Background
The urban heat island (UHI) phenomenon, as described by the authors, is "caused by a reduction in latent heat flux and an increase in sensible heat in urban areas as vegetated and evaporating soil surfaces are replaced by relatively impervious low-albedo paving and building materials," which replacement "creates a difference in temperature between urban and surrounding non-urban areas." Most studies of this phenomenon have evaluated its magnitude by means of ground-based measurements of near-surface air temperature made at urban and rural weather stations; and they have found that the urban-rural air temperature difference is expressed most strongly at night. In Imhoff et al.'s new UHI study, however, satellite-based measurements of surface temperature were employed; and they found that this alternative form of the UHI was most strongly expressed during the day.

What was done
In a study of 38 of the most populous cities in the continental United States and their rural surroundings, Imhoff et al. obtained land surface temperature (LST) data from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor on NASA's Earth Observing System (EOS) satellites, which they used in a spatial analysis to assess UHI skin temperature amplitude and its relationship to development intensity, size and ecological setting over three annual cycles (2003-2005), where urban impervious surface area (ISA) was obtained from the Landsat TM-based NLCD 2001 dataset.

What was learned
The researchers found that a city's fractional ISA was a good linear predictor of LST for all cities in the continental United States in all biomes except deserts and xeric shrublands, and that the fraction of ISA explains about 70% of the total variance in LST for all cities combined, with the highest correlations (90%) in the northeastern U.S., where urban areas are often embedded in temperate broadleaf and mixed forests. They also determined that the largest urban-rural LST differences for all biomes occurred during the summer around midday, and that the greatest amplitudes were found for urban areas that displaced forests (6.5-9.0°C) followed by temperate grasslands (6.3°C) and tropical grasslands and savannas (5.0°C). Last of all, they determined that the contrast between urban cores and rural zones was typically "accentuated during the time when the vegetation is physiologically active, especially in forested lands," and that "the amplitude of the UHI is significantly diminished during the winter season when vegetation loses its leaves or is stressed by lower temperatures."

What it means
Imhoff et al. conclude that "the use of ISA as an estimator of the extent and intensity of urbanization is more objective than population density based methods and can be consistently applied across large areas for inter-comparison of impacts on biophysical processes."