Remote Sensing of Urban Environmental Conditions - Remote Sensing of Urban and Suburban Areas - page 273

Fig. 14.2 Scatterplots of Surface Temperature and Vegetation Fraction for New York City and
Kuwait City. Darker areas correspond to greater number of pixels. Low temperature, unvegetated
areas generally correspond to water bodies. Note the Arabian Gulf is cooler in March than in New
York Harbor and the surrounding rivers in August. New York also has higher temperatures, in spite
of abundant vegetation, because the image was acquired during drought conditions when evapo-
transpiration was low
(Fig. 14.2 ). Pixels near the base of the triangle, which is parallel to the surface
temperature axis, attain a wider range of temperatures compared to the pixels in the
vertex of the triangles. Dark areas in Fig. 14.2 represent higher pixel density than
lighter tones.
According to Carlson et al. ( 1995 ), the warm edge is a sharply defined boundary
representing the locus of highest temperatures and different vegetation fractions.
On the other hand, the cold edge represents the locus of lowest temperatures for the
vegetation fractions. The triangular shape distribution in the scatterplots defines the
physical limits imposed by the vegetation cover, soil water content and different
combinations of surface materials. In rural areas, the shape of the Temperature/
Vegetation (TV) distribution results from spatial variations in vegetation cover and
soil moisture availability (Gillies et al. 1997 ; Crombie et al. 1999 ). In urban areas,
there is generally little exposed soil so the shape of the distribution is dictated by
variations in vegetation cover, albedo and shadow. The TV distributions for New
York and Kuwait City do not show the familiar triangular distributions usually seen
in rural areas. This is primarily a result of urban land cover heterogeneity and the
negligible fraction of exposed moist soil. The warm edge shows a less pronounced
cooling effect in New York as the image was acquired during drought conditions
with low moisture availability for many areas. The absence of a well-defined cold
edge is a consequence of extensive area of the partial shadow at all vegetation frac-
tions. The pervasive presence of shadow in the urban mosaic is a consequence of
the characteristic 20-30 m spacing of trees, streets and buildings (Small 2003 )
coinciding with the spatial scale of the Landsat IFOV. The increase in minimum
temperature is more pronounced in the Kuwait City because of the relatively low