Site Climate (Koenigsberger, et al, 1975)

Knowledge of the climatic zone to which a site belongs is indeed important as explained earlier. But every city, town or village and even a precinct in a town may have a climate of its own, which is called the site climate. Factors which may cause local deviations from the zonal climate are : 

1. topography - Slope, orientation, exposure, elevation, hills and valleys at or near the site, nearness to seaflake etc., 
2. ground surface (whether material or man-made) - IQ reflectivity, permeability and soil temperature which affect vegetation. 
3. objects - Such as trees, tree belts, walls and other buildings; they influence air movement or may cast shadows. 

A logical method will be lo follow the sequence of global climatic conditions, examined in the previous section, and see how each of the above factors will affect them. Following guidelines are available for guidance/decision making : 

• The air within 2 meters of the ground is significantly affected by solar radiation and reflection from ground. If the lower layers become hot enough, upward movement of an eddy of warmer, and hence, lighter air, will take place. At night the heat of the ground is lost faster than that of the air above. The direction of heat flow will be reversed from the air to the ground. The reflection factor of the underlying surface is important in this regard. Reflectivity of concrete is 30-45 %; of lime-stone is 50-65%; of rough gray asphalt is 25%; and of newly cut lemon grass is 20%. Thus, providing lemon grass around large buildings is not a luxury, but a necessity. The most difficult conditions are to be observed in urban areas where the surface of buildings and pavements provide additional sources of reflected heat. The mean ambient temperature of cities, is higher by 8C or so than in rural areas nearby. 

From the stand point of site climate, asphalt is the most unfavourable material. It contains noxious substances harmful to man; and it is air-tight and gas-tight. Thus, it has a negative effect on soil; a harmful influence on vegetation, and in general raises the ambient temperature. Also, accounting for higher temperatures in cities is the blanket of smoke over the cities which retards the escape of reflected radiation. It is advisable-to minimize the use of asphalt and smoke-emanating vehicles and industries in cities (Rimsha, 1976). 

• The phenomenon of heat flow from the air to ground which often occurs in the nights is referred to as temperature inversion. Cold air tends to settle in the deepest depressions and down the hill and along the floor of sloping valleys. Topography can thus, strongly influence air temperature, a difference of 5°C to 0°C can occur in a height of 7 to 8 m. 

It is better to avoid locating buildings in valleys and it is advisable to build large buildings on level grounds.

• Relative Humidity depends as much on the air temperature as on the actual amount of water vapour in the air. During the day, as the lowest layer of air is being heated by the ground surface, its relative humidity is rapidly decreased. The rate of evaporation is increased, if there is water available to be evaporated. An open surface of water or rich vegetation would provide an abundant supply of water. At night the situation is reversed as the lowest layer cools its relative humidity increases, the point of saturation is soon reached, and with further cooling the excess moisture condenses out in the form of dew. When the dew point point temperature is reached, the formation of fog will start, and if there is no air movement, a deep layer of (40 to 50 cm) fog can develop near the ground. This affects the visibility prevailing on uansportation network, particularly at the airports. 
• Where the ground changes level by more than 300 m, the wind-ward slope can be expected to receive more than the regional average of rain fall and the lee-ward slope correspondingly less. This is due to the phenomenon that the air mass is forced to rise up the hill by the prevailing  wind, and as it rises it cools and can no longer support the moisture carried by it and hence, rainfall is induced. 
• Regarding the behaviour of wind on a hilly site, the greatest wind speeds will be experienced on the crest of the hills. Small valleys and depressions will normally experience low velocities, except in valleys which are oriented in the direction of the wind. 

Precipitation on Hills

In regions where wind can provide a welcome relief from sultry weather, the crests and wind-ward slopes are preferable as building sites, to the lee-ward sides of hills. 

• Large stretches of water can give rise to the local coastal breezes. On-shore breezes (from water to land ) during the day may lower the temperature by as much as 10' C, but are likely to increase the humidity. 

It is obvous, buildings should be broadly spaced for adequate breeze penetration. As a rough guide spacing between long roofs of buildings should not be less than 5 times their height.