8. Climate on a local scale: Urban Climates
Climatic regions by definition are large areas characterised by similar climatic conditions that persist
over time, in reality they are not entirely homogeneous or constant. In order to understand climate on a
local scale geographers refer to the term microclimate which involves the study of the climate on a
much smaller scale. For instance you could study the differences in microclimate between a large deep
valley compared to a high altitude mountain range or compare the microclimate of a large conurbation
with that of its ‗more rural‘ hinterland.
Urban climates are arguably the most interesting microclimates as human interference has effects on
the climate on a local scale that then has an impact on the lifestyle / quality of life for the population
living in such cities.
(a) Temperature:
“Why are urban areas hotter than rural areas?”
E.g. Manchester can be 2oC hotter than surrounding rural
areas, why?
Temperature in cities is primarily controlled by:
Atmospheric Composition
Air pollution in cities makes light transmission leaving
the city surface (while rebounding back to space)
significantly less than nearby rural areas which have lower levels of pollution e.g. in Detroit
(manufacturing region), USA there is 9% less transmission of reflected radiation back to space
increasing to 25% less transmission of light on a calm day.
Daytime heating of the boundary layer occurs because aerosols (pollution) absorb solar radiation during
the day but this does not have as much effect up to the mean roof level (urban canopy layer). Incoming
solar radiation (short wave) is actually reduced by pollution but is counterbalanced (offset) by the lower
albedo (i.e. surfaces are darker and tend to absorb heat, therefore causing warming). Also cities have a
feature referred to as urban canyons (shape and arrangement of buildings) which effectively increases
the surface area of the area being heated as cities have a greater surface area of material that can be
potentially heated compared to those in the rural.
Urban Surfaces
The nature of the urban surface
controls how well it heats up:
-Character – i.e. type of
surface, some surfaces heat up
better than others and also
have higher heat capacities
leading
to
hotter
city
temperatures;
-Density of urban surface – i.e.
total surface area of structures
as well as building geometry
(shape) and arrangement.
City centres have relatively high heat absorption and therefore high temperatures; however at street
level the readings can be confusing and are often lower than expected due to shading from tall buildings.
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The geometry of urban canyons is important as it effectively increases the surface area by trapping
multiple reflected short wave radiation and also reduces reflection from the surface back to space as
there is a restricted “sky view” again due to building geometry.
Anthropogenic Heat Production
Traffic (cars and other vehicles), industry, buildings and even people release heat. Amazingly these
causes release similar levels of heat energy than that of incoming solar radiation in winter!
In the year 2000 the Boston-Washington DC Megapolis (great city) had an estimated 56 million
residents inhabiting a land area of 32, 000km 2 - which produced enough anthropogenic heat to account
for an equivalent of 50% of the winter radiation and 15% of the total summer radiation!
In the Arctic regions anthropogenic heat provides enough heat to provide a positive heat balance otherwise
conditions in such arctic urban areas would be much cooler.
Urban Heat Island Effect
The NET effect of urban thermal processes (human activity) is to make urban temperatures
considerably greater than those in surrounding rural areas.
The greatest effect occurs in the mid latitudes while under the influence of clear and calm conditions
(anticyclones) which prevents cloud formation and cloud cover.
The result is rural areas become
disproportionately cooler and make the effect appear more apparent.
 Factors contributing to urban heat islands:
1) Thermal heat capacity of urban structures is high. Canyon geometry – dominates the canopy
layer by heating from conduction and convection from buildings loosing heat and traffic;
2) By day there is absorption of short-wavelength radiation by pollution;
3) Less wind in urban areas due to more shelter from urban canyons therefore less heat
dissipation;
4) Less moisture in local atmosphere in cities due to there being less vegetation and quicker run-off
meaning there is less heat needed for evaporation of this moisture present so the remaining heat
not used up by the evaporation process has the effect of increasing temperatures.
The result of the above is that average urban temperatures can be 5-6 oC warmer than rural
areas and 6-8 oC warmer in the early hours of the morning during calm nights as the city radiates
the heat it absorbed during the day. It is heat loss from buildings that is by far the greatest
factor in controlling urban heat islands.
Urban heat islands show the greatest increase in mean temperatures in the largest cities that have
undergone huge population growth, for instance Osaka, Japan has a high population density which many
attribute as a cause for a 2.6 oC temperature rise over the last 100 years as well as it having very tall
buildings. Similarly many North American cities show a similar temperature rise and show the greatest
temperature difference between rural and urban environments – up to 12 oC for American cities with a
population over 1 million people. European cities show a much smaller temperature difference as
buildings here are generally lower and have shallower urban canyons. It is also now recognised that
urban population density has a greater effect on the city temperatures compared to simply city
population size i.e. higher population densities such as Tokyo (12 million population) would have a
greater temperature rise than a similar sized city with a lower population density such as Illinois (13
million population) for example.
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Case Study: London 1930-1960 average temperatures
City Centre 11oC
Suburbs 10.3 oC
Countryside 9.6 oC
Calculations suggest that London‘s domestic fuel use in
the 1950‘s increased temperatures by 0.6 oC on
average in winter. The regional wind speed needs to be
low and the city sheltered by topography for a heat
island to operate effectively. The heat island might be
so great that it may generate its own inward spiralling
wind systems at the surface.
Consequences of heat islands:
Large Cities tend to suffer badly during heat wave conditions when tarmac, paved surfaces and
bricks heat up and retain heat at night time making the effects of heat wave conditions worse.
For instance in 1987 Athens suffered tragic consequences through this process and hundreds
died through heat stress and dehydration.
Snow tends to lie for less time in city centres and near major roads in town centres but may lie
for longer in city centre parks where the surface materials are grass and heat up less quickly. This
might have possible ramifications for the continuation of transport development.
As discussed earlier heat islands might have a positive effect of actually making it possible to
inhabit inhospitable arctic areas in winter.
(b) Precipitation
Temperatures are generally higher in cities and therefore the air in cities can hold more moistures than
that in cooler rural areas and relative humidity levels are subsequently 6% lower in cities. Usually there is
less vegetation cover in cities and less surface water stores meaning lower evapotranspiration rates.
In terms of cloud cover cities often experience thicker cloud cover which is more frequent than that
experienced in rural areas. This is mainly because of convection currents are deflected upwards above
cities causing condensation and cooling as they rise above the urban boundary layer (area affected by
urban surface). Also cities contain more sooty particulate matter form factories and exhaust fumes
which form cloud forming nuclei in the atmosphere above cities. Excess cloud cover helps to explain
why on average large cities are 5-15% wetter based on their average rainfall totals.
In addition 30-60km downwind of a large city such areas receive on average 1/3 more monthly
precipitation than areas upwind of large cities. This can be explained by urban heat islands causing
increased air moisture content as temperatures are warmer and because evaporation of water from gardens
and cooling towers of power stations allows more moisture to be carried by the prevailing winds leaving the
city. It takes time for moisture to condense to sufficient size to fall as rain, so therefore rain is heavier
downwind of cities.
(Left) Photochemical Smog over Athens
Cities also suffer from a 400% increased probability of hail
storms resulting from intense convectional uplift from
rapidly warming man made surfaces such as tarmac and
concrete. Cities also are 25% more likely from suffering
thunderstorms in the summer for the same reason e.g.
London, in the Northern Suburbs thunderstorms are more
likely as rising thermals are encouraged by ridges of high
ground. Cities are also more likely to suffer from thicker
and more frequent fogs as firstly they release more
pollutants from industry and
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transport resulting in more particulate matter in the atmosphere. This particulate matter also
encourages fog formation as it allows condensation nuclii for water droplets, this linked with a greater
chance of calm conditions (due to urban canyons providing shelter) makes fogs a persistant problem
especially under anticyclonic conditions when air is still preventing fogs and smogs from being blown
away (e.g. Athens, Mexico city and Manchester pre 1970‘s due to burning vast amounts of coal).
Modern smog does not usually come from coal but from vehicular and industrial emissions that are acted
on in the atmosphere by sunlight to form secondary pollutants that also combine with the primary emissions
to form a form of man-made low level ozone called photochemical smog (e.g. Athens).
Case Study: Los Angeles
The smog that occurs is a result of a combination of a number of factors. The various forms of pollution
from vehicles (8 million in LA), industry and power stations become trapped in the lower atmosphere
due to the occurrence of a temperature inversion. This is a phenomenon which occurs during the summer
months prevents mixing of the upper and lower atmosphere trapping the pollutants. The pollution
consists of nitrogen oxides, ozone, sulphur dioxide, hydrocarbons and various other gases, brush fires
can add even more pollution to the atmosphere. The pollution exacerbates breathing problems such as
asthma and causes a huge increase in the
number of breathing associated admissions to
casualty and may even result death in very
sensitive or unwell people. City dwellers often
become upset by the high level of pollution due
to the risk to health that it poses, however
most people are also unwilling to give up their
car to help reduce pollution! The response of
the city government is to impose restrictions on
emissions by industry and cars, but many of the
large companies fear impact on their profits and
therefore prevent any effective cuts from
being made. Overall it seems as though the
political will to make a difference is not there.
(c) Air Quality
The quality of air in urban areas invariably is poorer than that of rural areas. Although in the UK air
quality has improved since the decline of the manufacturing industry in the 1970‘s as well as the
requirement for vehicles to be fitted with catalytic convertors. This said cities still have on average 7
times more dust in the atmosphere than those rural surrounding areas. The main factors contributing
to such particulate solid matter and gaseous pollutants is combustion (burning) of fossil fuels in power
stations and by private / public transport. Cities tend to have 200 times more sulphur dioxide (SO 2), 10
times more nitrogen dioxide (NO 2), 10 times more hydrocarbons and twice as much carbon dioxide (CO 2),
These anthropogenic pollutants increase likelihood of cloud cover and precipitation as well as increasing
the chance of photochemical smogs. All of the above absorb and retain more heat from incoming solar
radiation but conversely reduce the sunlight levels in cities.
Sulphur dioxide and nitrogen dioxide which are both considerably higher in urban areas cause acid rain
and can pollute areas downwind of major industrial cities and even major industrial countries (can have a
huge effect down prevailing wind e.g. UK acid rain blows over Scandinavia and causes acid rain over the
continent).
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There are primary pollutants that usually come from burning fossil fuels e.g. carbon monoxide from
incomplete combustion from car exhausts and secondary pollutants that combine with other molecules in
the atmosphere and can be acted on by sunlight to form new more poisonous substances such as the
formation of photochemical smog.
The main atmospheric pollutants are:
Sulphur Oxides (SOx): especially SO2 is released from burning fossil fuels.
Nitrogen Oxides (NOx): especially NO2 is released from high temperature combustion and can
be recognised as the factor causing the brown haze in the plume downwind of cities.
Carbon Monoxide (CO): is a poisonous gas released by incomplete combustion of hydrocarbons
or even wood with the main contributor being vehicle exhausts.
Carbon Dioxide (CO2): Odourless and colourless greenhouse gas emitted from combustion.
Volatile Organic Compounds (VOC‟s): Released from hydrocarbons and some solvents such as
paints.
Particulate Matter: is a measure of smoke and dust content of the atmosphere measures in
PM10 which is a particle 10 µm (microns) in diameter. Smaller than this will enter nasal cavity and
ultra fine particles less than 2.5 µm will enter the bronchial tubes in the lungs and cause respiratory
problems.
Unfortunately the highest levels of particulate matter occur
in developing countries where legislation on emissions is not
very strict such as India and China.
These are also the
manufacturing centres for the World.
To compound
problems in such LDC‘s they are also the countries that are
witnessing the most rapid population growth and already have
the highest populations.
This means that an increasing
proportion of the world‘s population are at risk form
particulate matter induced health problems (respiratory
problems) which will reduce life expectancy in these developing
countres and it is the poorest in the developing world who
suffer the costs of cheaply manufactured products (that
the developed World demands) the most.
(d) Winds
Urban structures have a significant effect on the microclimate in terms of wind patterns and wind
speeds.
(a) Air flowing in narrow streets: Causes air pollution as
turbulent eddies pick up dust and particulate matter reducing air
quality.
(b) Tall buildings: may create a downwash effect in the lee
(sheltered side) so that emissions from chimneys high up at
urban canopy level can‘t escape and become trapped at ground level.
Obviously the amount of pollution
will depend on the meteorological
conditions at the time which
control either air turbulence or
subsidence.
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(c) Air flowing narrow streets – Venturi Effect: is the effect of higher wind speed caused by
narrowing of streets creating ―wind tunnels‖.
Building also cause deflections to wind which causes circulations about tall buildings.
These are a collective
set of unpleasant effects that tightly packed city architecture suffers regularly from. Manchester‘s most
recent Spinningfields retail and leisure development just off Deansgate has been plagued by these effects
so much so that the purpose built pavement cafes here are very unpleasant to use indeed unless you like dust
or worse in your Mocha!
Developers must therefore try to reduce the
effects of spiralling air dynamics if they want
shoppers to linger for longer and therefore
spend money. If developers can‘t build the
structures any smaller they sometimes try to
build the building on a pedestal of one or two
stories so it is the pedestal that suffers the
unpleasant winds and not the entrances to
shopping malls that often suffer the worst
effects of wind tunnelling and increased wind
speed.
Manchester Spinning Fields Development – A Venturi Nightmare!