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of thermal energy exchanges, and the produc-
tion of anthropogenic heat is more uniformly
distributed spatially and is less intense than in
European and North American cities. In the dry
tropics, buildings have a relatively high thermal
mass to delay heat penetration and this, combined
with the low soil moisture in the surrounding
rural areas, makes the ratio of urban to rural
thermal admittance greater than in temperate
regions. However, it is difficult to generalize
about the thermal role of cities in the dry tropics
where urban vegetation can lead to 'oasis' effects.
Building construction in the humid tropics is
characteristically lightweight to promote essen-
tial ventilation. These cities differ greatly from
temperate ones in that the thermal admittance is
greater in rural than in urban areas due to high
rural soil moisture levels and high urban albedos.
Tropical heat island characteristics are similar
to those of temperate cities but are usually weaker,
typically 4°C for the nocturnal maximum -
compared with 6
The thermal characteristics of tropical cities
differ from those in mid-latitudes because of
dissimilar urban morphology ( e.g., building
density, materials, geometry, green areas) and
because they have fewer sources of anthropogenic
heat. Urban areas in the tropics tend to have
slower rates of cooling and warming than do the
surrounding rural areas, and this causes the major
nocturnal heat island effect to develop later than
in mid-latitudes - i.e., around sunrise ( Figure
12.31A ). Urban climates in the subtropics are well
illustrated by four cities in Mexico ( Table 12.4 ).
The heat island effect is, as expected, greater for
larger cities and best exemplified at night during
the dry season (November to April), when
anticyclonic conditions, clear skies and inversions
are most common ( Figure 12.31B ). It is of note
that in some tropical coastal cities (e.g., Veracruz;
Figure 12.31A ), afternoon urban heating may
produce instability that reinforces the sea-breeze
effect to the point where there is a 'cool island'
urban effect ( Figure 12.31A ). Elevation may play
a significant thermal role ( Table 12.4 ), as in
Mexico City, where the urban heat island may
be accentuated by rapid nocturnal cooling of
the surrounding countryside. Quito, Ecuador
(2851m) shows a maximum heat island effect by
C in mid-latitudes, and they are
best developed in the dry season. There are also
different timings for temperature maxima, and
with complications introduced by the effects of
afternoon and evening convective rainstorms and
by diurnal breezes.
°
8
8
(A)
(B)
6
6
Mexico City (U-S)
Mexico City (U-R)
4
4
Guadalajara (U-R)
Monterrey (U-S)
2
2
Veracruz (U-R)
Guadalajara (U-S)
0
0
Sea
breeze
DRY SEASON
WET SEASON
(DRY)
-2
-2
0
2
4
6
8
10 Noon
14
16
18
20
22
24
JFMAMJ
Months
JASOND
Hours
Figure 12.31 Diurnal (A) and seasonal (B) heat island intensity variations (i.e. urban minus rural or
suburban temperature differences) for four Mexican cities.
Source: Jauregui (1987). Copyright © Erdkunde. Published by permission.
 
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