- Every city, town or village and even a precinct in a town may have its own climate (‘microclimate’) slightly different from the climate described for the region (macro climate’).
- The term ‘micro-climate’ can imply any local deviation from the climate of a larger area, whatever the scale may be. The botanist – ‘microclimate’ may mean climate of a single plant leaf. The urban geographer – ‘microclimate’ may mean the climate of a whole town.
- The term ‘site climate’ implies the climate of the area available and is to be used for the given purpose, both in horizontal extent and in height. It establishes the scale.
The designer’s task
- Assess the nature and extent of climatic deviations – also the likely effects of the intended building.
- Identify the area most suitable for habitation.
- Design the buildings in such a way as to take advantage of favorable and mitigate the adverse characteristics of the site and its climate.
The factors which may cause local deviations from the climate of a zone are:
- Topography i.e., slope, orientation, exposure, elevation, hills or valleys, at or near the site.
- Ground surface, whether natural or man-made, its reflectance, permeability and the soil temperature, as these affect vegetation and this in turn affects the climate (woods, shrubs, grass, paving, water etc.)
- Three-dimensional objects such as trees, or tree-belts, fences, walls and buildings, as these may influence air movement, may cast a shadow and may subdivide the area into smaller units with distinguishable climatic features.
1) Air temperature
- At any point near the ground the air temperature is dependent upon the amount of heat gained or lost at the earth’s surface and any other surfaces with which the air has recently been in contact.
- Heat exchanges at surface varies between night and day, with the season, latitude and the time of year, always influenced by the amount of cloud cover.
- During the day, as the surfaces are heated by solar radiation, the air nearest to the ground acquires the highest
temperature, as the heat build-up of the lowest layer – an upward eddy of warmer, lighter air results.
- At night, particularly on clear nights, the ground loses much heat by radiation and the direction of heat flow is reversed: from air to the ground. The lowest air becomes cooler.
- This phenomenon is referred to as temperature inversion, as the day-time situation of decreasing temperature with increase of height is taken as normal.
The relative humidity depends as much on the air temperature as on the actual amount of water vapour present in the air.
- During the day, as the lowest layer of air is being heated by the ground surface its RH is rapidly decreased. With a lower RH the rate of evaporation is increased, there is water available to be evaporated.
- At night, especially on clear night with still air, the situation is reversed. The lowest layer (of the highest AH) cools, its RH increases, the point of saturation is soon reached and with further cooling the excess moisture condenses out in the form of dew (‘dew point’)
- When the dewpoint temperature is reached the formation of fog will start, and if there is no further rapid cooling and no air movement, a deep layer (up to 40 to 50 m) of fog can develop near the ground.
- When moisture bearing winds occur frequently from the same direction, the effect of hills on rainfall patterns can be very pronounced.
- The ground changes level by more than 300 m, the windward slope receives a rainfall more than regional average, and the leeward slope less.
- If rainfall generally occurs associated with high wind velocities, resulting in ‘driving rain’, the effect will be more pronounced on the windward side than on the leeward slope, the parallelogram of forces.
4) Sky conditions
- Normally sky conditions do not vary perceptibly over short distances, unless there is an abrupt and considerable change in topography, which may lead to an almost permanent cloud formation.
5) Solar radiation
The amount of solar radiation may be influenced by local factors three ways;
- The transparency of the atmosphere: atmospheric pollution, smoke, smog or dust and local cloud can reduce the intensity of solar radiation.
- The slope and orientation of site: Higher latitudes, a site sloping towards the pole will receive much less radiation than one sloping towards the Equator.
- Nearby hills or even trees and existing buildings: may cast a shadow over the site at certain times of the day-more pronounced when such obstruction lie on the east or west of the site.
- Radiation on a vertical building surface will be affected by its orientation, but not by the slope and orientation of the site. The factors under 1 and 3 above still show an effect. The magnitude of thermal effects of such incident radiation will depend on the surface qualities of the recipient ground or objects. If it is vegetation the heating is mitigated by evaporation, but a stone or concrete or especially an asphalt surface can reach a temperature up to 4.4 degC higher than the surrounding air temperature.
6) Air movement
Wind speed can be reduced after a long horizontal barrier by 50% at a distance of ten times the height and by 25% at a distance of twenty times the height.
- Air flowing across any surface is subject to frictional effects.
- The type of ground cover affects the wind speed gradient.
- Near the ground the wind speed is always less than higher up.
- With an uneven ground cover the rate of increase in speed with height is much more than with an unbroken smooth surface, such as water.
- On the hilly site the greatest wind speeds will be experienced at the crest of hills.
- Small valleys and depressions will normally experience low velocities, except where the direction of the valley coincides with wind direction.
- The effect of long, tall slabs or rows of buildings may be similar to the valley.
- The day-time heating of air over barren ground often gives rise to local thermal winds, especially in hot dry regions- whirlwinds or local breezes, hot and carrying fine dust.
- Largest stretches of water can give rise to local coastal breezes.
- On shore breezes (from water to land) during the day may lower the maximum temperature by as much as 10 degC, but are likely to increase the humidity.
- On lake shores these breezes are rarely effective beyond 400m inland, but on the sea coast the effect may reach much further inland if topography is favourable.
7) Special characteristics
- Local topography will influence thunder storms top hills, tall building- precautionary measures
- Local factors will influence dust and sand storms ground surface providing sand and dust particle and topography funneling or diverting wind or causing local eddies.
- Trees and vegetation form an intermediate layer between the earth’s surface and the atmosphere.
- Observation of existing vegetation can be guiding factor for landscaping and siting.
- With the knowledge of the soil, water, sun and wind requirements of common plants, the designer should identify the major areas of differences in site climate.
9) Urban Climate
- Man-made environments can create microclimates of their own, deviating from the macroclimate of the region to a degree depending on the extent of man’s intervention.
- The factors causing deviations of the urban climate from the regional macroclimate are the following:
- Changed surface qualities (pavements and buildings)- increased absorbance of solar radiation; reduced evaporation.
- Buildings- casting a shadow and acting as barriers to winds, but also channeling winds possibly with localized increase in velocity or by storing absorbed heat in their mass and slowly releasing it at night.
- Energy seepage- through walls and ventilation of heated buildings; the output of refrigeration plants and air conditioning (removing heat from the controlled space to the outside air); heat output of internal combustion engines and electrical appliances; heat loss from industry, especially furnaces and large factories.
- Atmospheric pollution- waste products of boilers and domestic and industrial chimneys; exhaust form motor-cars; fumes and vapours, which both tend to reduce direct solar radiation but increase the diffuse radiation and provide a barrier to outgoing radiation. The presence of solid particles in urban atmosphere may assist in the formation of fog and induce rainfall under favourable conditions. The extent of deviations may be quite substantial.
- Air temperature in a city can be 8 degC or 8 K higher than in the surrounding countryside
- Relative humidity is reduced by 5 to 10% due to the quick run-off of rain-water from paved areas, to the absence of vegetation and to higher temperature.
- Wind velocity can be reduced to less than half of that in the adjoining open country, but funneling effect along a closely built-up street or through gaps between tall slab blocks can more than double the velocity.
10) Site climatic data
- In most cases the regional data may be used with only some qualitative remarks regarding local deviations.