Geoscience Reference
In-Depth Information
March). In December, the harmattan dust haze
tends to reduce city maximum temperatures.
During this season, mean monthly minimum
temperatures are significantly greater in the urban
heat island than in rural areas (March +12
Table 12.4
Population (1990) and elevation for
four Mexican cities
Population
Elevation
(millions)
(meters)
°
C, but
Mexico City (19°25'N)
15.05
2,380
December only +2
C due to the atmospheric
dust effect). In general, urban-rural minimum
temperature differences vary between -2°
°
Guadalajara (20°40'N)
1.65
1,525
Monterey (25°49'N)
1.07
538
and
Veracruz (19°11'N)
0.33
Sea level
+15
C. Two other tropical cities exhibiting urban
heat islands are Nairobi, Kenya (+3.5°C for
minimum temperatures and +1.6
°
Source: Jauregui (1987).
C for maxi-
mum temperatures) and Delhi, India (+3 to 5°C
for minimum temperatures and +2 to 4
°
day (as much as 4
C) and weaker night-time
effects, probably due to the nocturnal drainage of
cold air from the nearby volcano Pichincha.
Ibadan, Nigeria (population over one million;
elevation 210 m), at 7
°
°
C for
maximum temperatures).
Despite insufficient data, there seems to be
some urban precipitation enhancement in the
tropics, which is maintained for more of the year
than that associated with summer convection in
mid-latitudes.
N, records higher rural
than urban temperatures in the morning and
higher urban temperatatures in the afternoon,
especially in the dry season (November to mid-
°
Small-scale climates are determined largely by the relative importance of the surface energy
budget components, which vary in amount and sign depending on time of day and season. Bare
land surfaces may have wide temperature variations controlled by H and G, whereas those of
surface water bodies are strongly conditioned by LE and advective flows. Snow and ice surfaces
have small energy transfers in winter with net outgoing radiation offset by transfers of H and G
towards the surface. After snow-melt, the net radiation is large and positive, balanced by turbulent
energy losses. Vegetated surfaces have more complex exchanges usually dominated by LE; this
may account for >50 percent of the incoming radiation, especially where there is an ample water
supply (including irrigation). Forests have a lower albedo (<0.10 for conifers) than most other
vegetated surfaces (0.20-0.25). Their vertical structure produces a number of distinct microclimatic
layers, particularly in tropical rainforests. Wind speeds are characteristically low in forests and trees
form important shelter-belts. Unlike short vegetation, various types of tree exhibit a variety of rates
of evapotranspiration and thereby differentially affect local temperatures and forest humidity.
Forests may have a marginal topographic effect on precipitation under convective conditions in
temperate regions, but fog drip is more significant in foggy/cloudy areas. The disposition of forest
moisture is very much affected by canopy interception and evaporation, but forested catchments
appear to have greater evapotranspiration losses than ones with a grass cover. Forest microclimates
have lower temperatures and smaller diurnal ranges than their surroundings.
Urban climates are dominated by the geometry and composition of built-up surfaces and by the
effects of human urban activities. The composition of the urban atmosphere is modified by the
addition of aerosols, producing smoke pollution and fogs, by industrial gases such as sulfur
dioxide, and by a chain of chemical reactions initiated by automobile exhaust fumes, which causes
