At the MLO, weather is monitored by automatic instruments. The data are available here

For our observing times we extract all available met data and inspect them:

Left: (top) Vertical temperature gradient (T2m – Ttop) in degrees C against fraction of Julian Day (fJD). fJD=0 at midnight, so each night’s observing starts at 0.8 or so and proceeds to the right, wrapping around at midnight. T2m and Ttop are temperatures measured on the MLO met tower at 2 m above ground level and at tower top (much more than 10 m up!). (middle) Relative Humidity in % against fJD. Ignore bottom panel.
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’.