"Col" wrote in message
...
I know of this effect as 'dayglow'. Clearly it won't be as pronounced
as at your latitude but for a few weeks around the solstice, even at
1 am (effectively midnight) there is a definite lightening of the northern
sky, in clear conditions at least.
I wonder how far south this can be observed?
I had a terrible time getting to sleep on Sunday night and spent much of the
time gawking out of the window to see whether there was any glow to the
north.

From what I remember (bearing in mind I was zombiefied by a lack of sleep
from Friday and Saturday nights as well!):

* At 11:30PM (BST) there was a fair bit of light to the north.
* At 1AM (BST), midnight GMT, it looked dark as far as I could see - there
are some 120-ft hills 3/4 mile to the north of me though.
* By 1:30 AM (BST), there was a faint glow to the north, suggesting the
coming dawn.

I'm pretty sure that if you had a flat horizon (eg looking over the sea),
you'd see a faint glow at midnight GMT even down here.

Reports from Leysdown, north Kent, 10m ASL.