EXPLORING THE ROLE OF MOISTURE IN ELEVATING NIGHTTIME TEMPERATURES IN MAJOR NORTHEAST U.S. CITIES

Rising temperatures, stemming from anthropogenic influences, have contributed to summertime heat-related fatalities in major cities. Hondula (2012), for example, established a statistically significant increase in mortality on extremely hot days in Philadelphia. This study explores how moisture contributes to elevated temperatures, particularly at night. Fifty years of hourly NOAA data, spanning the period 1973-2022, from several urban heat islands across the Northeast, including Philadelphia, are used in this study. Specifically, decadal trends in both dewpoint and cloud ceiling (defined as the lowest cloud base that is either broken or overcast) data are analyzed. Dewpoint is important since it reflects the amount of moisture in the atmosphere, and its combination with temperature -- commonly referred to as the heat index -- can impede the body's ability to cool. Cloud height is germane since lower clouds can increase the amount of longwave energy reaching the ground. Results from Philadelphia reveal temperatures have risen ~1.5-2°C since the 1970s during the overnight hours. The dewpoint has risen ~1°C during the same period. Yet, there has been no increase in the frequency of lower clouds, with suggestion of up to a 2% frequency decrease in clouds <1000 m (e.g., fog, stratus) from the ground. Thus, higher atmospheric moisture content can account for much, but not all, of the elevated temperatures overnight while no contribution stems from increased radiation due to low clouds.