A method of computing the intensities of direct, diffuse, and total solar radiation received on a horizontal surface of the earth is developed by consideration of sky radiation, terrain reflection, and depletion by dry air, water vapor, dust, and clouds in the atmosphere. The calculation is extended aloft, and a general equation is derived expressing the atmospheric transmission coefficient with cloudless sky at any altitude between 300 and 8000 meters as a linear function of solar zenith distance and logarithm of elevation. The contribution of solar radiation to the heat load on man is then evaluated and expressed as an equivalent increment of temperature.