From: liz@poppyrecords.invalid.invalid
Don Y wrote:
> On 11/19/2025 3:33 AM, Liz Tuddenham wrote:
> > Don Y wrote:
> >
> >> On 11/18/2025 2:39 AM, Liz Tuddenham wrote:
> >>> Don Y wrote:
> >>>
> >>>> Particularly, cloud cover.
> >>>>
> >>>> How can I quantify the extent and "density" (opacity) of cloud
> >>>> cover?
> >>>>
> >>>> And, to make it even more interesting, doing so AT NIGHT, as well?
> >>>
> >>>
> >>> At night the clouds would appear warmer than space, so an infra-red
> >>> scanning system would give you a picture of the clouds. I don't know
> >>> how you would calibrate it, because the apparent cloud temperature would
> >>> depend on air temperature and IR radiation from the earth.
> >>
> >> But the cloud cover wouldn't instantaneously change from observations
> >> made during the preceding daylight/twilight periods. I.e., "THIS is
> >> what cloud cover looks like and THAT is what clear skies look like,
> >> in the Ir" (assuming both are present)
> >
> > In the UK there can be hourly changes in the cloud cover from clear to
> > 90% cover as 'fronts' are blown across the country.
>
> Of course! But, I'm not just checking the skies once a day, etc.
> I will be watching continuously.
>
> During daylight hours, it is possible to determine which parts of
> town are seeing "daylight", "cloudy", "precipitation" and virga.
> My concern isn't with "those areas" but, rather, whether or not
> the "sky" will shift to bring those conditions to *me*.
>
> > I think you might have to measure daylight and darkness observations
> > differently. The apparent temperature of the clouds will increase when
> > they are in sunlight - but so will the apparent temperature of the space
> > in between them, which is filled with illuminated dust particles.
>
> But, during daylight, I can distinguish between blue skies and white/grey
> clouds. I don't have to use the same wavelengths for all my observations.
>
> >> Think about how "you" can tell the condition of the skies without
> >> even looking upward: "It's not *bright* enough for 1PM" or "The light
> >> is the wrong 'color'"...
> >
> > Recently I haven't even needed to look out the window to guess at the
> > weather. We had a period of several days of continuous thick black
> > cloud cover when the ambient light level was so low I needed to switch
> > on lights to see around the house. If you are only interested in the
> > oveall cloud cover, rather than mapping the clouds, an ORP12 and a 1.5v
> > battery would work in daytime.
>
> I can look at the "ground" to determine the *type* of cloud cover and
> whether it is transitional or not. But, that won't tell me what's
> on either side of my bit of sky. It won't help me *plan*.
Last Sunday I had parked the van on a local hill and a friend and I were
doing exactly what you seem to want to automate. The first problem was
the wind direction, which appeared to be different at different heights
- so different cloud layers were moving in different directions.
Then we noticed that there appeared to be a large patch of blue sky
appearing but there was no corresponding patch of sunlight on the ground
beneath it. We worked out that this was because we could see blue sky
through a diagonal gap between two layers - but in the direction of the
sunlight, the layers overlapped. This was confirmed by the tops of some
of the clouds (presumably in the lower layer) being illuminated by
sunshine.
We came to the conclusion that weather was extremely difficult to
predict from observations like that, even on a very short-term basis.
--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
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