Simulation studies on the performance of IR sounders under varying conditions of cloud cover and cloud heights are carried out for Nimbus 6. An analytic function is derived for calculating the relative response to cloud height errors for arbitrary cloud-sensing channels. Based on the values of the response function, the best choice of channels for determining cloud amounts are obtained. An algorithm is described for determining cloud heights and the sensitivity of cloud-height sensing channels are tested. It is found that for the HIRS instrument, the most transparent channel in the 4.3 μm band is optimal for adjusting cloud heights while the channel in the 15 μm band peaking closest to the surface is best for determining cloud amounts.