created an urban climate by building urban
areas. result from extra heat sources released
from industry, commercial and residential
buildings, and vehicles concrete glass and
tarmac.
urban climates are hotter with less water vapour
and lower humidity and less snow. more intense
storms and reduced visibility. slower winds but
can be wind tunnelling.
climatic changes occur in an urban area because…
hotter than surrounding areas because there’s a greater surface area to absorb
heat. albedo of tarmac and concrete is lower than grass areas so less insolation is
reflected. buildings absorb large heat quantities and release them at night. little
energy used for evapotranspiration so more is available to heat the atmosphere.
pollutants help trap radiation in urban areas
less moisture and lower relative humidity because there’s a lack of vegetation so
less transpiration and high drainage density that remove water.
more intense storms due to greater instability and stronger convection above
urban areas
less snow because higher temps
slower winds because wind is deflected by buildings
Case study: Vancouver’s microclimates
Canada’s 3rd largest city, open to the sea but backed by mountains broken by the
valley of the Fraser river which enters the coastal lowland to east of the city
Temperature…
on sunny summer days a surface temperature traverse across the city shows a
marked heat island. sea has the lowest temp (15ºC), followed by river (20º). Stanly
park has cool temps (20º) which is similar to the rural farmland of Fraser valley.
airport would record a higher temperature than the well vegetated Stanley park as
of terminal buildings and runways. temps are higher in suburbs of west Vancouver
(22º) and surrey (18º) but the dome of the heat island is centred on the downtown
(CBD) area where there is a high density of skyscrapers. downtown (30º).
when sea breezes blow on some summer afternoons, air temps near coast can be
5-10ºC cooler than in the Fraser valley beyond their influence, whereas in winter
it’s the inland areas that are cooler, by about 2º.
the CBD has the warmest area at 27ºC and is 10ºC warmer than rural area. steep
rise in temp at edge of the built-up area (25º). two peaks of temp, separated by
cooler temps over a small inlet of water.
difference in temp between urban and rural is high on summer nights too, with
calm and clueless anticyclonic conditions with no wind to remove heat. in those conditions temps can decrease steadily outwards. large urban areas have earlier
springs and later autumns and less frost than rural.
heat island effect caused by:
brick concrete and dark tarmac surface have a large specific heat capacity and low
albedos. rapidly absorb solar radiation. heat is conducted away from surface,
stored and re-radiated steadily into air at night as long wave radiation
tall buildings turn streets into canyons, reduced sky view and smaller angle of
direct heat loss at night. urban parks are cooler as the sky is much wider so
radiational heat losses are greater. greater the building density, greater the
heating so city centres have the highest minimum temps at night.
central heating, gas/electric fires and heat from industrial sources warm the urban
atmosphere
although pollutant particles and gases reduce the amount of short-wave radiation
reaching the surface by day, their main effect is reducing the loss of re-radiated
energy at night by absorbing and re-radiating long wave radiation
higher rates of evapotranspiration in rural areas lower their temps by absorbing
heat
Wind…
skyscrapers cause turbulence and vertical uplift with eddies in the lee of buildings.
winds less strong in centre as buildings obstruct and deflect airflow and increase
friction.
more calm in built up area, canyon like streets have the strongest gusts when
winds are fuelled between tall buildings.
Humidity…
absolute and relative humidity are both lower in city as less surface water and
vegetation is present for evaporation and transpiration to occur. rainfall runs off
rapidly down gutters and drains. humidity rises over the harbour, Burrard inlet and
Fraser river where more evaporation is possible. daytime humidity greater over
Stanley park where moisture is increased by transpiration.
at night humidity gets slightly higher over the city as in rural areas more dew is
deposited from the air, making it less humid.
Precipitation…
orographic effect is present because with every 100m increase in height,
precipitation increases by 100mm.
mountain edge of west Vancouver receives double of the main city.
snow falls there in winter but rare in lower areas (parts of the city nearer warmer
ocean and central city) where warmth melts snow as it falls.
relief is most important cause of variations in rainfall within the city but greater
amount of cloud and precipitation results from…
development of convection currents on calm clear nights when the urban rural
temp differences are largest. warm air rises over central city and airflow moves
towards rural area aloft and then sinks. air moves from rural area over the surface
to the lower pressure in the city. convection currents result in higher rainfall in
central city with more thunderstorms, 10% cloudier than surrounding rural area.
storm cells intensifying by contact with the warm surface as they pass over
buildings forcing moist air to rise, triggering convection
increased density of hygroscopic (condensation) nuclei strongly encourages
condensation Fog and smog
fogs are more frequent in the city, particularly in winter, because…
lower wind speeds and more frequent calm period allow air to remain in contact
with ground and to cool by conduction
greater number of hygroscopic nuclei around which condensation can occur
cities have x7 more particulates and 200x more gaseous pollutants than rural. 75%
of pollution from motor vehicles. lower Fraser valley is affected by photochemical
smog on warm summer days but the east suburbs have more ozone concentrations
than the downtown area as chemical reactions that form ozone take time. strong
ultraviolet sunlight causes chemical reactions in hydrocarbons and nitric oxides of
exhaust fumes, creating surface level ozone, ingredient of photochemical smog.
gradually produced as sea breezes move the pollutants inland. mountains trap the
pollutants in eastern suburbs.
worst pollution occurs during anticyclones as subsiding air warms and causes a
temp inversion above the city which acts as a lid on rising air and traps pollutants
below. light winds in anticyclones don’t disperse the pollutants so assist fog to form
and mix with pollutants to form smog.
