; Q) North Dakota; R) Ohio; S) Phoenix, Arizona; T) Roanoke, Virginia; U) St. Louis, Missouri; V) South Dakota; W) Texas; X) Tucson, Arizona; Y) Virginia; Z) Wilmington, North Carolina.

Acknowledgments. The authors are most grateful to Jane Feng and Suzanne Goodrich for cartographic assistance and to M. Lawrence Nicodemus, NOAA/NCDC, for providing data necessary for the com- pletion of this study. FIG. 11. Location of places discussed in this manuscript.

WSI were found in this area as well. Unlike a previous summer References evaluation using the WSI, where the greatest frequencies of extremely high apparent temperatures were found in an un- Balling, R. C., Jr. and S. W. Brazel, 1986a: "New" weather in expected part of the country (Kalkstein and Valimont, 1986), Phoenix? Myths and realities. Weatherwise, 39, 86-90. the winter findings were not unusual and somewhat approxi- Balling, R. C., Jr. and S. W. Brazel, 1986b: Temporal analyses of mated the latitudinal gradient of mean air temperature. summertime weather stress levels in Phoenix, Arizona. Archives for Results from the comparison of two extreme winters (1976- Meteorology, Geophysics, and Bioclimatology, Ser. B., 36, 331- 342. 77, 1981-82) using the WSI were more interesting. Although Balling, R. C., Jr. and S. W. Brazel, 1987: Temporal variations in the coldest individual days occurred in January 1982 (11 Jan- Tucson, Arizona summertime atmospheric moisture, temperature, uary 1982 was the most-stressful day on record, with much of and weather stress levels. J. Climate AppI. Meteor., 26, 995-999. the East experiencing a WSI of 100 percent), the WSI indicated Court, A., 1981: Windchill over 40 years. In Preprint Volume of Extended Abstracts: Fifth Conference on Biometeorology, American that the winter of 1976-77 was, by far, the more severe. The Meteorological Society, Anaheim, CA, pp. 82-85. most-stressful weather during 1976-77 occurred in an area Harries, K. D., S. J. Stadler, and L. S. Kalkstein, submitted. The extending from Ohio to North Carolina, where about one-third impact of weather stress on summer crime patterns. Annals of the of the days during this winter recorded WSI values exceeding Association of American Geographers. 90 percent. With the exception of January 1982, which was Kalkstein, L. S., 1982: The weather stress index. NOAA Tech. Proced. Bull, 324, 1-16. colder than normal, much of the winter of 1981-82 was not Kalkstein, L. S., 1983: The development of indices for climate/socio- unusual, and only about one-tenth of the days in the eastern economic assessment: The "weather stress index" as an example. United States experienced WSI values exceeding 90 percent. The Penn. Geogr., 21, 7-17. The WSI provided information about these winters that would Kalkstein, L. S. and K. Valimont, 1986: An evaluation of summer discomfort in the United States using a relative climatological index. have been indeterminable using an absolute index, such as Bull. Amer. Meteor. Soc., 67, 842-848. windchill, or only raw weather data. For example, the region Lally, V. E. and B. F. Watson, 1960: Humiture revisited. Weatherwise, experiencing the most-stressful weather during the winter of 13, 254-256. 1976-77 was not located in the region with the lowest air Quayle, R. and F. Doehring, 1981: Heat stress: A comparison of in- temperature, windchill, or apparent temperature. In addition, dices. Weatherwise, 34, 120-124. Siple, P. A., 1945: General principles governing selection of clothing the WSI isolated the most-stressful winter day over the past 40 for cold climates. Proceedings of the American Philosophical So- years in the country based on the extent of the area covered ciety, 89, 200-234. by 100-percent WSI conditions. The WSI is not a perfect in- Steadman, R. G., 1971: Indices of windchill of clothed persons. J. dex—there is still disagreement among researchers concerning Appl. Meteor., 10, 674-683. US Department of Commerce, 1982a: US economic and social impacts the importance of relative versus absolute impacts of climate of the record 1976-77 winter freeze and drought. Climate Impact on human activity—but it continues to provide information that Assessment, United States, 1-50. would be important in climatic-assessment research and in- US Department of Commerce, 1982b: US economic impact of the terregional evaluations of climate impacts. severe winter of 1982. Climate Impact Assessment, United States, I-47. US Department of Commerce, 1983: Impact assessment of weather stress; July 1983. Climate Impact Assessment, United States: July, Appendix. Locations of places mentioned in the text. The II-14. locations referred to in the text are shown in Fig. 11. Cities Weiss, M. H., 1983: Quantifying summer discomfort. Bull. Amer. are designated by circles; states are identified by letter only. Meteor. Soc., 64, 654-655. •

Unauthenticated | Downloaded 10/05/21 08:45 AM UTC