For the development of long range infrared sensors, it is important to have an accurate assessment of the transmission of IR radiation in the atmosphere. Measured experimental data to date have been obtained over tens of kilometers. In this paper a theoretical analysis of factors affecting clear band atmospheric transmission over ranges exceeding 200 km is presented. The analysis considers altitudes below 10 km (important for detection of low altitude targets) in the IR and uses a Grumman line-by-line code, LINETRAN, which incorporates the AFGL Line Parameter Atlas. Multiple atmospheric layers in LINETRAN account for the effects of refraction and altitude-dependent absorbers. The absorption contribution from each layer is calculated and summed to give the total transmission between the source and the observer. LINETRAN molecular absorption (smoothed to 20 cm-1 resolution)is compared to the molecular absorption predicted by the band-model code LOWTRAN for several clear bands and several ranges. The relative effects of humidity and visibility on band integrated transmission are then shown. Molecular line absorption, molecular continuum absorption, and aerosol extinction are included for two clear bands. The need for measurement of atmospheric parameters such as humidity and visibility during long range transmission measurements is seen to be essential for further progress in long range atmospheric modeling.