Unfortunately, lack of regulations against light pollution makes it significant in most cities, so on humid nights transparency drops easily.
Negative effects of low transparency are two:in one hand part of the light from celestial objects is absorbed, reducing its brightness and on the other hand artificial lighting (light pollution) is reflected resulting in sky background brightness that covers fainter deep space objects.
You can see in this photo taken at my home in a low transparency night that artificial light scattered by air humidity makes it impossible to see the dimmest stars.
In contrast with that, in a good transparency night, although artificial lighting is the same, many more stars can be seen, since not only the stars look brighter but the background is darker.
For Deep Space Astrophotography sky transparency must be good.
Even wispy high cloud layers invisible to the naked eye at night reduce the transparency and make it impossible Deep Space astrophotography.
The following photos show the difference between two images produced with the same camera, one taken with poor transparency and the other with good transparency.
For Canada and USA you can get forecast of several very useful astronomical weather parameters at Clear Sky Chart. There is a chart for San Diego, California at the bottom of the pages of this site.
Astronomical sky transparency of several locations used to be available at http://astroforecast.org/ but this site has been off line for a while.
Other very good site is http://7timer.y234.cn/V3/ where you can create an "APanel" containing 72 hour forecast of several very useful astronomical weather parameters. It can also be seen for Montevideo at the bottom of the pages of this site.
Another useful site for amateur astronomers is http://www.meteoblue.com/ where you can see one week forecast of atmospheric seeing, cloud cover, etc.
Another useful site for amateur astronomers is http://www.meteoblue.com/ where you can see one week forecast of atmospheric seeing, cloud cover, etc.
