"Urbanization" is to blame for many of the high minimums. Lawns,
roads, roofs...all man-caused activities...all absorb heat and release
it later, which causes higher minimums, and gives a "head start"
toward higher maximums during the day.
I have been told that the climate in the Phoenix area is
an example of man made changes. Thanks to lawn watering, non native
vegetation, and air conditioning - the humidity levels are a far cry from
the 50s when allergy sufferers craved the dry pollen free relief of the
area.