The Advanced Baseline Imager (ABI) aboard the GOES-R series will have channels centered at 6.185 µm and 10.35 µm, which are within the water vapor absorption and atmospheric window regions of the spectrum, respectively. Previous studies have shown that positive brightness temperature difference values between these two bands are occasionally observed above cold thunderstorm tops. These positive values may be associated with water vapor absorption in the relatively warm lower stratosphere. This study uses an observational operator to simulate radiances at these two channels. Sensitivity tests are performed to demonstrate that brightness temperature differences depend primarily on the strength of the tropopause inversion, and to a lesser extent the amount of water vapor above cloud top. Additionally, it will be shown that an optically thin cloud layer above a deep cumulonimbus can also affect brightness temperatures. Studies of this nature are extremely important in preparation for the launch of GOES-R.