Paul Hyett wrote:
What are the main causes of this (in situations where no fronts are involved, of course)?

Would this phrase "diurnal" refer to a division of the day that runs
from 4 to 10 am as the heaviest or morning dew? The coldest part of the
day; even in full sun in the winter it is still bitterly cold until
that magic shut off time (when there are "no" fronts involved of
course.)