Summary: In spring 2012, 135 temperature and humidity sensors were deployed on light and utility poles 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 2013, 15 locations were added for a total of 150, which recorded measurements every 15 minutes for three years. Spatially, the physical density of the built environment was the primary driver of temperature patterns, with local modifying effects of lake proximity and topographic relief. Temporally, UHI intensity tended to be higher during the warmer summer months and lower during the colder months of the year. These patterns appeared to be related to regional vegetation and snow cover conditions setting seasonal baselines for UHI intensity, with factors like wind, clouds, relative humidity, soil moisture, and snow cover modifying daily UHI intensity around that baseline. During the first two years of study, the sensors recorded a historically hot summer (3rd hottest on record) and historically cold winter (coldest in 35 years) in and around Madison. In both cases, Madison's UHI substantially altered the intensity and duration of extreme temperatures. Temporally, the UHI also tended to be slightly more intense during more extreme heat days compared to average summer days. Spatially, there was a positive correlation between population density and UHI intensity. Madison's urban climate also altered freeze dates by up to several weeks, and decreased heating degree days and increased cooling degree days by up to several hundred per year, with significant implications for heating and cooling energy demand. Finally, I conducted a preliminary analysis of the relationship between temperature and daily hospitalization rates at the zip code scale across Dane County during the hot summer of 2012. Although hotter temperatures slightly increased hospitalizations in Dane County as a whole, warmer zip codes did not have higher hospitalization rates. This dissertation provides novel insights into the fundamental spatial and temporal nature of urban climates, particularly during extreme temperature conditions. The implications of these results, especially in light of global climate change, are described and discussed.