Bio-climatic Design
The design of a habitat which reflects the consideration and analysis of environmental variables like dry bulb temperature, relative humidity, air movement and radiation as they effect human perception and response, can be called Bio-climatic Design.

Bio-climatic Analysis can be carried out with the help of either the Bio-climatic chart or the psychrometric chart. This tutorial will work with the psychrometric chart for bio-climatic analysis. The psychrometric chart was developed for engineering calculations for the design of HVAC systems. The psychrometric chart helps to determine the comfort zone, in terms of the four environmental variables for a person engaged in a specified activity and wearing a specified amount of clothing. We are going to adapt its use in an architectural context.
Psychrometry (from the Greek: psukhros = cold) is the study of moist air (which is mostly oxygen, nitrogen and water vapor) and of the changes in its condition. We are going to use the psychrometric chart to determine comfort conditions.

Human response to heat
Heat, or rather the thermal state of the environment, is perceived by the skin surface of a living body. The temperature sensors on the skin are most sensitive around 34°C (93.2 °F), where quite small changes or differences in the temperature can be perceived. The sensitivity decreases towards the limits of tolerable temperatures.

The heat produced by a resting adult is about 85.83 Kcal/h (340 Btu/h). Because most of this heat is transferred to the environment through the skin it is often convenient to characterize metabolic activity in terms of heat production per unit area of skin. For the resting adult, this is about 50 kcal/(h.m²) (18.4 Btu/h.ft²) (average person has a skin surface area of about 1.82 m² (19.6 ft²) and is called 1 met). Higher metabolic rates are often described in terms of the resting rate.

Heat from the environment can reach the body surface by any or all of the three ways of heat transfer (conduction, convection and radiation) and the skin perceives the total, combined thermal effect. Thermal balance exists when the heat produced by the body is fully dissipated to the environment. The heat produced in deep-body tissues is continuously transported to the skin surface (and to the lungs), whence it is emitted to the environment by convection, radiation and evaporative heat transfer (some also by conduction).

Effects of changes in core-body temperature

42°C (107°F)		Likely death
41°C (105.8°F)		Sweating stops. Coma sets in. May damage the brain.
40°C (104°F)		Heat stroke
		Hyperthermia (inevitable body heating) occurs.
		Evaporative cooling - sweat. (for short periods at the rate of upto 4 litres/hr)
		Vasodilation (skin surface dilates). Increase in heat transportation to the surface. Rise in skin temperature and dissipation of radiant and convective heat.
37°C (98.6°F)		Normal, healthy condition
		Vasoconstriction. Lower skin-temperature, goose-pimples (erection of hairs)
		Shivering (increase in muscular metabolism)
35°C (95°F)		Hypothermia (inevitable lowering of deep-body temperature)
25°C (77°F)		Likely death

Thermal Comfort
Thus, the limits of existence can be defined in terms of deep-body temperature as lying between 35 and 40°C (95-104°F), the normal being about 37°C (98.6°F). The skin temperature (normally at 33.7°C) must always be lower than the deep-body temperature. The temperature of the environment, in turn must be below the skin temperature, if heat is to dissipate. The range of environmental temperatures which will allow sufficient, but not excessive heat dissipation, and will therefore be judged as 'comfortable', is referred to as the comfort zone.

In the next SECTION, we will discuss factors that influence thermal comfort.