This slide will show the effect of moisture on the seeing. The animation will not work in the online version so I will replace the animation with text.
If you were to view the sky from Kitt Peak (elevation 7000+ ft) in the Arizona desert the sky would be strikingly uniform from the zenith almost all the way to the horizon. Only when you got to very low angles would the transparency seem to fade. This is caused by two effects. First you have gone high getting above a lot of atmosphere. Second you have selected a place with very low humidity (at least if your night on Kitt Peak was more fortunate then mine).
The drawing shows the more dramatic effects of low altitude and moisture. This might be typical of a spring evening in San Jose that had a relatively high dewpoint. It would be worse in a site having a higher dewpoint.
The visual magnitude of a sky (which is an indication of the quality of a site) is measured looking straight up. For the sake of this discussion let us say that the dimmest star you can see is magnitude 7. This might correspond to a top site like Lake San Antonio (where we hold CalStar).
As you look at progressively lower angles you will notice that you can no longer see magnitude 7 stars. In fact as you look to the horizon you can only see the brightest.
I call this “looking through a lifesaver” because looking up you are viewing through the hole. At increasing angle from the zenith in all directions you are looking through more of the lifesaver. In this case the crud in the air is preventing you from seeing the dimmer light.