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2. Factors controlling urban climate, urban
climate scales, layers, energy balance
URBAN CLIMATOLOGY
2.1 Factors controlling urban climate
• Urban climate is a typical example of the local climate. However,
it can be studied on different scales from mesoclimate to
microclimate (see further for urban climate scales)
• For these categories it is typical that processes in lower layers of
the atmosphere are primarily formed by radiative, thermal,
aerodynamic, and moisture properties of active surfaces
Main factors controlling urban climate are:
• Thermal properties of the surface materials
• Surface geometry
• Surface waterproofing
• Anthropogenic heat
• Air pollution
Thermal properties of the surface materials
(radiation balance)
• Thermal and radiative properties of active surfaces control
intensity of short-wave (absorption and reflection) and longwave
radiation (emission)
• Albedo of urban areas is lower (10–15 %) that that of rural areas
• Thermal properties of typical urban surfaces cause accumulation
of thermal energy during the day ant its release during the night
Thermal properties of the surface materials
Comparison of selected thermal characteristics for typical urban and rural surfaces
(modified after Oke, 1987 and Zmarsly et al. 2002)
Surface geometry
Sky View Factor (SVF)
Urban SVF Rural SVF
Surface geometry
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Surface roughness
source: http://www.mfe.govt.nz/
• general decrease of wind speed in „strong flow“ situations
Surface roughness
source:https://www.quora.com/
• higher turbulence due to higher air instability (strong local winds in „UHI
situations“)
Surface waterproofing
Higher proportion of impervious surfaces in urban areas
Air pollution
Average air pollution concentration over an urban area
Factors increasing urban air pollution:
• Higher concentration of sources (vehicles, industry, heating)
• Lower wind speed due to surface roughness
• Role of relief (basins, concave shapes of urban relief)
• Higher stability of urban atmosphere (temperature inversions)
Energy balance of urban/rural areas
The city energy balance
can be simplified to:
2.2 Urban Climate Scales (Oke 1997)
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Typical features of urban climate
(Landsberg 1981)
2.3 Final remarks and questions
• important role of local geography (relief)
• role of weather types (radiation dominated vs. advection dominated)
2.3 Final remarks and questions
1. What are the main factors controlling urban climate?
2. What are the main terms of urban climate energy balance?
3. How we can define urban climate scales?
4. What are other factors forming typical urban climates (besides the
relief and weather types)?