Transcript Microclimates - GEOCITIES.ws

Weather and Climate
Lecture 6: Microclimates
Introduction
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Microclimatology:
– Study of climate over a small area
– Urban
– Forest
– Mountains and valleys
 Changes can be seen in:
- Temperatures
- Wind speeds, humidity and precipitation
Introduction
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Microclimates are of importance to:
Planners
Architects
Farmers
Geography Students
Bare Soil
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Temperatures can vary due to the nature of soils:
– Humus rich (dark)
– Absorb more heat than light coloured soils, hence
higher temperatures
– Moist soils vs dry soils
– Specific heat capacity differences
– Moist soils will warm up more slowly, lower
temperatures than dry soils
– Ridge vs furrow in ploughed soils
Bare soil
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Air as a poor conductor of heat
– Soil with a high proportion of air (sand)
– Hot at surface, cool rapidly with depth
– Excess moisture in the soil will take it a longer time to
heat up than fairly moist soil
Rainforest Microclimates
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Trees and Forests have a marked effect on climate
 Due to trees which act as wind-breaks:
– Air movement in a forest ecosystem is much less
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As a result of shade provided by the canopy:
– More humid than open land
– Vapour pressure higher
Rainforest Microclimates
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Incoming radiation
– Absorbed by canopy layer
– In a rainforest, up to 5 layers
– Trees with large, waxy leaves also have higher albedo,
reflect incoming insolation
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At night:
– Vegetation traps and retains outgoing radiation
– Aided by the high humidity (high number of water
vapour molecules
Deciduous Forest
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Deciduous Forests:
– Will tend to have their own microclimate in Summer
when vegetation is full
– During winter, leaves are lost
– May take the climate of the surrounding area as canopy
is absent to affect temperatures, humidity and winds
Deciduous Forest
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Areas with large leaved trees as opposed to areas
with small leaved trees:
 Sycamore vs birch or oak
 Sycamore absorbs more energy, hence lower
temperatures in areas associated under these trees
 Oak trees have higher density of leaves than birch,
hence more light reaches the ground under birch,
warmer temperatures
Urban Microclimates
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Large cities and conurbations:
– Climatic conditions differ from countryside
– Atmospheric composition
– Temperature
– Sunlight
– Wind
– Humidity
– Cloud Cover
– Precipitation
Urban Microclimates
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Atmospheric Composition
– 3 to 7 times more dust particles
– Absorb radiation to give off heat
– 200 times more sulphur dioxide
– 10 times more nitrogen oxide
– 10 times more hydrocarbons
– 2 times more carbon dioxide
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These aid in increases in cloud cover and precipitation,
smog, higher temperatures and reduced sunlight
Urban Microclimates
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Temperature
– Tower blocks: low albedo and non reflective, absorb
heat during the day time; higher temperatures than in a
rural area
– during the night; due to high thermal capacity of
concrete structures and buildings, heat is slowly
released; warmer temperatures
– Further heat is obtained from car fumes, factories,
power stations, central heating units, people
Urban Microclimates
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Urban Heat Island Effect
– Warmer temperatures in the more built-up city centre
– Cooler temperatures towards the suburbs and
countryside
Temperature
Rural
City
Rural
Urban Microclimates
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In urban areas:
– Day temperatures on average 0.6 deg C warmer
– night: 3 to 4 deg C warmer
– Dust and cloud act as a blanket
– Mean winter temp 1 to 2 deg C warmer
– Mean summer temp 5 deg C warmer
Urban Microclimates
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Sunlight
– Less insolation and more cloud cover than rural areas
– Dust and other particles absorb and reflect insolation
– High rise buildings block out sunlight
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Wind
– Wind velocity is reduced; buildings act as windbreaks
– Urban mean annual velocities may be up to 30 percent
lower in rural areas
– Calm periods 10 to 20 percent more than rural area
Urban Microclimates
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On the other hand:
– Skyscrapers which are very closely spaced and without
‘void decks’
– Form canyons
– Wind eddies and currents are channeled along these
narrow canyons
– Strong enough to cause tall buildings to sway and
pedestrians to be blown over
Urban Microclimates
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Relative Humidity
– Lower in urban areas than in rural areas
– Warmer air in urban areas (increasing saturation
specific humidity)
– Lack of vegetation and surface storage to contribute to
evapotranspiration
– Therefore relative humidity lower
Urban Microclimates
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Cloud Cover
– Thicker and more frequent cloud cover
– More condensation nuclei
– warm, unstable air
– clouds therefore form easily
Urban Microclimates
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Precipitation
– Mean anuual precipitation total, number of days with
less than 5mm of rainfall are 10 percent higher in major
urban areas
– turns snow into sleet, limits the number of days with
snow on the ground
– high frequency and intensity of smog due to
concentration of condensation nuclei and pollutants
Mountain and Valley Microclimates
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Mountains and valleys can be said to create their
own climates:
– Foehn/Chinnok
– Anabatic winds
– Katabatic Winds
– Associated Fog
– Rainfall