Pannus is an accessory cloud made of ragged shreds — torn, irregular fragments often forming a near-continuous layer below another cloud, sometimes attached to it. These shreds are the species fractus (stratus fractus or cumulus fractus), informally called scud. They form in the cool, rain-moistened air beneath a precipitating cloud: falling rain saturates the air below the base, and condensation produces low ragged cloud that the wind tears into shreds.
Worth knowing
Pannus is genuinely born of the rain: precipitation falling from the parent cloud saturates and cools the air below the base, so the scud you see is effectively the rain making its own low cloud beneath the storm.
Note
Still unsure it's the same cloud? Our app, Cloud Point— on iOS and Android — reads a photo and returns the likely type with altitude, useful when a formation is fading faster than you can look it up.
