a giant tethered helium balloon that displays real-time reports of atmospheric pollution by a colorful lighting system, which can be seen from more than 20 km (12.4 miles) away. the balloon's color signifies the ambient air quality using 3 projectors that are located in the middle of the balloon. for example, red signifies highly polluted air, orange for polluted, yellow for moderate, light green for clean, & green for very clean.

[link: aerophile.com|via physorg.com & ecofriend.org]

see also:
. nuage vert
. atmostpheric pollution
. eco river
. public air quality indicator