The UK did the same between 1940 and 1979 in various tests with zinc cadmium sulphide, e.coli and bacillus globigii. I suspect these kind of things are largely the basis for the Chemtrails conspiracy theories. More: http://www.theguardian.com/politics/2002/apr/21/uk.medicalsc...
https://www.youtube.com/watch?v=jf0khstYDLA
It's one of the many conspiracy type videos out there, and I don't endorse the content in any way.. I just find this stuff fascinating