Measurements have been made of the near sea infra-red transmission of the atmosphere over paths of 5 and 9 km in a temperature environment. Seven broadband spectral regions have been used and variations with absolute humidity, temperature and visibility are investigated. The data are compared with the predictions of the AFGL computer model LOWTRAN3B. A discrepancy has appeared between the measured and calculated data for the 4.4 to 5.4 micrometers region due, possibly, to either no water continuum absorption being included in the model or an under-estimation of the N2 continuum absorption. Indications are that the dependence of water vapour continuum transmission on precipitable water and temperature in the 8.2 to 11.8 micrometers region may not be accurately described by the LOWTRAN3B model. (Author).

Equipment is described that has been developed to investigate the propagation of infra-red radiation through ground layers of the atmosphere over various broad-band wavelength regions from 1 to 12 micrometers for pathlengths up to 10 km. Propagation studies undertaken to date have included absolute transmission and relative transmission measurements in a terrain environment, and terrestrial scintillation observations. The various errors associated with transmission measurements are discussed. (Author).

This report investigates the applicability of the standard LOWTRAN atmospheric models to four typical Australian environments and considers the effects to these environments upon the transmission of 3 to 5 um radiation. It is found that the LOWTRAN model atmospheres can be useful in Australian conditions, and advice is given as to the most appropriate models to use depending upon the path and environment conditions. The extent to which model data is used rather than real data will affect the transmittance predictions; the magnitude of this effect is given, indicating that there is particular value in measuring water vapour content (shown to be the most significant meteorological parameter) in the lowest few kilometers through which the path passes. Transmittance as a function of range (and vice versa) is considered for all slant path angles from vertical to horizontal. Graphical results are presented in full as a resource to be consulted given a particular site ad path transmittance prediction requirement. Keywords: Infrared radiation; Atmospheric models; Slant paths; Australia. (JHD).

Measurements are reported on infrared transmission through the atmosphere at a coastal marine site in a temperate environment. Ten broadband spectral regions were used and variations of transmittance with absolute humidity and visibility over paths of 5 and 9 km investigated. All data have been compared with the predictions of the AFGL computer code LOWTRAN 5. The discrepancy reported previously between the measured and calculated data in the 4.4 to 5.4 micrometer region still exists, although new data on the water continuum absorption coefficients for this region have reduced this. Reasonable agreement has been shown to exist in the 8 to 12 micrometer region for absolute humidities up to 12 gm/cu m. The results of the comparisons for the 1 to 2.5 micrometer region showed a greater discrepancy. Some of this discrepancy could be arising from point measurements of aerosol extinction using equipment which is underestimating the aerosol attenuation coefficient. Data on scintillation fluctuations recorded during the transmittance measurements are also presented.