Weather and Climate

1
Weather and Climate

• Lecture 6 Microclimates

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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

3
Introduction

• Microclimates are of importance to
• Planners
• Architects
• Farmers
• Geography Students

4
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

5
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

6
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

7
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

8
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

9
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

10
Urban Microclimates

• Large cities and conurbations
• Climatic conditions differ from countryside
• Atmospheric composition
• Temperature
• Sunlight
• Wind
• Humidity
• Cloud Cover
• Precipitation

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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

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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

13
Urban Microclimates

• Urban Heat Island Effect
• Warmer temperatures in the more built-up city
  centre
• Cooler temperatures towards the suburbs and
  countryside

Temperature
Rural
Rural
City
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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

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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

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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

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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

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Urban Microclimates

• Cloud Cover
• Thicker and more frequent cloud cover
• More condensation nuclei
• warm, unstable air
• clouds therefore form easily

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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

20
Mountain and Valley Microclimates

• Mountains and valleys can be said to create their
  own climates
• Foehn/Chinnok
• Anabatic winds
• Katabatic Winds
• Associated Fog
• Rainfall