Pannus is an accessory cloud appearing as a chaotic layer of dark fragments just below the base of precipitating clouds like cumulonimbus or nimbostratus. It consists of stratus or cumulus fractus generated in the saturated air beneath rainfall.
Formation and appearance
When rain falls through relatively dry air, some water evaporates, saturating the lower layer and forming these accessory clouds. Pannus gives the sky a threatening, chaotic appearance during storms and persistent rain. It is sometimes confused with funnel clouds or tornadoes, but lacks organised rotation.