There was a thread on this here a long time ago but I can’t find it.
I am certain there must be a correlation between rain and night-time, because the temperature drops at night (obviously) while the dewpoint will stay about the same (same airmass) so water is more likely to fall out – no?
Certainly true. With fog, this correlation is nearly universal. With rain, it may be limited so some places and some periods of the year (e.g. early autumn). I remember 2-3 weeks like this in Prague a couple of years ago, with rain most nights and a perfect CAVOK most days. In midsummer, however, the correlation may be different, with the weather changing in mid-afternoon due to convection from the sun-heated surface.
I seem to recall that my original idea was that the passage of a cold front would correlate with the terrain underneath it being in darkness.
IOW, does the wind speed (it is mid-level wind e.g. FL100) that drives frontal wx, mostly) go up and down with day/night?
I have a gut feeling the effect would be very minor, but it’s fairly easy to assess by comparing the travel distances of a given isolated cold front from morning to evening and from evening to morning for a few consecutive days.
Makes sense, but: in London (which may or may not have it’s very own micro climate 
) there is a pretty established pattern whereby the days start bright and sunny – however by early afternoon it rains. As London is in the west wind zone, the usual suspect of onshore flow doesn’t work. Dry – at least by UK standards, meaning damp – land to the west. I’ve never managed to figure this out – ideas?
The movement of cold fronts are driven by large-scale pressure differences which are not affected by day/night differences.
Makes sense, but: in London (which may or may not have it’s very own micro climate ) there is a pretty established pattern whereby the days start bright and sunny – however by early afternoon it rains. As London is in the west wind zone, the usual suspect of onshore flow doesn’t work. Dry – at least by UK standards, meaning damp – land to the west. I’ve never managed to figure this out – ideas?
This is simply a description of classic low pressure instability, and is not specific to London or any other specific location.
At the end of the night the land mass is relatively cool and in the absence of frontal activity the weather fine. As the sun rises and the land heats up then in low pressure ascending air masses, thermals develop and moist air condenses out at the dew point typically producing showers in the early afternoon. Not related at all to frontal weather.
