MACRO AND MICRO CLIMATE

FACTORS AFFECTING LOCAL CLIMATE

LOCAL FACTORS

Factors affecting the local deviation are:

•Topography: Slope, orientation, exposure, elevation, hills or valleys at or near theground.

•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 asthese may influence air movement, cast a shadow, etc.

AIR TEMPERATURE

•The air temperature near the ground is dependent on heat gained/lostby the earth surface.

•heat exchange varies with day/night, season, time of the year, latitudeand cloud cover.

•During the day, with the heating of the earth surface, the air nearest tothe ground(within 2 meters) gains most temperature. At night, thedirection of heat flow is reversed.

•A difference of 7‐8 m in height can cause 5‐6 deg drop in temp.

HUMIDITY

•Relative humidity is inversely proportional to temperature. •During the day, when the lowest layer of air is heated by the groundsurface, 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.

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 noloner able to support the moisture carried. This effect can also be caus

ed by towns. The opposite is also true. SKY CONDITIONS •Abrupt change in topography may cause a permanent cloud cover likein the rock of Gibraltar.

SOLAR RADIATION

•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. Atmid latitudes, a site sloping towards the pole will receive much less radiationthan 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.

• 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 400mbut 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.