Convergence of microclimate across diverse cities in the US
Urban activities, land use, and the built environment significantly alter biophysical properties of air, water, and soil within cities and beyond. Early observations about the distinct characteristics of urban air, documented more than a century ago, have since developed into an extensive field of urban climatology focused on the patterns and drivers of atmospheric phenomena at multiple scales in cities. Worldwide, similar biophysical trends such as the urban heat island have emerged, driven by common building materials and comparable land cover, among other factors. However, few studies have explored the extent to which similar human preferences and land cover decisions in cities lead to convergent local climates. Are microclimates in diverse cities more similar to each other than climates of their surrounding native landscapes? In 2013-14, we compared relative humidity and air temperature in residential yards and native ecosystems in six metropolitan areas, Phoenix, Miami, Baltimore, Boston, LA, and Minneapolis-St. Paul. We expected that residential microclimates across diverse cities would converge. In other words, we expected that arid southwestern cities would be cooler and more humid than their surrounding native lands, and cities in the humid east would be warmer and drier than the ecosystems that these cities replaced.
Urban properties and processes lead to convergence of microclimate among cities located across diverse climate zones. During an 8-day period in August, the afternoon relative humidity (RH) of residential yards in the Phoenix metro area was 40% higher than in the nearby, arid Sonoran Desert (19% RH in yards vs. 13% RH in the desert). Conversely, the humidity of yards in Baltimore, Boston, Miami, and Minneapolis was 9% lower than in the forests and grasslands surrounding these cities (average 66% RH in yards vs. 74% RH in surrounding native ecosystems). Average Phoenix afternoon temperatures in August were an uncomfortable 41°C and similar to undeveloped desert land. Yards in Baltimore, Boston, Miami, and Minneapolis (24-31°C) were not as hot as Phoenix but were ~2°C (8%) warmer than surrounding native ecosystems (22-29°C). We expect that patterns in microclimate are driven by variation in vegetation cover, open water, impervious surface area, and irrigation practices between urban and native landscapes across cities, and between residential properties within cities. These data suggest that common yard characteristics and management activities across diverse US households result in ecological homogenization at continental scales.