Let's begin with Stratus, abbreviated St or ST — Page 199, Lesson 170

I want to walk you through the cloud types that form the core of any meteorology syllabus for professional pilots. We're starting with the low-level stratiform clouds — the layered, sheet-like clouds — and then moving to the first of the convective, heaped clouds. Let's begin with Stratus, abbreviated St or ST. The name comes from the Latin stratum, meaning "strewn" — think of something spread out like a blanket. Stratus is generally a grey, layered cloud with a fairly uniform base. It may produce drizzle or light snow. Its vertical extent — how thick the layer is from base to top — can range from a few hundred feet up to several thousand feet. Stratus is usually the lowest of all cloud types, meaning it sits closest to the ground. The main hazard associated with stratus is poor visibility. It often covers high ground, concealing hill tops from pilots — that's a direct safety issue — and it produces what hikers call hill fog. When stratus is at its thinnest, the sun can be clearly seen through the layer, so don't assume a visible sun means no low cloud. Next is Stratocumulus, abbreviated Sc or SC. This is probably the most common form of cloud in the skies of the United Kingdom. It appears grey or whitish, but usually has distinct dark parts — so it's not a uniform grey blanket like stratus. Stratocumulus can be seen as patches or in a continuous layer. Its thickness is usually no more than 2000 to 3000 ft, but it may become 5000 to 6000 ft deep in certain conditions. The cloud base — the height of the bottom of the cloud above ground — is usually between 1000 ft and 4500 ft. So if you're flying at 2000 ft and see a grey, patchy layer with dark sections, that's likely stratocumulus. Now we move to Nimbostratus, abbreviated Ns or NS. The prefix "nimbo-" means rain-bearing. Nimbostratus is a dense, dark-grey, stratiform cloud — so it's layered, not heaped — and it produces extensive and long-lasting continuous or intermittent precipitation. That means steady rain or snow that goes on for hours, not just a quick shower. Usually the cloud base is between the surface and 6500 ft above ground level. Nimbostratus is generally found at the warm front in polar front depressions — that's a key operational point: if you're flying into a warm front in a mid-latitude depression, expect Ns with prolonged precipitation and a low base. Finally, let's look at Cumulus, abbreviated Cu or CU. This is the most common form of convective cloud — meaning it forms from rising thermal currents, not from a large-scale lifting of a stable air mass. It's classified as heaped cloud, from Latin cumulare meaning "to heap up." For glider pilots, a developing cumulus is regarded as a reliable indication of the presence of thermal upcurrents which, if skilfully exploited, can enable the glider to gain height. That's the positive side. For pilots of light aircraft, on the other hand, on a day when the sky is peppered with fine-weather cumulus, flight below cloud base is turbulent, whereas above the cloud tops, the air is likely to be very smooth. So the turbulence is underneath the cloud, not on top of it. A developed cumulus cloud is generally dense, with sharp outlines — you can see its edges clearly. As it continues to develop vertically, a cumulus cloud forms mounds, domes or towers, of which the upper parts often — and the excerpt cuts off there, but you get the picture of a cloud that builds upward like a tower. That figure shows how we find cloud base with a balloon, but for now, focus on these four types: Stratus, Stratocumulus, Nimbostratus, and Cumulus. Each has a distinct appearance, base height, thickness, precipitation type, and hazard profile. As a professional pilot, you need to identify them from the cockpit and know what weather and turbulence to expect.