Weather and Climate

1. Weather and Climate Lecture 6: Microclimates

2. 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

11. 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

12. 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

14. 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

15. 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

16. 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

17. 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

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

19. 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