Scales of Climate in Designing with the Desert - Design with the Desert: Conservation and Sustainable Development - page 66

TABLE 3.4
Radiative Surface Temperatures (10:00 a.m.) and Areal
Coverage in Phoenix, on June 24, 1992

Area
(Miles 2 )
Water 86.1 12.3 0.04 2
Irrigated agric. 88.8 21.1 0.56 80
Irrigated res. 100.2 18.4 0.30 126
Dry res. 110.6 19.4 0.13 178
Commercial 111.9 18.7 0.02 355
Desert 116.9 19.3 0.07 226
Barren 113.7 25.1 0.03 53
Unclassified — — — 669
Source: Derived from Lougeay, R. et al., Geocarto Int ., 11, 79,
1996.
*NDVI, normalized difference vegetation index = Landsat TM
(band 4 − band 3)/(band 4 + band 3).
Albedo, reflected incoming solar radiation from the surface.

Surface
Temp. (°F)

Albedo
(%)

NDVI
Index* Surface Type

and (7) the land-use geography of the city. All of these aspects require shared knowledge
from specialists across an array of disciplines.
Table 3.4 provides results from a remote-sensing project, in which three characteristics—
surface temperature, albedo, and a Normalized Difference Vegetation Index (NDVI)—
were calculated from Landsat 1992 radiance imagery on June 24, 1992, at 10:00 a.m. over
Phoenix. 12 The imagery allows us to estimate an integrated surface ground temperature
for a 394 ft resolution scale. NDVI is an estimate of surface wetness and biomass that can
be derived from specific wavelengths of energy received by the satellite system (except val-
ues for a water surface are inappropriate and should be ignored). Note considerable range
in wetness across the region. Generally, the hotter surfaces are the drier surface types at
this time of the day (10:00 a.m.). Typically, the city commercial and industrial zones retain
heat later in the day and at night due to thermal heat storage properties and canyon-like
trapping of heat. 13
Let us simulate the local effects at this scale by (1) increasing the amount of shade in the
city; (2) changing areas of parks, watered areas, and open areas; (3) changing albedo by
increasing the amount of pavement on the desert; (4) spreading out buildings and affect-
ing the wind-affected roughness at ground level over differing surfaces; and (5) creating
more and more deep urban “canyons” of the city. 14 Tables 3.5 and 3.6 demonstrates these
typical urban effects on the local climate. The bottom line in response is: (1) increasing the
area of pavement increases energy absorption and raises the temperature later in the day,
(2) less shade can cause significant heating, (3) reducing greenspace heats the city, (4) actu-
ally increasing the overall roughness could aid in increasing wind flow and pushing away
bad air (but this is a complex question), and (5) creating deep urban canyons may shade the
surface during the day, but trap heat to higher levels at night than would normally occur.
Wind ventilation is also a key to ameliorating the street-level urban canyon heat excesses
and developing more daytime coolness in the hot desert. Table 3.7 shows the importance
of wind processes and street-level ventilation in relation to above roof flows.15 15 Whether
heat excess is ventilated depends on the speed and type of flow across the variable urban
landscape.