Measurement of sky clarity using FIR radiometers as an adjunct to   atmospheric Cerenkov radiation measurements
  A technique for detecting the presence of cloud in the field of view of an atmospheric Cerenkov telescope using a far infra red radiometer is described. Models for the radiative emission from clear and cloudy skies are tested and found to represent the measurements. 
D.J. Buckley, M.C. Dorrington, P.J. Edwards, T.J.L. McComb, S.P.   Tummey and K.E. Turver (University of Durham)