Download climate on a local scale

CLIMATE ON A LOCAL
SCALE-URBAN CLIMATES
Cities create their own climate.
Geographers call it the climate dome,
where weather is different from the
surrounding rural areas.
MODIFICATIONS TO URBAN
CLIMATES
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TEMPERATURES
WIND
SUNLIGHT
PRECIPITATION
CLOUD
ATMOSPHERIC COMPOSITION
RELATIVE HUMIDITY
In comparison to rural areas cities
have:
• Higher max and min temperatures-greater
temp range,
• Lower wind speeds, with exception of
tunnelling,
• Greater uplift of air,
• Greater cloud cover and precipitation,
• Poor visibility, with haze and smog more
likely.
COMMENT UPON THE
TEMPERATURE DISTRIBUTION
SHOWN IN THIS MAP
Map of temp dist over Montreal in
Canada,7 March, 7am, with winds
of 0.5ms
• Lower temps on side of prevailing wind
• The plateau-like temp zone running from
the city edge towards the city centre,
which has the higher temp
• The correlation between the highest
density of building and the highest temps
• The fall in temp over the park, which has
an open vegetated space.
The effects of urban areas on local
climate.
The URBAN CLIMATE DOME
THE DOME HAS THE
FOLLOWING FEATURES:
• Extends upwards 250-300m and its influence
may continue for tens of kms downwind.
• There are 2 levels within the urban dome-below
roof level there is an urban canopy where
processes act in the space between buildings
(canyons) and above this is the urban boundary
layer. This extends downwind and at a height as
a plume into the surrounding rural areas.
WIND
• Wind speed is reduced by 12-14mph by the
presence of buildings, which create friction drag
and act as wind breaks.Create
turbulence,causing changes in wind speed and
direction
• Wind-tunnelling can occur due to high-rise
buildings channelling air into canyons between
them. Called the VENTURI-EFFECT and can be
avoided by building on stilts.
• As wind hits buildings it can cause eddies and
turbulence at street level.
• On calm ,clear nights there is a surface inflow
from cooler areas outside the city to the warmer
areas in the city centre.
AIRFLOW MODIFIED BY A
SINGLE BUILDING
• Air is displaced upwards and around the
sides of the building and is also pushed
downwards in the lee of the structure.
AIRFLOW MODIFIED BY MORE
THAN 1 BUILDING
• If buildings are close each will interfere
with the airflow around the next structure.
• Important that chimneys are built high
enough to release pollutants into
undisturbed air .flowl
The VENTURI EFFECT
• In the lee of the building there is a zone of
lower pressure, causing vortices behind it.
• If 2 buildings allow flow between them
then the venturi effect is set up, in which
pressure within the gap causes wind to
pick up speed.
• However, a reasonable flow of air is
needed to remove air pollution.
Vortex flows
• As air flows around buildings suction is set
up.On windward side overpressure causes
a descending flow.Forms a vortex when
reaches ground and sweeps around the
windward corners.
THE URBAN HEAT ISLAND
EFFECT
• Over the year London is 1.3c warmer than
the surrounding rural area.
• Temp differences can vary from 0.6cin the
day to 3-4c at night- WHY?
• Most noticeable when wind speeds are
low as they tend to reduce temp diffs by
dispersing the build-up of warm air or
cloud cover.
DEFINITION:
A REGION OF WARMER AIR
TEMP RELATIVE TO THE
SURROUNDING RURAL AREA IN
A METROPOLITAN AREA.
Temperatures
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Raised because heat from energy sources escape into atmosphere:
Industries, vehicles and people all generate heat
Air conditioning units release hot air into the atmosphere
Building materials such as concrete, bricks and tarmac act like bare
rock surfaces, absorbing heat in the day and slowly releasing it at
night. Some buildings with lots of glass have a high reflective
capacity, and multi storey buildings tend to concentrate the heating
effect in the surrounding street by reflecting energy downwards.
• Air pollution increases cloud cover and creates a pollution dome,
which allows in short-wave radiation but absorbs a large amount of
outgoing radiation as well as reflecting it back to the surface.
• Water falling to the surface is disposed of quickly and so this
changes the urban moisture and heat budget – reduced
evapotranspiration means more energy available to heat the
atmosphere.
Urban heat islands vary:
• Over time –WHEN CONDITIONS ARE HIGHPRESSURE WHY? ALSO WHEN THERE IS A
TEMP INVERSION.
•
- IN WINTER ?
• Over space – TEMPS RISE ABRUPTLY AT THE
RURAL-URBAN BOUNDARY= A CLIFF! FROM
THIS POINT TEMPS RISE STEADILY TO A
PEAK IN THE CITY CENTRE (2-4C PER KM)
THIS IS CALLED THE PLATEAU. WITHIN THIS
PLATEAU THERE ARE VARIATIONS.
PLATEAU VARIATIONS
• Distribution of power stations, water areas and open
spaces,
• Due to ALBEDO. (amount of insolation that is reflected
by the Earths surface and the atmosphere). Dark
surfaces absorb insolation and then re-radiate it as longwave energy that heats up the urban area.Reflective
surfaces absorb little and reflect back into atmosphere
keeping cool, of reflect it into small areas, which heat up.
• Surfaces in cities are less reflective than in rural areastarmac! Snow reflects and stays much longer in the
countryside.
HOW CAN THE URBAN HEAT
ISLAND BE REDUCED?
• Create green spaces within the city.
• Use lighter coloured materials for roads
and roofing to reflect the heat.
PRECIPITATION
• Higher urban temps cause low pressure and
convection rainfall, which is heavier and more
frequent.
• Also causes thunder and lightning.
• WHY?
• URBAN HEAT ISLAND GENERATES
CONVECTION,
• HIGH RISE BUILDINGS GENERATE VERTICAL
MOTION,
• WATER VAPOUR FROM INDUSTRIAL
SOURCES AN DPOWER STATIONS,
• POLLUTANTS ACT AS HYGROSCOPIC
NUCLEI AND ASSIST CONDENSATION.
THUNDERSTORMS
• Develop in hot humid air. In urban areas the
chance of them is increased.WHY?
• Because of the convectional uplift created under
conditions of extreme instability.
• During uplift cooling, condensation and latent
heat production occurs,further fueling the
convectional uplift.
• Raindrops are split and positive electrical charge
builds up, when the charge is high enough to
overcome resistance in the cloud a discharge
occurs to areas of negative charge in the cloud
or to earth.This is lightning.Thunder is heard as
a result of rapid expansion of the air.
FOG
• Occurrence of fog increased with
industrialisation.1700s 20 days of fog, by 1950s
risen to over 50 days.
• Particulate pollution creates condensation
nuclei,encouraging fog formation at night,
usually under high pressure conditions.
• The clean air act of the 1950s reduced smoke
production and particulate emissions, reducing
the number of foggy days.
• As LEDCs industrialise more fog is
experienced.Eg New Delhi and Beiijing suffer
winter fogs, crippling transport networks and
affecting human health.
CLOUD COVER
• Clouds are thicker and 10% more frequent
• Due to more rising air due to higher temps
and a larger number of condensation
nuclei.
• All the same reasons that precip is higher.
• Humidity is lower in cities because there is
a lack of water surfaces or veg to emit
moisture through transpiration.
SUNLIGHT
• Despite higher temps a city receives less
sunshine.
• Pollutants may reflect up to 50% of the
insolation.
• High rise buildings also block out sunlight.