Although I have known this for many years, It really hit home this spring when we had some really dry air over us. A few weeks ago we had a rather dry air mass pass over us and I could see stars that I don’t usually get to see. I know that air with humidity in the 30% to 40% range is not very dry for some places but here in North Carolina, where the normal summer time readings are in the 70% to 80% range, it was very dry for us. It was wonderful observing from home with such low humidity readings where I could easily see the Messier galaxies and many NGC galaxies.
In contrast to the low humidity, this past Monday I was out observing with much higher humidity readings. With the humidity at a still low 63%, I tried to find many of the galaxies that I had observed only a week before, only to find that I could just detect the presence of those galaxies and no longer really see them. This was very disappointing, knowing that the humidity was only going up in the months ahead.
What causes this to happen? Summers in the southeastern United States tend to be very humid and that humidity holds pollen and dust particles in the air. These particles and the moisture in the air tend to scatter the light pollution, that is so common today, making the sky appear brighter. The end result is that the summer sky is much brighter than in the winter months. This brightening of the sky makes it almost impossible to see faint objects or the fainter outer regions of galaxies. Only the brightest portions of the inner areas of galaxies may be seen. For example, I observed M104 in Virgo. This galaxy has a listed size of 9′x4′ but what I could see was only approximately 3′x1′ in size. Obviously I was only seeing the brighter inner core region of the galaxy. Here is a sketch of this portion of M104 as seen Monday night. I was suprised to see the dust lane at magnification of 200x.