At the MLO, weather is monitored by automatic instruments. The data are available here http://www.esrl.noaa.gov/gmd/dv/data/index.php?site=mlo.
For our observing times we extract all available met data and inspect them:
Right: (top) Histogram of vertical temperature gradient for all observing times (black) and for every hour of 2012 (red). (bottom) Histogram for RH.
We note that the vertical temperature gradient for our observing times almost always are negative - that it is warmer higher up than near the ground - this implies vertical stability. The opposite condition certainly occurs during 2012 but does not correspond to observing times - probably because conditions were bad (convectively unstable or cloudy?). We also see that the gradient becomes less negative through the night implying cooling and equilibration of temperatures. Changes in RH are small. RH during observations show a skewed distribution relative to what was available - so something about observing picks out not the driest nights.
We shall see if the few nights with large RH (say, above 40%) tend to correspond to particularly bad conditions, by inspecting observed images and checking for 'drifting wiggles' and plain old 'large halo'.