I'm contemplating a major purchase --- an upgrade to a modern, large aperture, computer-controlled telescope. It will be expensive.
It occurred to me to ask, before it invest so heavily in a new telescope. just how often would I haul the instrument into the car, drive out of town to a remote dark site, and set up for an evening of stargazing?
Sky conditions are far from ideal in central Pennsylvania, but I recall viewing conditions were much more favorable and more frequent in my younger days. Our weather seems to be dominated by more frequent and heavier rains and more humid tropical airflow patterns.
So just how frequent are the skies clear enough to warrant setting up a large telescope?
I decided to answer this question by making nightly observations of the sky conditions. Basically, would the sky be clear enough to make setting up the telescope worthwhile?
Starting in December 2017, I went out every evening to view the sky and appraise its prospects for astronomy. I used a simple numerical code to summarize the sky conditions, taking into account transparency, darkness, and steadiness. If the night was acceptable, I estimated the percent of cloud/haze blockage, and I used an infrared thermometer to measure the ground temperature on that part of the lawn where I would have set up the telescope. I used a green laser pointer to judge air quality and haze. I also took note of other environmental (comfort) conditions such as dew, frost, wind, high cirrus clouds, haze, and the presence of upwind skunks.
If it was not raining or completely overcast, I rated the sky by the following scale:
1 = only the brightest (1st magnitude) stars visible
2 = 3rd magnitude stars visible, or more than 50% cloud/haze coverage
3 = transparent between opaque cloud patches
4 = clear, but turbulent or interference from a bright Moon
5 = clear, dark, and steady - rare
A 1, 2 or 3 rating was judged to be a "marginal" evening. I might stay out awhile and look around, maybe with binoculars, but I definitely would not set up a telescope.
A 4 or 5 rating was judged to be an "acceptable" evening. Setting up the telescope would be worth the effort.
(Overcast/rain/snow nights were given a 0 rating.)
Understand, at my location, the sky is never completely clear and never completely dark. I live in a suburban valley within miles of car dealerships and multiple shopping centers. The sky is always bright. So my ratings are relative to this constant background.
Here are my results for 2018.
"5" nights 26 7.12% "4" nights 31 8.49% "3" nights 18 4.93% "2" nights 46 12.60% "1" nights 23 6.30% "0" nights 221 60.55%
In 2018, there were 221 impossible nights, overcast, rain or snow. That's 60.6% of the time.
87 nights were marginal. It would not be worth setting up a telescope 23.8% of the time.
So that's 84.4% of the year when the telescope stays indoors.
57 nights were acceptable. I might set up and use the telescope only 15.6% of the time.