A high resolution oxygen A-band and water vapor band spectrometer has been developed and deployed for a field campaign at the ARM SGP site. The resolution of this instrument is 0.025 nm with out of band rejection better than 10^-5. Given the performance of the instrument, we are able to retrieve both mean and variance of photon pathlength distributions. Studies are underway of the joint statistics of photon pathlength moments (mean and variance) and cloud optical properties that were observed during the deployment of the spectrometer at the ARM site. Preliminary results demonstrate that information derived from photon pathlength distributions is a powerful diagnostic tool for sky homogeneity and can provide constrains for radiative transfer calculation in cloudy atmospheres.