Cloud Formation and Precipitation — Page 217, Lesson 182

I want to walk you through how clouds form and the different ways air gets lifted to create them. This is foundational meteorology for your ATPL, so let's take it step by step. Cloud forms when air is lifted and cools adiabatically — that means it cools because it expands as it rises, without exchanging heat with the surrounding air — until the water vapour in it condenses out as water droplets. The height at which this condensation first happens is called the condensation level, and that is also the height of the cloud base. So whenever you see a cloud base, you're looking at the condensation level for that particular rising air. Now, there are five ways that initial lifting of air can occur. Let me list them precisely: 1. Turbulence — mechanical stirring of the air, usually by friction with the ground. 2. Orographic uplift — air forced to rise over high ground like hills or mountains. 3. Convection — air rising because it's warmer than its surroundings. 4. Slow, widespread ascent — also called frontal uplift, where one air mass is forced over another. 5. Convergence — air flowing together horizontally, which forces it to rise because it has nowhere else to go. There's an important note here: strictly speaking, all of these lifting processes are forms of convection. The third one — what we normally call convection — is free convection, where the air rises on its own because it's buoyant. The other four are forced convection, where some external mechanism pushes the air upward. Let's go back to the condensation level and be precise about what it means. The condensation level is the height at which rising air, cooling adiabatically, has cooled to the dew point temperature. The dew point is the temperature to which air must be cooled for condensation to begin. So once the rising air reaches that temperature, any further ascent and further cooling will result in condensation and the formation of cloud. That height is the base of the cloud. Now let's look at the first lifting process in detail: turbulence cloud. In stable conditions, vertical movement of air is limited. The upcurrents created by surface friction — the turbulence — are limited in how high they can reach. If the rising air reaches its dew point before it reaches the top of the friction layer, then cloud will form. But because vertical development is restricted, the cloud tends to develop horizontally, giving layers of ST — that's Stratus — or SC — that's Stratocumulus. Because Stratus and Stratocumulus are formed in these conditions, they are known as turbulence cloud. There will normally be an inversion — a layer where temperature increases with height — above turbulence cloud, which caps the vertical development. Next, orographic cloud. When air meets a ridge of high ground, it is forced to rise. If the air is sufficiently humid, the condensation level will appear below the crest of the ridge, and cloud will form. If the air is stable and precipitation occurs, the air will descend on the lee side — the downwind side of the ridge — and the cloud base will be higher on the lee side than on the windward side. This descending air also generates warmer surface temperatures — that's the Föhn effect, a well-known phenomenon where the downslope wind is warmer and drier than the upslope wind. If the air is dryer, then the cloud base will be above the ridge, and lenticular cloud would result — those lens-shaped clouds that often form over mountains in stable, dry conditions. Now, lifting in unstable conditions can produce Cu — Cumulus — or Cb — Cumulonimbus clouds, and also thunderstorms if there is enough water vapour present. Strong winds with moist air can cause convective instability and produce Cb and thunderstorms. The Cb can be embedded in other cloud types, for example frontal cloud or turbulence cloud — meaning you might not see it clearly from a distance, but it's there, and that's a significant hazard for aviation. Finally, let's talk about convection cloud and the concept of critical temperature, also called the convective temperature. Before we deal with the formation of convection cloud, we need to understand this. Imagine air rising and cooling at the DALR — the Dry Adiabatic Lapse Rate, which is about 3°C per 1,000 feet — at three different times: 0700, 0800, and 0900 hours. In the first two ascents, the air cools to the environmental temperature before it reaches its dew point — so no cloud forms. But at 0900, the air cools to its dew point, cloud forms, and the ELR — the Environmental Lapse Rate — allows the air to continue rising, now cooling at the SALR — the Saturated Adiabatic Lapse Rate, which is slower, about 1.5°C per 1,000 feet — and forming Cumulus-type cloud. The critical temperature is the surface temperature that must be reached for this to happen — the temperature at which convection will produce cloud. That's the core of how vertical motion creates cloud, from the condensation level through turbulence, orographic lifting, and convection.