2. Microclimate
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Lecture presentation of Climatology subject presented at the MBS School of Planning and Architecture.
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2. Microclimate
- 3. Scale of microclimate • To a botanist, Microclimate can be of a single plant leaf, with its temperature and moisture conditions, its population of insects and micro organisms, on the scale of a few centimetres. • To an urban geographer, micro climate may mean the climate of a whole town. • SITE CLIMATE: Climate of the area available and is to be used for the given purpose, both in horizontal extent and in height. • The task of the designer is to take advantage of the favourable and mitigate the adverse characteristics of the site and its climatic features
- 4. FACTORS AFFECTING LOCAL CLIMATE
- 5. LOCAL FACTORS Factors affecting the local deviation are: • Topography: Slope, orientation, exposure, elevation, hills or valleys at or near the ground. • Ground surface: (Whether natural or man‐made): affect in terms of reflectance, permeability and the soil temperature as these affect the vegetation and this in turn affects the climate. (woods, shrubs, grass, paving, water, etc.). • Three dimensional objects: such as trees, tree belts, fences, walls and buildings as these may influence air movement, cast a shadow, etc.
- 7. AIR TEMPERATURE • The air temperature near the ground is dependent on heat gained/lost by the earth surface. • heat exchange varies with day/night, season, time of the year, latitude and cloud cover. • During the day, with the heating of the earth surface, the air nearest to the ground(within 2 metres) gains most temperature. At night, the direction of heat flow is reversed. • TEMPERATURE INVERSION • The phenomenon of decreasing temperature with increase in altitude. • Cold air tends to settle in the deepest depressions and behaves as a liquid. It does not flow quite readily like water but like a highly viscous liquid. • A difference of 7‐8 m in height can cause 5‐6 deg drop in temp.
- 8. Altitude/Elevation This is why hill stations are preferred locations during summer. Elevation Temperature Jaisalmer 830’ 45º C Mt. Abu 4003’ 30º C
- 9. Jaisalmer
- 11. HUMIDITY • Relative humidity is inversely proportional to temperature. • During the day, when the lowest layer of air is heated by the ground surface, RH decreases rapidly. This leads to higher rate of evaporation(If there is water body, vegetation, etc.) leading to increase in Absolute Humidity. • At night, When dew point temperature is reached, fog formation takes place and if there is no further rapid cooling nor air movement, a deep layer (40‐50m) of fog develops.
- 12. PRECIPITATION • When ground level changes by more than 300m, the windward side receives more rainfall than the regional average. • In an extreme case, on a large site located on the top of a hill and extending down to both the slopes, the leeward side receives only 25% of the rain received by the windward side. • REASON: hill forces the air mass to rise which cools it down and it is no loner able to support the moisture carried. This effect can also be caused by towns. The opposite is also true. SKY CONDITIONS • Abrupt change in topography may cause a permanent cloud cover like in the rock of Gibraltar.
- 16. SOLAR RADIATION • Three ways: • Transparency of the atmosphere: Cloud formation, air pollution, smog, smoke, dust, etc. affect the intensity of solar radiation on the horizontal plane. • Slope and orientation of the site: Intensity on the actual ground surface. At mid latitudes, a site sloping towards the pole will receive much less radiation than the site sloping towards equator. (Effect is negligible around equator) • Obstructions like hills, trees, existing buildings which may cast a shadow on the site. The effect is most pronounced wen the objects like on east or west of the site.
- 17. AIR MOVEMENT • Air flowing across any surface is subjected to frictional forces. Wind speed near the round is less than the speed than higher up and the difference depends on the smoothness/evenness of the surface. • On hilly sites, the greatest speeds are experienced at the crest of the hills. The valleys may experience I wind speed if their direction coincides wit the direction of the wind flow. • Large stretches of water can give rise to local coastal breezes. On‐sore breezes (from water to land) during the day may lower the maximum temperature by 10 deg Celsius but are likely to increase humidity. • On shore lake breezes are rarely effective beyond 400m but the sea breezes are much more effective. • Wind speed can be reduced after a long horizontal barrier by 50% at a distance of ten times and by 25% at a distance of 20 times the height.
- 18. Jaisalmer
- 22. Lakes & Oceans Warm temperatures are reduced in summer Cool temperatures are raised in winter 10ºF 10ºF 72ºF 80ºF 35ºF 30ºF
- 24. VEGETATION • Vegetation is usually regarded as a function of climate but in itself it can influence the local or site climate. • Forming an intermediate layer between the earth surface and the atmosphere, they have a moderating effect on air temperature, humidity, radiation and air movement. • increase the surface of contact to a higher layer by a factor of 4 to 12 times. Trees provide shade and affect the micro climate of the place. EVAPOTRANSPIRATION: Trees and shrubbery give off moisture that increase humidity
- 25. GroundCover–Vegetation Trees, shrubbery, and grasses provide shade that prevents moisture from evaporating Permeable surfaces reduce temperatures through evaporative cooling Reeves House, Lopez Island, WA Cutler-Anderson, 2002 http://www.greenroofs.com/projects/ pview.php?id=562
- 26. URBAN CLIMATE • Man‐made environments create micro climate of their own deviating from the regional climate. The factors causing deviation: • Changed surface qualities: (pavements and buildings) : Increased absorbance of solar radiation, reduced evaporation. • Buildings: Casting a shadow, acting as barriers to winds, channelizing winds, storing heat in their mass and releasing it at nit. • Energy seepage: through walls and ventilation of heated buildings • Atmospheric pollution: Presence of solid particles in urban atmosphere may assist in formation of fog and induce rainfall. • Temperature can be 8 deg higher in cities tan the surrounding country side, humidity can be reduced by 5‐10%, wind velocity can be reduced by half.