On a clear day, you use an infrared thermometer to measure a blackbody temperature of 273 K looking towards the horizon and 255 K looking directly upwards. Estimate the longwave emissivity and optical depth of the atmosphere. Noting that and assuming that the slant path optical depth T -> infinity looking towards the horizon.
Given that:
The infrared thermometer to measure a black body temperature of 273 k looking towards the horizon and 255 k looking directly towards.
On a clear day, you use an infrared thermometer to measure a blackbody temperature of 273...