HEAT INDEX CLIMATOLOGY FOR THE NORTH-CENTRAL UNITED STATES Todd Rieck National Weather Service La Crosse, Wisconsin 1. Introduction middle Mississippi River Valleys, and the western Great Lakes. Also, the physiological Heat is an underrated danger, with an response to heat will be briefly investigated, average of 175 Americans losing their lives including a review of how heat acclimatization annually from heat-related causes. According to affects the human body’s biology. This the Centers for Disease Control and Prevention, protective biological response is an important from 1979-2003 excessive heat exposure consideration when evaluating the impact of the caused 8,015 deaths in the United States. heat on those that are, or are not, acclimatized During this period, more people died from to the heat. extreme heat than from hurricanes, lightning, tornadoes, and floods combined. In this study, 95°F will be used as the start for the climatological analysis as prolonged Heat kills by taxing the human body beyond exposure to heat this warm increases the risk of its ability to cool itself. Cooling is primarily sunstroke, heat cramps, and heat exhaustion accomplished by the evaporation of perspiration. (Table 1) . How efficiently this process functions is directly related to the amount of water vapor in the air. 2. Data High moisture content reduces the evaporative cooling rate of perspiration, making it difficult for All available weather observations from the the body to maintain a steady and safe internal National Climatic Data Center were used from temperature. One way to measure the 192 locations (Fig. 1), extending from Utah to combined effect of temperature and moisture on Michigan, and from the Canadian-U.S. border the human body is the heat index. south to a northern New Mexico to Arkansas line. Initially, the period of record used was the In 1979, R. G. Steadman constructed an last set of climate normals, 1971-2000. Due to apparent temperature table using temperature, some data unreliability and station closures, a relative humidity, and other factors, based on a majority of the sites only had data extending number of published research papers over a 54- back to 1973. This study focused on the year span. The National Weather Service summer months of June, July and August, when (NWS) developed a heat index equation based the combination of heat and humidity produces on Steadman’s work as guidance for heat- the highest heat indices. Some areas of study related advisories or warnings. Heat advisories were under-represented by the original data set. or warnings are issued to the public when values In an attempt to fill these data voids, “newer” approach dangerous levels. locations were used and therefore more recent data (1990 to the present) were incorporated This study will attempt to show the frequency into the data set. These additional data were of various heat index values for a section of the used sparingly, trying to keep the data set as United States, focusing on the Plains, upper and true to the 1971-2000 period as possible. 1 Fig 1. Observation sites used in this study. The data were quality controlled and geographical and climatic location (humid observations were eliminated where the continental to semi arid; Fig. 7) favor warm to temperatures and dew points were suspect or hot summer temperatures. The maximum over missing. If there was any question to the the Badlands of South Dakota is also a favored integrity of an observation, it was eliminated location for warm to hot summer temperatures from the data set. due to terrain and relative lack of vegetation (Fig. 8). Appendix A contains a full listing of the sites used and related data. Within this broad maximum are a few areas where the percentage of warm air temperatures 3. Results is reduced. Across southern Missouri, the somewhat higher elevation and more densely The frequency of occurrence of heat indices vegetated Ozarks keep this region cooler than from 95°F to 110°F (every 5 degrees) was locations to its south and west. The higher calculated over the data domain, along with air elevation of the Black Hills in western South temperatures of 95 degrees or greater (Figs. 2- Dakota keep that area relatively cooler 6). A 110°F heat index was used as the compared to the warmer Badlands to the south extreme value for analysis as data indicated that and east. South of the Badlands are the Sand heat indices warmer than 110°F occurred very Hills of Nebraska, which is another minimum. infrequently (generally less than 5 observations The Sand Hills is a region of mixed-grass prairie out of 10,000). Even in the warmest, most covering just over one-quarter of Nebraska, and humid areas, percentages of observations with is the largest and most intricate wetland heat indices over 110°F were predominantly ecosystem in the United States. It’s this under 0.5%. ecosystem that keeps it cooler than surrounding Examining the frequency of occurrence of territories. 95°F degree or greater air temperatures (Fig. 2), Inspecting the heat index maps (Figs. 3-6), a a broad maximum is located across much of the maximum is noted from the Central Plains east Central Plains into portions of southern South to the middle Mississippi River Valley. The Dakota. This was expected as the latitude and 2 Central Plains are a favored location for warm to itself, the localized minimum is not as hot summer temperatures and high heat indices widespread as the figures would suggest. are expected. Meanwhile, the middle Mississippi River Valley is generally not as warm One more maximum of note is located over as locations to the west, but dew points during southeast Colorado, and is only apparent on the the summer are high. This is largely the result 95°F air temperature and heat index maps (Figs. of two processes: moisture from the Gulf of 2-3). A likely explanation for this would be deep Mexico transported north, and mixing in the afternoon leading to warm evapotranspiration from vegetative cover. temperatures but low humidities. Further Looking more closely at this broad area, the investigation of the data points responsible for influence of the Ozarks is noted with a relative this maximum (PUB, LAA, and LHX) revealed minimum of high heat indices in this area (Figs. that all the instances of 95-degree temperatures 4-5). or heat indices occurred late in the afternoon, during peak heating. Relative humidities were A few other, more minor maxima are evident, less than 30% in nearly all cases with especially on the 100°F or greater heat index temperatures above 100°F about 60% of the map (Fig. 4). Over central South Dakota, a time. This supports the maximum indicated in maximum is indicated near Pierre and areas to Figures 2 and 3. However, at temperatures this the east. While there are not many data points warm, further moisture would be required to in this area, this maximum appears to be valid raise the heat indices higher. In a deeply mixed for a couple reasons: geography and irrigation. environment, this is difficult to accomplish In South Dakota the terrain transitions from a without some form of moisture advection. Since hilly, drier, and more arid land in the west (Black this is rare on days this warm over southeast Hills and the Badlands), to flatter terrain, more Colorado, a maximum is not evident for heat vegetation and crops in the east. The higher indices above 95°F (Figs. 4-6). percentage of crops and vegetation requires more irrigation resulting in additional evaporation In the more northern latitudes, temperatures and evapotranspiration. are generally not as warm, nor is there a nearby large source of moisture (i.e., Gulf of Mexico). Another minor maximum is located over Therefore, the frequency of high heat indices is southwest Minnesota. This area is located on much less. the lee side of the Buffalo Ridge, which is a large expanse of rolling hills and the second 4. Physiological Effects of Heat highest point in Minnesota. Down sloping winds The body tries to maintain a consistent off this ridge can lead to warmer temperatures temperature by using various means to cool or compared to surrounding portions of Minnesota. warm itself. When blood is heated above 98.6°F, In addition, agriculture is predominant across the body attempts to dissipate this extra heat by southern Minnesota, adding moisture via losing water through the skin and sweat glands, evapotranspiration. These influences likely play varying the rate and depth of blood circulation, a role in the higher heat indices. and as a last resort – panting. About 90 percent A minimum in the heat index is indicated over of the body’s heat is lost through the skin, mostly southeast Minnesota (Figs. 3-4). Rochester is through perspiration. Above 98.6°F, heat can the data point associated with these lower only be lost through sweating. values, and this observation site is located at the airport, on an open and unsheltered ridge south of the city. Its location makes it cooler than surrounding areas, with lower dew points. Therefore, while representative for the data point 3 Sweating itself does not cool the body; or circulatory diseases. rather, it’s the evaporation of this water that performs this function. Energy is required to Overall, the inability of the body to shed change liquid water into a vapor, and the body heat, and the loss of fluids and minerals through provides this in the form of heat. However, sweating, can result in various heat-related some water vapor condenses back onto the disorders and illnesses, such as heat cramps, body, returning some of that heat energy back to heat exhaustion, and heat stroke (Table 1).