Nuage Vert (Green Cloud) is part of Hehe's (http://hehe.org.free.fr/) free research into altering our perception of pollution.

"Whenever we see a chimney emitting smoke or vapour we think of it as pollution. But these emissions function also as weather-veins and landmarks in the urban landscape. If such smoke signals were to signal recycling, displaying the local effort made by citizens when they recycle more (or less), the chimney would then be a sign of environmental effort as well as waste. The waste incineration site at St Ouen (near Paris) treats the waste that comes from ordinary household bins and the waste which has been rejected from the local recycling site. The constant plume emitted from the site is visible for miles. It consists mostly of water vapour and emits small levels of dioxins. The aim of the Nuage Vert project is to raise the level of awareness and participation amongst consumers.
The colour image on the cloud will change depending upon the ratio between the amount of daily waste collected for recycling and that collected for incineration."

found via www.we-make-money-not-art.com/archives/2005/03/nuage-vert-1.php