Certain molecules will absorb certain frequencies of IR. During daytime, an IR sensor pointed at the sun (on heliotropic platter) could therefore determine the amount of certain molecules in the atmospheric column between the sensor and the sun.

More infos: https://www.wikiwand.com/en/Infrared_window

Same applies to other bands of EM spectrum: https://www.wikiwand.com/en/Optical_window

For example, with H2O (water) the sensor could estimate cloud cover within the column.

It's feasible that similar could be done at night by making heliotrope point at moon?