2. Literature review
2.6 Climatic Imperatives
This section will discuss about the scientific notion of ‘thermal comfort’ in outdoor spaces. The various determinants of thermal comfort will be presented and the relevant determinants will be selected for studying the environmental design thinking of Vedic urban designers.
2.6.1 Thermal comfort
“Thermal comfort is that condition of mind, which expresses satisfaction with the thermal environment” (ASHRAE, 1958). Although, comfort for both indoor and outdoor spaces is expressed in terms of a number of quantifiable environmental variables like air temperature, radiant temperature, relative humidity, air flow and their effect is synergistic, non-thermal factors are also works as determinants (Ahmed, 1996).
Thermal comfort judgement can be significantly influenced by olfactory, acoustic and visual environment. Hence, comfort is not only a sensory phenomenon but largely one of perception (Fisher, 1978; Bansal, 1994; cited from: Ahmed, 1996).
It is possible that the requirements for the determinants of thermal comfort may vary from person to person within the same environment. The indeterminacy of categorizing comfort in a changing environmental condition that is of urban climate is quite apparent with regard to a group of people. The designer’s task is to provide a comfortable environment to a majority of people, with a minority of people still remaining dissatisfied (Ahmed, 1996).
62 2.6.2 Determinants of thermal comfort in outdoor urban environment The ranges of factors that influence thermal comfort in outdoor urban environment can be broadly categorized into the following three:
a. Physiological determinants b. Behavioral determinants c. Environmental determinants 2.6.2.1 Physiological determinants
‘Metabolic rate’ and ‘Acclimatization’ are the major physiological determinants. The body generates energy through the process of metabolism, a little part of which is utilized by the body, while the rest is discharged to the environment. Metabolic rate means the rate of heat liberation. It is directly related to the age, sex, nutrition, level of activity and relevant factors. The relative thermal tolerance of a person is adjusted through the process of acclimatization. For example, the adaptive mechanism of human body increases the sweating and lowers the metabolic rate when exposed to a hot environment and it takes about 1 to 6 weeks for a person to be acclimatized in such condition (Ahmed, 1996).
2.6.2.2 Behavioral determinants
Psychological adaptation enables a person to be alert when exposed to an extreme environment which results in behavioral adjustment ranging from simple action of changing the cloth to complex action of migration. The thermal resistance of clothing (measured in Clo; 1 Clo=.0155 m2 ℃/ W) and the level of albedo of a person (depends on pigmentation of the skin and color of clothing) are the measurable determinants of
‘clothing’. ‘Migration’ can be observed in both outdoor and indoor environment as a means of altering to a favorable environmental setting by changing location. When people are subjected to a variety of environmental conditions within a same setting - which vary from warm to cool or from dark to light, they often seek out preferred condition. This behavior is known as ‘Tropism’ (Ahmed, 1996).
63 In tropical urban environment and in many Mediterranean cities, it has been observed that people tend to follow the shaded paths while walking, which are generated by the building geometry and foliage of the trees planted along them. Very often people choose to walk on a crowded but shaded sidewalks in outdoors, while the other side of the road remain unoccupied, irrespective of the ambient temperature. Thus seeking shade is a spontaneous behavior of ‘Tropism’ which is stimulated be solar radiation (Ahmed, 1996).
2.6.2.3 Environmental determinants
The principal environmental factors are dry bulb temperature, radiant temperature, relative humidity and air flow. Their effect on comfort is not individual but synergistic.
Air temperature: Heat exchange by free or forced convection is directly related to the dry bulb temperature of the environment. Heat loss is induced when the air temperature is less than the skin temperature (Ahmed, 1996). Expressed in ℃ (degree Celsius).
Radiation: All thermally excited surfaces in an environment emit radiation in different directions. Radiant temperature indirectly indicates the intensity of radiation field. In outdoor environment the sun is the source of short wave radiation and all other terrestrial bodies or surfaces emit long wave radiation. Radiation, whether long wave or short, can directly excite the thermal receptors of human body (Ahmed, 1996).
Solar Irradiance is a variable which can indicate the level of solar radiation. Solar Irradiance is the sun’s radiant energy incident on a surface of unit area, expressed in W/m2 or kW/m2.
Incident solar radiation, also termed insolation, refers to the wide spectrum radiant energy from the Sun which strikes an object or surface. This includes both a direct component from the Sun itself (sunshine), a diffuse component from the visible sky (skylight) and a reflected component from other vertical surfaces and the ground.
Insolation is expressed in Wh/(m2 day).
Relative Humidity: The moisture content in the air is expressed in terms of relative humidity. It is the ratio of the actual amount of moisture present to the amount of moisture air could hold at the given temperature expressed as percentage. Heat loss by
64 means of skin diffusion, respiration and sweat evaporation is directly affected by the relative humidity. It varied diurnally and may also vary between different microclimate within a given period of time. In urban environment, humidity can be increased locally by passive or semi-passive means (Ahmed, 1996).
Air Flow: Air velocity also influences the convective and evaporative cooling. But if the air temperature is greater than skin temperature, it induces heat gain of the body. In warm humid climate, where air temperature is below skin temperature, increasing the air velocity can cause considerable heat loss. The Beaufort scale provides the mechanical effects at various flow rate (Ahmed, 1996). Air velocity is expressed in m/s (metre per second).
Temporality: Time is an important factor in determining the comfort boundaries because the extent of response may vary with the length or duration of a particular stimulus which can directly influence the tolerance level. Also, the perception of comfort is not a fixed experience. People tend to find comfort in various environmental conditions (Ahmed, 1996).
Among the range of determinants of thermal comfort discussed above, the variables which are consistent with the rationale of Vedic architects are solar insolation, shadow condition and wind flow in case of open space.