ancient cities

850 Urban Climate
(three-dimensional motion of the flow of air).
Rough surfaces are also slightly better absorbers of
sunlight compared to flat surfaces with equivalent
surface properties.
Surface Reflectance. Overall, urban areas tend to
be slightly better absorbers of sunlight; that is, the
reflectance of urban areas is slightly less than that
of their surroundings. Within urban areas, there is
large variability in the solar reflectance for urban
materials, ranging from dark asphalt roads to
whitewashed walls. Variations in surface reflectance,
combined with shading from buildings and trees,
means that there is a large variability in the amount
of sunlight received and absorbed by individual
components of urban surfaces and, consequently,
in their temperature.
Urban Thermal Properties. The materials used in
urban construction for the most part are good at
storing and retaining heat. Individual urban materials
exhibit a wide variety of thermal properties, ranging
from well-insulated roofs with a poor ability to
conduct or retain heat to massive amounts of
concrete that can store large amounts of heat by day
and release their store of heat slowly at night.
Moisture. Most materials used in urban construction
are impervious, so that precipitation quickly runs
off, meaning that many urban surfaces are dry
except during or immediately after precipitation.
Overall, cities tend to be drier than their surroundings.
However, cities may also be home to irrigated parks,
lawns, and gardens, so that in some cases, cities
may contain areas where moisture is present even
when the prevailing climate has little precipitation.
Urban Atmosphere. Additions of pollutants, heat,
and humidity from combustion processes within
the city (such as from space heating, human
activities, and traffic) modify the urban atmosphere
directly and contribute to urban climate modifications.
Polluted urban atmospheres reduce the
transmission of solar radiation, leading to slightly
less radiation being received by the urban surface,
and change the character of the received sunlight: It
is usually depleted in ultraviolet wavelengths and
more radiation is scattered by pollutants, leading
to more indirect or diffuse sunlight received by
urban surfaces. Absorption of sunlight by pollutants
warms the urban boundary layer. Additions of
humidity and other gases also affect transfer of
infrared radiation and serve to make the urban
atmosphere warmer.
Urban Surroundings. Finally, the nature of urban
climates, and in particular their difference from
the climate of their surroundings, depends on the
nature of their surroundings. For example, in the
winter season of cities in mid to high latitudes,
strong contrasts in surface reflectance may occur
after snowfalls; while rural areas are mostly snowcovered,
urban areas exhibit a large fraction of
snow-free surfaces. Desert cities may exhibit
higher humidities than their surroundings due to
irrigation. Cities on small islands surrounded by
ocean have nonurban surroundings with much
different thermal characteristics than those of
cities in continental interiors.
Surface Radiation and Energy Balances. The
surface radiation and energy balances provide the
energetic basis for understanding urban climates.
The radiation balance describes the input of
radiation energy to the surface from both the sun
and the atmosphere and how much of this energy
is reflected, absorbed, and emitted. The energy
balance describes how the net absorbed radiation is
partitioned into heating the air, evaporating water,
or stored as heat in urban materials. Researchers
use instrumentation mounted on tall towers to
directly measure and study urban radiation and
energy balances. Numerical representation of the
surface radiation and energy balances are also our
primary means of modeling urban climates. Several
nations are currently working to implement urban
surface radiation and energy balance models as
part of the standard numerical weather forecasting
models used for weather prediction with the hope
of ultimately providing urban residents with better
forecasts.
Urban Heat Island
The most studied urban climate effect is that of the
urban heat island. The urban heat island represents
the difference in temperature between urban areas
and their nonurbanized surroundings. Urban heat
islands exist in both the air temperature of the
urban atmosphere (urban canopy layer and urban