Without the proverbial "Clear Skies", you can't see the stars above. For wide-angle & landscape shots, terrestrial clouds can add interesting texture, for narrow shots, even a small patch of clouds can block the entire view for a long time.

While I'm not a trained meteorologist, but after living in the same place for more than two years, I recognize the weather patterns. And I'm sure, you too have similar experience. In addition, I use mostly just two websites with frequent updates from local weather stations, doppler radar & satellite images (careful, those can be 60...180min old). These sites help me to spot rain & clouds even as they are one or two hours away. To confirm my weather map observations, I use a few webcams, preferably close to my observation site, again to check for rain or clouds before I leave.

In preparation for a (planned) shot, I roughly follow these steps :
  • Starting ~ a week in advance, I keep an eye on CALSKY's "Meteogram" -- it has sufficient detail on wind, weather & clouds in an easy to read diagram and you can use a map to define the location : www.calsky.org
  • You might listen to your TV weather speculator .... when it comes to 3...7 day forecasts, they usually tell you the same as the weatherpages
  • In the day & hours before the observation, I check an online-map showing rain & visual cloud cover (Wunderground.com) -- this map has MANY options. By looking out my window and reading the map at the same time, I have tweaked the settings to fit my needs.and saved that long link as a bookmark. You will need to make your own adjustments.
  • If you have, bookmark webcams close to your shooting location -- unfortunately few are sensitive enough to show fog & clouds at night
Especially by looking at the satellite images and wind patterns, I can see cloud banks off the coast and can somewhat judge if the risk of driving & getting fogged in is worth the possible reward.

Besides the obvious coastal fog & clouds, other factors impact the quality of the observation :
  • hot temperatures increase air movements, more blurry vision
  • high humidity causes unwanted reflections and scatters light
  • different layers of hot & cold air cause cause changing refractions
  • these layers often have winds in different directions, further increasing the (dynamic) disturbances
  • changing winds not only stir the air above but possibly also shake the telescope
  • consider how low an object is in the sky -- the lower it is, the more atmosphere and thus the higher the chances of interferences.
  • Air pollution, like humidity, further degrades quality and amplifies the effect of light pollution

Speaking of the atmosphere, I suggest you closely look at a setting moon :

When you use binoculars or large magnification camera-lens (or a simulation in Stellarium) you will notice the moon's shape changes. It is not round anymore when it sets. The air refracts light and the airmasses' thickness increases closer to the horizon. That's why the air doesn't act like a clear piece of window glass. Instead it distorts everything like a bad prescription lens.
More about the effect or Air Mass : /stargazer95050/21434571


Many of these factors can be reduced by picking the right location for your observation. High altitude helps. Get away from cities with their light- & air-pollution. Or at least, stand upwind from the pollution and hide behind some hills blocking the lights.

One such prime location is the Atakama desert high in the Andes mountains, with its extremely dry air all year round and hundreds of miles from a metropolis. Mt. Kilauea on Hawaii has similar but not as good advantages. However, it is much easier to reach. If you can afford it, put your telescope into space and avoid the hassle of atmosphere all together :-))

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