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Marcel, yes, I think you are right!

If I could relate sun angle to the probability of seeing the sun
through a "cloudy", "clear sky", etc, then if I could assume that
everywhere had similar clouds (I'm laughiing!) then I could work out
the probability of seeing the sunrise/set!

I tried to find out the height of typical clouds and failed! However,
midlevel clouds are between 2000-5000m high.

So, if the cloud is 2-5km high, for there to be a view through the
clouds there must be a hole such that the top of the clouds at 5km
high are some 250km away and the bottom of the cloud is some 160km
away. To see a complete view of the sun, the hole in the clouds must
be such that every observer along a 32'= 32nautical miles path away
from the sunset is able to see the sun through this hole, so the
nearest cloud base must be 160km-32*1.852 (= 100km) must be able to
see.

So, assuming the clouds are from 2-5km then the hole in the clouds
must be some 250-100 = 150km long by 60km wide whilst a similar cloud
at midday need be only 17m wide!

CALCULATIONS!

Earth's radius R = 6370km
Angle of arc around earth to base of cloud = cos-1(R/R+2) = 1.4degree
Angle of arc around earth to Top of cloud = cos-1(R/R+5) = 2.3degree
Distance = R*tan(angle)

Or using Pythagooras:

R=earth's radius
H=Cloud height

Distance = ((R+H)^2 - R^2)^.5
         = (H^2 + 2*H*R)^.5
which is approx 112*square root of cloud height in km







On Dec 6, 1:07 pm, "Marcel Tschudin" 
wrote:
> Mike,
>
> On Dec 6, 2007 2:33 PM, Isonomia  wrote:
>
> > ...
> > So, whilst cloud cover should give a very good indication of the
> > probability of seeing the midday sun (when the sun is itself high due
> > to latitude and season), I can't see how to relate it to the
> > probability of seeing a sunset/rise.
>
> You could e.g. put the daily data of the Wunderground Web-page (where
> you likely may have hourly information on cloud coverage) in relation
> to the sun's position above the horizon. This may allow you to gain
> eventually an idea on which of all the different meteorological
> parameters are important for your problem. However, I don't think that
> statistical analyses will be able to tell you more than very rough
> tendencies, this as a result of the large (temporal) variations in
> cloud coverage and thus also of the hole sizes.
>
> Marcel
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