Pannus are the dark shreds of low cloud, appearing in the saturated air beneath this layer of rain cloud. The atmosphere below a precipitating cloud can become very humid, on account of all the moisture falling through it. Only the slightest rising gust can then cool the air enough for some of this moisture to condense into tiny droplets, which hang around as wisps of thin cloud, which generally appear darker than the layer of cloud above. If it is not raining or snowing when you notice shreds of pannus below a forbidding sky, you can be confident that it very soon will be. Pannus are the five-minute-precipitation-warning of the cloud world.