International journal of biometeorology
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The rates of convection and evaporation at the interface between the human body and the surrounding air are expressed by the parameters convective heat transfer coefficient hc, in W m-2 degrees C-1 and evaporative heat transfer coefficient h(e), W m-2 hPa-1. These parameters are determined by heat transfer equations, which also depend on the velocity of the airstream around the body, that is still air (free convection) and moving air (forced convection). The altitude dependence of the parameters is represented as an exponential function of the atmospheric pressure p: hc approximately pn and h(e) approximately p1-n, where n is the exponent in the heat transfer equation. ⋯ The rate of convective and evaporative heat losses from the human body surface at various altitudes in otherwise identical conditions depends on the following factors: (1) during convection--the thickness of the boundary layer, plus the decrease in air density, (2) during evaporation (mass transfer)--the thickness of the boundary layer, plus the increase with altitude in the diffusion coefficient of water vapour in the air. The warming rate of the air volume due to convection and evaporation is also considered. Expressions for the calculation of altitude dependences hc (p) and h(e) (p) are suggested.