I second Joe's recomend. I've been using this for the last year and it is excellent!

John

I often use a web site at http://www.cleardarksky.com/csk/ that I have found to be quite valuable in determining/predicting the quality of light over the next 24 hours.

The site was designed for astronomers to predict sky visibility at night, but because it provides information on a 24 hour day, it is equally valuable for photographers working in daylight.

There are over 1500 Clear Sky Clock locations throughout North America. Rather than try to explain it, I suggest you take a look at the site and determine if the information can help you.

http://www.cleardarksky.com/csk/

Cool!! Joe - thanks for sharing that site.  I think I will use it quite often now.