following text is taken from University of Strathclyde's Energy Systems
Research Unit ESRU's web site (file type '16-Climate.pdf')
Macro and Micro Climate
The climate of the earth consists of a series of interlinked physical systems powered by the sun.
In the built environment we are generally concerned with local climatic systems in particular:
· Macro-climate the climate of a larger area such as a region or a country
· Micro-climate the variations in localised climate around a building
The macro and micro climate has a very important effect on both the energy performance and environmental performance of buildings, both in the heating season and in summer.
The site and design of a building can have a profound effect upon the interaction between a building and its environment.
The building site affects exposure to the prevailing wind, the solar radiation the building receives, pollution levels, temperatures and rain penetration.
Site and macro climate
The orientation of the building affects solar gains and exposure to the prevailing wind (ventilation).
The location of neighbouring trees and buildings affects the solar gains (shading) and wind patterns.
Neighbouring trees and buildings also protect the building from driving rain.
The macro climate around a building cannot be affected by any design changes, however the building design can be developed with a knowledge of the macro climate in which the building is located. General climatic data give an idea of the local climatic severity:
· Seasonal accumulated temperature difference (degree day) are a measure of the outside air temperature, though do not acount for available solar
· Typical wind speeds and direction
· Annual totals of Global Horizontal Solar Radiation
· The driving rain index (DRI) relates to the amount of moisture contained in exposed surfaces and will affect thermal conductivity of external surfaces.
This Metereological data gives a general impression of the climate at the site of a building and the building design can be planned accordingly. However the building itself and surrounding geography will affect the local climate.
The site of a building may have a many micro climates caused by the presence of hills valleys, slopes, streams and other buildings.
Micro Climate – Effect of Local Terrain
Surrounding slopes have important effects on air movement, especially at the bottom of a hollow. In hollows air warmed by the rises upwards due to buoyancy effects (anabatic flow), to be replaced by cooler air drifting down the slope (katabatic flow).
The result is that valey floors are significantly colder than locations part way up the slope. Katabatic flows often result in frosts persisting for longer in low lying locations. The most favourable location in a valley is known as the thermal belt, lying just above the level to which pools of cold air build up, but below the height at which exposure to wind increases.
The crests of hills and ridges have unfavourable wind velocity profiles, the wind flow is compressed (as happens with an aerofoil) leading to high wind velocities.
Micro-Climate – Effects of Buildings
Buildings themselves create further micro-climates by shading the ground, changing wind flow patterns.
One example of how buildings affect the local climate is the heat island effect in large cities where the average temperature is higher than the surrounding area:
Solar energy absorbed and re-emitted from building surfaces, pavements roads etc. creates a warming effect on the surrounding air. Also the large quantities of buildings break up the wind flow, reducing wind speeds and causing the warm air to remain stagnant in the city. This also causes increased pollution as well as temperatures.
The presence of local high rise buildings can degrade the local climate as wind speed at ground level can be significantly increased, while extensive shadows block access to sunlight for long periods, increasing space heating costs in surrounding buildings.
Improving Micro Climate through Design
The aims of enhancing Micro-Climate around Buildings:
· Reduce costs of winter heating
· Reduce summer overheating and the need for cooling
· Maximise outdoor comfort in summer and winter
· Improve durability of building material (reduced rain penetration)
· Provide a better visual environment in spaces around buildings
· Encourage growth of plants
· Discourage growth of mosses and algae
· Facilitate open air drying of clothes
Means of enhancing the micro climate around a building include:
Allow maximum daylight into space and buildings
Allow maximum solar radiation into space and buildings
Shade space and windows from prolonged exposure to summer sun
Protect space and windows from glare
Protect space and buildings from prevailing winds and cold (e.g. North/East) winds.
Prevent buildings and terrain features from generating turbulence
Protect spaces and buildings from driving rain and snow
Protect space and buildings from katabatic flows, while retaining enough air
movement to disperse pollutants
Provide thermal mass to moderate extreme temperatures
Use vegetation for sun shading and wind protection (transpiration helps moderate high temperatures).
Provide surfaces that drain readily.
water for cooling be evaporation (pools and fountains)
Factors Affecting Micro Climate
Two main possibilities for influencing Micro Climate are Solar Access and Wind
Solar access to a site is often a case of minimising solar overheating in summer while maximising solar access during the winter.
Buildings with a heating requirement should be orientated north south with maximum glazing on the south face.
Deciduous trees offer an excellent means of site shading, with shading being reduced in winter when the trees lose their leaves.
The colour of surrounding surfaces will have a pronounced effect on the solar
radiation available to the building. Light coloured paving will increase the radiation reflected from the ground into the building. Paving stones will also provide external thermal mass, moderating temperature swings immediately adjacent to the building.
Grass planted outside a building will reduce the ground reflected solar.
Use of courtyards and water can also moderate the effects of high temperatures on summer.
The form of the building can have a great effect on the impact of the wind:
· Avoidance of the building flank facing the wind
· Avoidance of funnel-like gaps between buildings
· Avoidance of flat roofed buildings and cubical forms
· Avoid piercing buildings at ground level
· Avoid abrupt changes in building heights
· Orientate long axis of the building parallel to the direction of the wind
· Use podium to limit down draught at ground level
· Use pitched rather than flat roofs and stepped forms for higher buildings
· Groups of buildings can be arranged inirregular patterns to avoid wind tunneling.
Coniferous trees and fencing and other landscape features such as mounds of earth and hedges can also reduce the impact of wind and driving rain on the building
Enhanced Micro Climate and Energy Saving
Increased external air temperature leading to reduced space heating reduction:
increase solar access to site, wind protection, external thermal mass, quick drying surfaces.
Reduced Air Change Rate, internal air movement and decreased external surface connective heat transfer: reduced pressure driven ventilation by wind protection.
Reduced moisture effects on fabric: less wetting of fabric and energy loss due to evaporation from wet surfaces by protecting from driving rain and providing adequate surface drainage.
More information on macro-micro climate is provided by the University of Strathclyde's Energy Systems Research Unit web site Environmental Engineering Science 1
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