Investigations of the internal variability in the urban heat island in San Jose, California

Most cities have heterogeneous land uses, which produce distinct differences in urban form and vegetation density at the neighborhoods scale, leading to internal variability in the Urban Heat Island (UHI). Urban heating coupled with rising global temperatures leads to an increase in heat-related illnesses, decrease in human comfort and increase in energy consumption. This study uses vehicular transects to investigate spatial variability of the UHI at the neighborhood scale in San Jose, California. San Jose is the third largest city in California and part of the SFBA conurbation. The transects ran approximately east-west across the city, maintaining a relatively uniform elevation and distance from the surrounding bay and hills, allowing comparison between neighborhood temperatures based on differences in the urban form. Observations included air temperature and humidity mounted on a roof rack at 2 m above ground level, as well as a radiometer directed at the landscape to the side of the road. Temperatures were corrected for the temporal change in temperature over the 60-minute transect time. Relative variations in UHI were determined by variability of temperature samples collected in 6 different land-use types, representing a range in urban form. Sampling sites were classified using the Local Climate Zone Classification (LCZ) scheme proposed by Stewart and Oke (2012), based on vegetation and impervious fractions and building morphology. Internal variability of the UHI is expected to be controlled most strongly by paved surfaces, building heights and density as well as variations in vegetation cover and presence of street trees. UHI assessment studies using LCZ mapping are limited in Mediterranean climates and San Jose is a critical region of development within the Silicon Valley and an example worldwide of urban development with a growing population.