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    Re: How often can you see the sunrise/sunset - PLEASE HELP?
    From: Mike L
    Date: 2007 Dec 6, 06:15 -0800

    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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