At low level, defined as surface to 6 500 feet, we have Stratus, abbreviated… — Page 207, Lesson 175

I want to walk you through the cloud types that matter for professional flying. This is a big reference table, so let me take it systematically by stability category and altitude band. First, the table splits clouds into two fundamental categories: stable air clouds and unstable air clouds. That distinction drives everything about the weather you'll encounter inside them. Let's start with stable air clouds, working from low level upward. At low level, defined as surface to 6 500 feet, we have Stratus, abbreviated St. Stratus is composed of water droplets. The flying conditions inside it are: no turbulence, light to moderate icing, and poor visibility — specifically less than 30 metres. It's what we call a turbulence cloud — that doesn't mean it's turbulent itself, but it's associated with drizzle or light rain. So if you're in stratus, expect smooth air but icing risk and very poor visibility. Still in stable air, at medium level — that's 6 500 to 23 000 feet — we have Altostratus, abbreviated As. Altostratus contains both water droplets and ice crystals. Turbulence is light to moderate, icing is light to moderate, and visibility is fair — defined here as less than 1 000 metres. It's associated with a warm front and produces light to moderate rain. At high level in stable air — from 16 500 feet up to the tropopause — we have two types. First, Cirrus, abbreviated Ci. Cirrus is made of ice crystals. There is no turbulence, no icing, and fair visibility — greater than 1 000 metres. It's also associated with a warm front. Second at high level is Cirrostratus, abbreviated Cs. Also ice crystals, no turbulence, no icing, fair visibility greater than 1 000 metres, and again a warm front association. Third at high level is Cirrocumulus, abbreviated Cc. Ice crystals, light turbulence, no icing, fair visibility greater than 1 000 metres, and a warm front association. Now let's move to the unstable air side of the table. At low level — surface to tropopause — we have Cumulus, abbreviated Cu. Cumulus contains water droplets and ice crystals. Expect moderate to severe turbulence, moderate to severe icing, poor visibility, and heavy showers. This is a significant weather producer. Still at low level in unstable air is Cumulonimbus, abbreviated Cb. Same composition — water droplets and ice crystals. Same hazards: moderate to severe turbulence, moderate to severe icing, poor visibility, and heavy showers. Cumulonimbus is the most hazardous cloud you'll encounter. At medium/high level in unstable air — from 6 500 feet upward — we have Nimbostratus, abbreviated Ns. This one contains water droplets and ice crystals. Turbulence is moderate to severe, icing is moderate to severe, visibility is poor — less than 20 metres. It's associated with a warm front and produces heavy precipitation. Also at medium/high level is Altocumulus, abbreviated Ac. Water droplets and ice crystals, light to moderate turbulence, light to moderate icing, fair visibility less than 1 000 metres. And Altocumulus castellanus, abbreviated Acc — this is specifically indicative of medium level instability. That's a key warning sign for a pilot. Also at medium/high level is Stratocumulus, abbreviated Sc. Water droplets, light turbulence, light to moderate icing, poor visibility less than 30 metres. It's a turbulence cloud producing drizzle or light rain. There's an important note: orographic uplift — that's when air is forced upward by terrain — can give moderate to severe turbulence and icing in stratocumulus, even though the table lists it as light. Now let me cover the notes at the bottom of the page. First, Stratus Fractus, abbreviated St Fra. This is the ragged cloud found in the precipitation below nimbostratus. It forms because the evaporation of water droplets saturates the air, and low-level turbulence then leads to cloud formation. So if you see ragged stratus fragments beneath a precipitation layer, that's stratus fractus. Then we have the stages of cumulus development, which is critical for recognising how a cumulus cloud is evolving. Cumulus Fractus, Cu Fra — small ragged cloud, usually forming in the early morning. It may also be found in the precipitation zone below cumulonimbus. Cumulus Humilis, Cu Hum — also known as Fair Weather Cu. Small cumulus, the next stage of development, usually seen early in the morning. Cumulus Mediocris, Cu Med — larger cumulus. It's distinguished by light upper areas having a cauliflower appearance but very dark underneath. This is moderate vertical development. Cumulus Congestus, Cu Con — also known as Towering Cumulus, abbreviated TCU. This is strong vertical development. This is the stage just before it becomes a cumulonimbus. Then we have the two cumulonimbus species. Cumulonimbus Calvus, Cb Cal — the tops are now rounded but do not have a fibrous appearance, and there is no anvil. Cumulonimbus Capillatus, Cb Cap — any cumulonimbus having an anvil. That anvil shape is the ice-crystal plume spreading out at the tropopause. Finally, a critical convention: by convention, a cumulus cloud is reported as cumulonimbus if accompanied by lightning, thunder, hail, or any other precipitation of a showery nature. So even if the cloud looks like a towering cumulus, if it's producing any of those phenomena, you report it as cumulonimbus.