Seems to happen *every* time there is a thundery spell in summer. As
soon as the storms have moved away, there is always between 24 and 48
hours of impenetrable, gloomy low cloud which refuses to lift no matter
how close to the summer solstice you are. Seems to be independent of
wind direction too... no matter what the characteristics of the air
mass behind the storms is, easterly or westerly, warm or cool, it's
always the same.
The pattern that seems to occur is... last storm clears, sky still
covered by castellanus, castellanus breaks up, sky starts to clear....
then the gloom descends.

I'd imagine it must be something to do with moisture left behind by the
storms, but in that case:

- why do areas that missed the storms get the gloom too?
- why does it not happen following other types of rain? Even warm
sectors, for all their moisture, show more of a clearing trend in
summer.

Nick