Data are provided on the heating effect of the sun at any position within the Earth's atmosphere. No provision is made, however, for cloud cover or reflected irradiance. The spectrum of irradiance outside and within the Earth's atmosphere is presented accounting for the attenuating effects of dust particles, water vapour, etc. Graphs are provided relating the solar zenith angle to various degrees of latitude and times of year. Transmission factors are presented enabling the irradiance to be determined when the solar rays pass through the atmosphere at angles other than normal to the Earth's surface. The sea level water vapour content is superimposed upon world maps and the variation of water vapour with altitude is shown. Two worked examples are given illustrating the use of these data.