What properties of the atmosphere reduce its transparency? Air by its own right scatters light. That is why the sky is blue and sunsets are red. But that is only the start and other factors can reduce the transparency much further.
One of the most common problems is moisture. The dewpoint will crudely tell you how much moisture is in the air. I am currently studying whether the dewpoint or relative humidity is a better indication of the impact on viewing. In any case humidity is why Arizona has such clear skies and Costa Rica looked like we were in the middle of a huge doughnut. The next slide illustrates this point.
Other factors can also reduce the transparency. Particulates or Smog can make a clear sky a muddy brown.
High, thin clouds, as we discussed earlier, could give the impression of a clear night. In reality, dim objects that should normally be visible at this site will invisible.
The dewpoint is a direct measurement of the surface air. Contrails and Sky Color can be used as a crude measurement of the moisture content of the upper air. A milky sky color and persistent contrails hint at high moisture above. Of course you would already know that if you had consulted the water vapor satellite image.