  A Low Humidity and Temperature Profiling (LHATPRO) microwave radiometer,
manufactured by Radiometer Physics GmbH (RPG), is used to monitor sky
conditions over ESO's Paranal observatory in support of VLT science operations.
In addition to measuring precipitable water vapour (PWV) the instrument also
contains an IR camera measuring sky brightness temperature at 10.5 {\mu}m. Due
to its extended operating range down to -100 {\deg}C it is capable of detecting
very cold and very thin, even sub-visual, cirrus clouds. We present a set of
instrument flux calibration values as compared with a detrended fluctuation
analysis (DFA) of the IR camera zenith-looking sky brightness data measured
above Paranal taken over the past two years. We show that it is possible to
quantify photometric observing conditions and that the method is highly
sensitive to the presence of even very thin clouds but robust against
variations of sky brightness caused by effects other than clouds such as
variations of precipitable water vapour. Hence it can be used to determine
photometric conditions for science operations. About 60 % of nights are free of
clouds on Paranal. More work will be required to classify the clouds using this
technique. For the future this approach might become part of VLT science
operations for evaluating nightly sky conditions.
