Paul Hyett wrote:
What are the main causes of this (in situations where no fronts are
involved, of course)?
--
Paul Hyett (change 'invalid83261' to 'blueyonder' to email me)

The main cause of a rise in dewpoint during the day is probably
evapotranspiration and is confined to a shallow layer, as frequently
confirmed by soundings. If any dew is deposited at night the dew-point
must fall because water is (temporarily) removed from the atmosphere.
Some of the high dewpoints observed in this country in hot summer
weather can partly be accounted for in this way as they are sometimes
higher than the sea temperature.

Tudor hughes, Warlingham, Surrey.