Description: Clouds are visible masses of water droplets or other particles above skyline. Level of cloud cover varies considerably, typically moving across the skyline from left to right of the image.
 Features / Attributes: The slower moving clouds often do not appear in the background subtracted images because of the low rate of change in color and movement. Time periods with higher cloud activity tended to have high activity across the entire sky.
Source of contamination: For detection of polluting plumes, clouds pose a source of noise when they are dense, moving quickly and follow the same trajectory as a partially dissipated black smoke plume follows after it has risen above the city skyline. These are particular sources of signal we are interested in tagging so model can learn to distinguish as non-plumes.
Issues / challenges: The objective of tagging clouds is not to identify images containing clouds, but rather those which may serve as a source of noise for our model aimed at detecting plumes. Efforts were concentrated on tagging clouds which have similar features to the plumes of interest.