Here in southern Arizona the winter cloud cover tends to typically be closer to 100% on the days we have it. Those days are the highs are moderated and difference between highs and lows is moderated. The clouds hold off the heat of the day but also holds in the heat at night. This can be seen in the graph below as the lower highs were cloudy days:
What is your cloud cover scenarios during the two seasons? Do you have the solar radiation sensor, and can you overlay that on your charts?
Just a wild guess on my part. Also can you compare prevailing winds during the two seasons? Our winds from the continent offer much more dynamic temps than our Gulf of California winds which are ocean moderated? Just another wild guess on my part...
