Urban Heat Island

• Surface UHI: pattern decides by geometry, radiative and thermal properties of surface facets

  • greatest during clear daytime conditions (in parts of citiy with little vegetation)

• Canopy layer UHI: difference between near surface below roof air temperature in city with that one over non-urban landscape

  • greatest after sunset (air above urban cools slower)

  • daytime is usually small or even negative

  • sky view factor is inversly related to rate of nocturnal cooling (-> if sky blocked, canopy layer UHI is stronger)

• boundary layer UHI: maintained by enhanced sensible heat flux from city (promotes mixing in lower atmosphere during daytime and sustains overnight (-> urban boundary layer is warmer than rural one))

  • wind transports urban heat well beyond built-up area

• Subsurface UHI: forms due to transfer sensible heat from urban surface and infrstructure into the ground (heat stored subsurface represents accumulation of long periods, even decadal timescales)

• depends on sky view factor and canyon aspect ratio and more (materials, geometry, …)

• sky view factor shows linear relationship to max UHI

• canyon aspect ratio relationship to max UHI magnitude

• cities heat up slower because of canyon shading and thermal admittance of urban fabric (at some point in the afternoon, cities may even be cooler -> urban cool island (doest exist for less dense/low rise cities))

• UHI effect is strongest in the night (heat stored in buuildings is released + low sky view factor)

• temperature in cities varies greatly (park, schade, surface, …)

• mean winter temperature is higher -> prevents cold-deaths

• during summer more clouds and precipitation in cities

• increase frequency of thunderstorms and hailstorms

• due to surface roughness, aerosols and UHI

BUT most weather affecting citites is forced by much larger scales than city -> influences are only subtle

-> more enhancing than initiating

• cities are moister (especially in autumn and winter) -> from combustion sources

• Dewfall is light or absent (warmer)

-> warm and moist atmosphere and high pollutant concentration results in more longwave radiation

• vegetation provides very versatile services -> can control radiation (intercept 70-90%in summer) exchange, airflow, ventilation of air pollutants, air qualtiy, evaporation, temperature, erosion, runoff and noise levels

• emit specific air pollutants, but also promote deposition and adsorption of other pollutants

• varies with plant architecture (canopy form, branch and root systems) and physiology (species, age, health)

• seperating traffic

  
  

• Decidious are ideal in high latitudes -> sunlight in winter and shade in summer:

  • summer: 70-90% interception of radiation

  • winter: 20-50% interception of radiation

• Coniferous: intercept 70-90% the whole year (can clean the air all year around

conflitcting demands!

• value of H/W: sunshine (daytime warming), raduative heat loss at night (nocturnal cooling), shelter from the ambient wind and street ventilation (air quality and air flow)

• -> values between 0.4 - 0.6 are good