If you have students looking for an interesting science project, the May Scientific American has a nice one. A sun photometer can be used to determine the amount of haze in the atmosphere, and this article describes one that can be built in a couple of hours for less than $20 (although you also need to have a voltmeter). One of the neat things about this instrument is that it exploits the fact that ordinary light-emitting diodes (LED's) can be used in the reverse mode, as detectors of a narrow band of the visible spectrum. There is a very well- designed Web page at http://www.concord.org/haze that supplements the SA article, and is intended to facilitate collection by amateurs of data for a regional, national, or global database. On the other hand, the published link for the "Society of Amateur Scientists" was disappointing when I visited it. Only two of the "hot" items had been added in the last year!