Urban climate effects on extreme temperatures in Madison, Wisconsin, USA

As climate change increases the frequency and intensity of extreme heat, cities and their urban heat island (UHI) effects are growing, as are the urban populations encountering them. These mutually reinforcing trends present a growing risk for urban populations. However, we have limited understanding of urban climates during extreme temperature episodes, when additional heat from the UHI may be most consequential. We observed a historically hot summer and historically cold winter using an array of up to 150 temperature and relative humidity sensors in and around Madison, Wisconsin, an urban area of population 402 000 surrounded by lakes and a rural landscape of agriculture, forests, wetlands, and grasslands. In the summer of 2012 (third hottest since 1869), Madison’s urban areas experienced up to twice as many hours ⩾32.2 °C (90 °F), mean July TMAX up to 1.8 °C higher, and mean July TMIN up to 5.3 °C higher than rural areas. During a record setting heat wave, dense urban areas spent over four consecutive nights above the National Weather Service nighttime heat stress threshold of 26.7 °C (80 °F), while rural areas fell below 26.7 °C nearly every night. In the winter of 2013–14 (coldest in 35 years), Madison’s most densely built urban areas experienced up to 40% fewer hours ⩽−17.8 °C (0 °F), mean January TMAX up to 1 °C higher, and mean January TMIN up to 3 °C higher than rural areas. Spatially, the UHI tended to be most intense in areas with higher population densities. Temporally, both daytime and nighttime UHIs tended to be slightly more intense during more-extreme heat days compared to average summer days. These results help us understand the climates for which cities must prepare in a warming, urbanizing world.


Introduction
As Earth's climate warms, the frequency and intensity of extreme heat is rising both globally (Luber andMcGeehin 2008, Seneviratne et al 2014) and in cities (Tan et al 2010, Habeeb et al 2015, Mishra et al 2015, where 54% of the world's population lives (United Nations, Department of Economic and Social Affairs, Population Division 2015). By 2030, the global urban population is projected to grow from 3.9 to 5 billion (United Nations, Department of Economic and Social Affairs, Population Division 2015), and urban land cover could triple its 2000 extent (Seto et al 2012). The simultaneous global trends of urbanization and climate change present a pressing challenge to create livable cities that are well prepared for future climates.
Extreme heat poses significant risks to Earth's growing urban population. Heat waves in Chicago in 1995 (Changnon et al 1996), Paris and other European cities in 2003 (Garcia-Herrera et al 2010), and cities across Russia and Eastern Europe in 2010 (Barriopedro et al 2011) caused hundreds, thousands, and even tens of thousands of deaths. Such events have been described as social disasters (Klinenberg 2002) due to the importance of social and demographic vulnerability in explaining patterns of mortality (Semenza et al 1996, Vandentorren et al 2006, Keller 2013). However, cities not only concentrate vulnerable populations, they also raise temperatures through the urban heat island (UHI) effect. Land surface and air temperatures both experience UHI effects (Arnfield 2003); this study focuses on urban effects on near surface air temperature.
The UHI refers to cities being warmer than their rural surroundings due to the built environment absorbing, retaining, and/or producing more heat than the natural landscape it replaces (Oke 1982). Many studies have reported greater UHI effects during the summer (Arnfield 2003, Schatz andKucharik 2014, but see other references in Arnfield 2003 reporting other peak seasons), when higher urban temperatures could coincide with summer heat waves. Diurnally, the UHI typically peaks at night, when stored daytime heat prevents cities from cooling as much as rural areas (Oke 1982). During heat waves, this can produce longer, unbroken stretches of stressful temperatures, which pose greater public health risks than isolated hot days (Schwartz 2005, Tan et al 2007, Kalkstein et al 2011. Under normal temperature conditions, UHIs may not create significant risks to urban communities. During extremes, however, UHIs could have critical impacts by raising already stressful temperatures during heat waves or by providing relief during severe cold. To improve our understanding of UHIs during these conditions, our study describes UHI effects during a historically hot summer, a record setting heat wave, and a historically cold winter in Madison, Wisconsin using one of the densest urban climate networks ever deployed (Schatz and Kucharik 2014).
Our study contributes to a growing body of literature on UHI effects on extreme heat.
Several studies have used remotely sensed land surface temperature (e.g., Zaitchik et al 2006, Schwarz et al 2011 or weather research and forecasting model simulations (e.g., Li and Bou-Zeid 2013, Meir et al 2013, Chen et al 2014, Gutiérrez et al 2015 to understand urban effects on extreme heat. Other studies, like ours, use urban sensor arrays to record urban effects on near surface air temperatures during extreme heat events (e.g., Harlan et al 2006, Bornstein and Melford 2009, Basara et al 2010, Kershaw and Millward 2012, Meir et al 2013. Our study contributes several novel elements to this literature. First, little to no comparable research has been conducted in mid-sized cities like Madison with populations of 100 000-500 000, which together comprise over half of the global urban population (Cohen 2006). Another key element is that we observed not only a record setting heat wave, but also the historically hot summer in which it was nested. This provides a window into the mid-to late-21st century when both average temperatures and episodic extreme heat are projected to increase globally (IPCC 2014) and in Wisconsin (Kucharik et al 2011). Additionally, our temperature and humidity measurements allow us to describe both temperature and apparent temperature (AT) (Steadman 1984). This is one of the first urban studies to report both metrics at such high spatial resolution, providing novel insights into how UHIs affect heat exposure. Finally, to our knowledge, our study is the first to focus on urban effects on extreme cold. Climate change has decreased the incidence of extreme cold over recent decades in many parts of the world (Vavrus et al 2006, Mishra et al 2015, but cold remains a significant health risk factor (Mercer 2003), and we are aware of no other studies describing UHI effects during persistent, regionally extreme cold temperatures.
Using these observations, our objectives are to describe (1) how the UHI affected the intensity and duration of hot and cold conditions, (2) whether the UHI was stronger or weaker during more extreme temperatures, and (3) where UHI effects occurred spatially with respect to where most people lived.

Data
Madison, Wisconsin is a city of 233 000 in the northcentral United States with an urban agglomeration population of 402 000 (US Census Bureau 2012). It has a humid-continental climate (Köppen: Dfa), 1981-2010 mean annual precipitation of 876 mm, and mean temperatures of −7°C in January and 22°C in July (NCDC 2014). Madison is surrounded by lakes and a rural landscape of agriculture, forests, wetlands, and grasslands (figure 1).
In March 2012, 135 HOBO ® U23 Pro v2 temperature/relative humidity sensors in solar shields (Onset Computing 2010) were installed on streetlight and utility poles across the study area (figure 1). Sensor accuracy is 0.21°C from 0 to 50°C for dry bulb temperature and 2.5% from 10 to 90% for humidity (Onset Computing), though errors nearly twice as large have been reported for sensors within shields in field conditions (Nakamura and Mahrt 2005). Additional locations were added in 2012 and 2013 for a total of 150 sensors. The sensors were installed at 3.5 m height to minimize risk of disturbance. This differs from standard meteorological heights of 1.5-2 m, and the possible impacts of this are discussed in our results. Sensors were positioned on the north side of poles except in six cases to avoid the road right-of-way. Instantaneous measurements were recorded every 15 min.
AT reflects the interacting effects of temperature and humidity on physiological heat stress and is commonly used to measure heat exposure in epidemiological studies (Basu 2009). We calculated AT as: AT=−1.3+0.92 T+2.2e (Steadman 1984), where T is dry bulb temperature (°C) and e is vapor pressure (kPa), calculated after Buck (1981). Although our study focuses on temperature and humidity, it is important to remember that other micro-climatic factors, such as wind and sun exposure, also affect heat stress (Steadman 1984) and are also sensitive to urban development (Landsberg 1981).
Our study describes the summer of 2012 and winter of 2013-14, when 133 and 148 sensors were active, respectively. The summer of 2012 was Madison's third hottest since 1869 when records began, with temperatures at the Dane County airport (MSN) reaching 32.2°C (90°F) on 39 days compared to the 1981-2010 average of nine days (NCDC 2014). In late June and early July, Madison experienced a severe heat wave with seven consecutive days over 35°C, three consecutive days reaching 38.9°C, and five consecutive days with record high temperatures (NCDC 2014).

Calculating UHI intensity
UHI intensity (ΔT) is classically defined as the temperature difference between a city and its rural surroundings (Stewart and Oke 2012). However, the degree of urban development varies continuously across landscapes, as will the magnitude of UHI effects. Defining sites as simply urban or rural forces a continuum into a category, oversimplifying both cities and their UHIs. Recently, Stewart and Oke (2012) proposed a wider range of urban and rural land cover categories in order to better describe measurement sites, contextualize UHI intensity, and compare among studies. We offer an alternative definition of ΔT that does not rely on categories, but rather on continuous empirical relationships between temperature and the density of the built environment. Figure 2 illustrates this definition using simulated data from 20 hypothetical measurement sites. Among  Illustration of the calculation of UHI intensity (ΔT) using simulated data. In our study, ΔT represents the fitted temperature difference between areas with 0% impervious surface coverage and 100% impervious surface coverage (i.e., the slope of the temperature-impervious relationship multiplied by 100). these 20 sites, there is a positive relationship between temperature and percent impervious surface coverage (IMP), with a slope of 0.05. For this paper, we define ΔT as the fitted temperature difference between areas with 0% IMP (i.e., rural) and 100% IMP (i.e., dense urban). In figure 2, ΔT is therefore 5°C. Urban-rural AT differences (ΔAT) are defined in the same way. This definition puts UHI intensity explicitly in terms of urban development and avoids qualitative urban and rural land cover categories. Other measures of urban physical density, such as height-width ratio or sky view factor (Unger 2004), could be used instead of IMP, but for our study area, IMP consistently provided better model fits than sky view or height-width ratio.
We calculated ΔT on each day of our study period using linear regression models. The response variables were either daily T MAX , T MIN , AT MAX , or AT MIN at each sensor. The explanatory covariates were average IMP around each sensor (with water masked out), lake proximity, and topographic relief, which all influence temperatures in our study area (Schatz and Kucharik 2014).
Average IMP within circular buffers ranging in radius from 100 to 2000 m (in increments of 100 m) were tested to see which produced the best model fits across the entire study period. Of all radii tested, 600 m provided the best average fit of the temperature and AT data. It was more important to have a consistent set of covariates across the entire study period, enabling direct comparisons of daily ΔT/ ATs, than to fit optimal models for each individual day. For lake proximity, we tested various linear and exponential relationships between lake proximity and temperature. The best fitting relationship across the entire study period was e −8d , where d is kilometers from the nearest lake shore. Topographic relief was the difference between local elevation and average elevation within a 0.8 km radius on a three meter resolution elevation model.
Daily ΔT/AT models were well behaved for normality, constant variance, and independence of predictors, but significant spatial autocorrelation occurred in 21% of daily models, for which we used spatial regression (Anselin 2002). The spatial models used inverse distance weighted neighbor matrices with maximum neighbor distances of 10 km in either spatial error or spatial lag models, which were selected using Lagrange multiplier tests (Anselin 1988). Using this same method, we explored diurnal UHI patterns during the July 2012 heat wave by calculating ΔT and ΔAT every 30 min.

UHI intensity versus extreme temperature
This analysis addressed two questions. First, did extremely hot or cold days have stronger UHIs? Second, were these relationships due to the extreme temperatures themselves, or to the weather conditions coinciding with extreme temperatures?
To answer the first question, we used linear mixedeffects models to test the relationship between temperature and UHI intensity. To test for extreme heat effects, we used data from the 50 consecutive days of the year with the highest average temperature across our study period (15 June-3 August for 2012-2014; n=150 days) and fit models between daily ΔT and T MAX and between daily ΔAT and AT MAX . To test for extreme cold effects, we used data from the 50 days of the year with the lowest average temperature across our study period (23 December to 13 February for 2012-2014; n=100 days) and fit models between daily ΔT and either minimum temperature or wind chill temperature (WCT). The WCT incorporates the perceived temperature effect of moving air increasing the rate of heat loss from the skin surface, and was calculated as where T is temperature (°C) and V is wind speed (km hr −1 ; Osczevski and Bluestein 2005). In all mixed models described in this section, year was included as a fixed effect, and a first order autoregressive correlation structure accounted for serial autocorrelation.
To answer the second question, we used a series of two linear mixed-effects models (Pinheiro et al 2014) on the extreme heat or cold data. The first model controlled for weather effects, with daily ΔT or ΔAT as the response variable. Covariates were daily average wind speed, percent sun, and either soil moisture (warm weather model) or snow depth (cold weather model), which are the primary meteorological determinants of UHI intensity in our study area (Schatz and Kucharik 2014) and elsewhere (Oke 1982, Runnalls and Oke 2000, Malevich and Klink 2011, Smoliak et al 2015. The second model used the residuals from the first model as the response variable, with daily T MAX or T MIN as the explanatory covariate. This series of models allowed us to test whether extreme temperatures had independent effects on UHI intensity beyond their correlation with key weather conditions. For the weather covariates, temperature, wind speed, and snow depth came from MSN (NCDC 2014). Percent sun was calculated as measured insolation (300-1100 nm; S-LIB-M003 pyranometer, Onset Computing) averaged over three sites in our study area as a percent of potential insolation, which was calculated after Allen et al (1998). Soil moisture was measured at 10 cm depth (EC-5 sensor, Decagon) and also was averaged over three sites in our study area (figure 1).

Interpolations
The intensity and duration of extreme temperatures were visualized on a 400×400 m resolution grid using regression kriging, which uses information about the land surface to inform data interpolation (Hengl et al 2007). For covariates, IMP, lake proximity, and topographic relief were used (each as described in section 2.2). Lake effect and topography were averaged within each 400 m grid cell; IMP was averaged over a 600 m radius of each grid centroid (with water masked out). Spherical or exponential variogram fits were selected using Akaike Information Criterion. In winter 2013-14, lakes were frozen (Wisconsin State Climatology Office 2015) and there was no significant effect of lake proximity on temperatures, so lake effects were not included.

Urban effects on heat intensity and duration
Madison's most densely built urban areas are primarily concentrated in the isthmus located between the region's two largest lakes, near the center of out study region (figure 1). In summer 2012, these densely built urban areas experienced mean July T MAX up to 1.8°C higher; mean July T MIN up to 5.3°C higher; and up to twice as many hours 32.2°C (90°F) than rural areas (figures 3(a)-(c). The physical density of the built environment, represented by IMP, was the primary spatial driver of these differences (table 1, figures 4(a)-(c)). The lakes tended to decrease adjacent temperatures, though these effects were mostly restricted to shoreline locations, as reflected by the rapid exponential decay of lake influence with distance from shore (section 2.2). Local topography was only statistically significant with respect to minimum temperatures (table 1), presumably due to cold air drainage to low lying areas.
The UHI had similar effects on AT as on dry bulb temperature (figures 4(a)-(c)). However, the slope of AT MAX versus IMP was lower than the slope of T MAX versus IMP, indicating that the UHI increased AT less than it increased T. This may relate to urban areas tending to have drier air, causing the UHI to raise AT MAX less than it raised T MAX . Calculating urbanrural differences of mean July 2012 maximum dew point confirms this, yielding a ΔDP of −0.5°C, indicating that urban areas averaged lower dew points.
Our sensors were at 3.5 m height, complicating comparisons with the official NOAA observing station at MSN (figure 1), which is at 2 m. However, comparing observed daily T MAX at the airport to T MAX interpolated from our sensors for the same location indicates that at 3.5 m, downtown Madison experienced up to 10 more days 32.2°C than MSN in 2012 (figure S1). If this 10 day difference held at 2 m as well, downtown Madison experienced approximately 49 days over 32.2°C compared to the 39 days officially recorded at MSN in 2012. This underscores the impact of urban climates on heat exposure, as well as the potential for rural or semi-rural airport stations to underestimate heat wave severity for nearby urban populations.

Urban effects during heat wave conditions
During the summer of 2012, Madison also experienced a record setting heat wave. From 1 to 7 July, the densest urban areas, which are near the center of our study region, experienced daily T MAX from 1.6 to 2.2°C higher than rural areas, with similar urban effects on AT MAX (figure 5). Urban effects on nighttime temperatures were particularly striking. Figure 5 shows the growth of each day's UHI during the evening and nighttime hours, prolonging high temperatures and diminishing nocturnal relief from extreme heat. Minimum temperatures in the densest urban areas averaged 4.4°C higher than rural areas. This is reflected in the number of consecutive nights without temperatures falling below 26.7°C (80°F), which is a heat stress threshold used by the National Weather Service (NWS) (Robinson 2001). While most rural areas cooled below 26.7°C every night, the densest urban areas spent over four days (temperature) and nearly 7 days (AT) without falling below 26.7°C (figures 6(a) and (b)).

UHI intensity during extreme temperatures
Madison's UHI also affected exposure to cold temperatures. In winter 2013-14, Madison's most densely built areas experienced a mean January T MAX up to 1°C higher and a mean January T MIN up to 3°C higher than rural areas (figures 7(a) and (b)). Remarkably, urban areas also experienced up to 40% fewer hours −17.8°C (0°F) than rural areas, a difference of nearly 200 h ( figure 7(c)). The physical density of the built environment, represented by IMP, was the primary spatial driver of these patterns (table 1; figures 8(a)-(c)). Lake proximity was not significant in winter, when the lakes were frozen (Wisconsin State Climatology Office 2015), and local topography was only statistically significant with respect to average January T MIN , presumably due to cold air drainage to low lying areas.

Population density and the UHI
More densely populated areas tended to experience larger UHI effects. The log of average population density (2010 US Census block group data) within a 1000 m radius of each sensor was linearly related to UHI intensity (figure 9). This was true during both the winter (figures 9(c)-(e)) and summer (figures 9(a), (b) and (f), with minimum temperatures showing the strongest relationships (figures 9(c) and (d)). Table 1. Percent of spatial variation in the duration and intensity of hot and cold temperatures explained by percent impervious surface coverage (IMP), lake proximity (lake), and topographic relief (topo).  3.5. Was the UHI more intense during more extreme temperatures? During our warm temperature study period, there was a positive relationship between daily T MAX and ΔT at T MAX compared to summer average UHI intensity (figure 10(a)) with a similar relationship between AT MAX and ΔAT ( figure 10(b)). Put simply, the daytime UHI tended to be stronger on hotter summer days, with most days over 32.2°C experiencing above average UHI intensities (figures 10(a) and (b)). There was a weaker positive relationship between T MAX and UHI intensity at T MIN ( figure 10(c)), but no significant relationship between AT MAX and ΔAT at AT MIN ( figure 10(d)). Although these are weak relationships, they nonetheless indicate that UHI effects tended to be slightly stronger on hotter summer days. These relationships appeared to be due to hotter summer days coinciding with weather conditions that favor stronger UHIs. After accounting for the variation in ΔT that was explained by percent sun, wind, and soil moisture, T MAX no longer had significant effects on UHI intensity. Of these weather factors, only percent sun was significant (p<0.0001). During our cold temperature study period, most of the strongest UHIs occurred at T MIN −17.8°C, while days with a T MIN over −5°C experienced no above average UHIs (figures 11(a) and (b)). This created a negative relationship between daily T MIN and ΔT, such that colder days and particularly nights tended to have stronger UHI effects. Using WCT, however, rendered these relationships non-significant (α=0.01), presumably because the strong winds that cause low WCTs also tend to weaken UHIs (Oke 1982). As with extreme heat, any significant relationships between T MIN and ΔT were due to colder days coinciding with weather conditions that favor stronger UHIs. Percent sun and snow depth were each significantly related to both T MIN and ΔT and accounted for any significant correlations between the two, Figure 5. Temporal patterns of urban and rural temperatures during the 1-7 July 2012 heat wave in Madison, Wisconsin. The red filled series represents ΔT, with the upper bound representing average temperature in areas with 100% impervious cover (dense urban), and the lower bound representing average temperature in areas with 0% impervious cover (rural). The upper and lower dotted lines represent the same for AT. The horizontal line marks 26.7°C, which is a nighttime heat stress threshold used by NWS. Numbers represent urban-rural temperature differences (ΔT) at each day's T MAX or T MIN .

UHI effects on extreme temperatures
Madison's UHI substantially increased extreme heat exposure, particularly at night. Heat stress is cumulative, and prolonged heat exposure poses greater risks to public health than isolated hot days (Schwartz 2005, Tan et al 2007, Kalkstein et al 2011. As such, perhaps our most alarming finding was that during the July 2012 heatwave, nearly all rural areas cooled below  26.7°C every night, but the densest urban areas near the center of our study region spent over four days (temperature) and nearly a full week (AT) without falling below 26.7°C. This exemplifies the power of urban climates to fundamentally alter the severity of heat waves by prolonging and intensifying hot conditions. The UHI had similar effects on AT as on temperature, though during the daytime, urban effects on AT tended to be smaller than urban effects on dry bulb temperature. This is presumably because evapotranspiration from rural soils and vegetation raised rural humidity relative to the urban landscape, particularly during the daytime when evapotranspiration typically peaks.
At the other end of the temperature spectrum, Madison's UHI reduced exposure to cold temperatures, which can be a significant health risk factor (Mercer 2003). Urban warming during severe cold could save lives and significantly reduce winter heating demand (Kolokotroni et al 2012). In regions susceptible to cold temperatures, it is important to consider these potential cold weather benefits when designing UHI mitigation measures to lower heat risk.

Extreme heat effects on UHIs
Not only did the UHI increase the severity of extreme heat in Madison, but UHI intensity was greater than the summer average during periods of extreme heat. Several other studies have also reported that UHI intensity is as strong or stronger during heat waves compared to summer background conditions (Harlan et al 2006, Li and Bou-Zeid 2013, Meir et al 2013, Li et al 2015. In Madison, we attributed this to hot summer days coinciding with clearer skies, which favor stronger UHIs (Oke 1982). In Baltimore, Li and Bou-Zeid (2013) attributed the phenomenon to low wind speeds and drier soils during heat waves, which also facilitate stronger UHIs (Oke 1982). In general, heat waves result from stagnant high pressure systems (Loikith and Broccoli 2012) that are associated with clearer skies, calmer winds, and drier soils, all of which favor greater UHI intensity (Oke 1982, Schatz andKucharik 2014). This suggests that heat wave conditions may generally facilitate stronger UHIs.
Further, Li et al (2015) explored the synergy between heat waves and UHIs using an energy budget approach in Beijing. They found that heat waves increased sensible heat flux relatively more in urban areas and latent heat flux relatively more in rural areas, leading to greater urban-rural temperature divergence on hotter days. Essentially, heat waves tended to magnify existing differences in urban and rural energy budgets, with more energy partitioned to latent heat in rural areas and more to sensible heat in urban areas. These traits of urban and rural energy budgets are typical of cities (Oke 1982), providing further support for the hypothesis that UHIs strengthen during heat waves. Anthropogenic heating from combustion and air conditioning may also increase during heat waves (Stone 2012), further strengthening UHIs. More research is needed to test these hypotheses, but taken together, this represents a growing body of evidence that UHIs tend to strengthen during extreme heat. Oke (1973) first described the positive relationship between the log of a city's population and its maximum UHI intensity. Our study is the first to show a similar relationship within a single urbanized area, such that the most densely populated parts of our study region tended to experience the strongest UHI effects. This does not imply that large concentrations of people create intense UHIs (but see Sailor 2011), but rather that densely built urban areas tend to have both high population density and intense UHI effects. We suspect that this will hold true in many cities, although population density and the physical density of the built environment certainly can be decoupled. People may not live in commercial or industrial areas, for example, and informal urban settlements can have extremely Figure 10. Daily maximum temperature versus UHI intensity during the period 15 June-03 August from 2012 to 2014. The y-axes represent daily UHI intensity (ΔT or ΔAT) at each day's maximum T or AT. The x-axes represent the corresponding daily maximum T or AT at MSN. Horizontal dotted lines represent seasonal mean UHI intensity. ns=not significant at α=0.01 in linear mixed effects models. Figure 11. Daily minimum temperature versus UHI intensity during the period 23 December-13 February from 2012 to 2014. The y-axes represent UHI intensity (ΔT), as calculated in section 2.2, at each day's maximum or minimum T. The xaxes represent the corresponding daily minimum temperature at MSN. Horizontal dotted lines represent seasonal mean UHI intensity. All relationships are significant at α=0.01 in linear mixed effects models. high population densities (Streatfield and Karar 2008) without the dense, tall buildings associated with strong UHIs. Nonetheless, urban residents tend to spend much of their time in densely built areas, whether to live or to work, where they may encounter not just urban warming, but relatively strong urban warming.

The UHI and climate change projections
Climate change projections commonly report future changes in the number of days above high temperature thresholds. For example, Madison currently averages 9 days per year 32.2°C, but this is projected to reach 29-37 days by mid-century and 37-65 days by late century (Kucharik et al 2011), depending on the greenhouse gas emissions scenario (B1, A1B, A2; IPCC 2000). However, these projections do not account for urban climate effects, which is concerning for two reasons. First, cities are where most people will encounter future warming. Second, we observed substantially more time above high temperature thresholds in urban areas compared to rural areas (figures 3, 6, S2), including up to 21 more days 32.2°C in urban versus rural areas in 2012 (figure S1). This suggests that projections failing to account for UHIs could considerably underestimate the amount of heat for which urban communities need to prepare (Fischer et al 2012, Argüeso et al 2015, Oleson et al 2015, with the largest underestimates in the largest cities with the highest populations and strongest UHIs.

Conclusion
Using a dense network of temperature and humidity sensors, we recorded UHI effects on historically hot and cold conditions in Madison, Wisconsin. More densely built and populated urban areas experienced significantly greater exposure to extreme heat and lower exposure to severe cold. Further, UHIs tended to be more intense during extreme heat episodes due to hot days coinciding with weather conditions favorable to strong UHIs. These results advance our understanding of urban climates during thermal extremes and help urban communities understand the risks they face in a warming, urbanizing world.