Microclimates - GEOCITIES.ws
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Transcript Microclimates - GEOCITIES.ws
Weather and Climate
Lecture 6: Microclimates
Introduction
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
Microclimates are of importance to:
Planners
Architects
Farmers
Geography Students
Bare Soil
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
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
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
As a result of shade provided by the canopy:
– More humid than open land
– Vapour pressure higher
Rainforest Microclimates
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
At night:
– Vegetation traps and retains outgoing radiation
– Aided by the high humidity (high number of water
vapour molecules
Deciduous Forest
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
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
Large cities and conurbations:
– Climatic conditions differ from countryside
– Atmospheric composition
– Temperature
– Sunlight
– Wind
– Humidity
– Cloud Cover
– Precipitation
Urban Microclimates
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
These aid in increases in cloud cover and precipitation,
smog, higher temperatures and reduced sunlight
Urban Microclimates
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
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
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
Sunlight
– Less insolation and more cloud cover than rural areas
– Dust and other particles absorb and reflect insolation
– High rise buildings block out sunlight
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
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
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
Cloud Cover
– Thicker and more frequent cloud cover
– More condensation nuclei
– warm, unstable air
– clouds therefore form easily
Urban Microclimates
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
Mountains and valleys can be said to create their
own climates:
– Foehn/Chinnok
– Anabatic winds
– Katabatic Winds
– Associated Fog
– Rainfall