Now let's talk about cloud amount — Page 199, Lesson 166

I want to walk you through the chapter on clouds. Let's start with the introduction. Clouds are like signposts in the sky — they give us warning of what the weather is doing or what it's likely to do. For aviation, clouds are a source of several hazards: turbulence, poor visibility, precipitation, icing, and lightning. Because of these hazards, you as a pilot must be able to recognize the different types of clouds and identify the hazards associated with each. There's a summary table at the end of the chapter that lists the properties and hazards for each cloud type. Now let's talk about cloud amount. Cloud amounts are reported in oktas — that's eighths of the sky. We assume the sky is divided into eight equal parts, and we assess how many of those eighths are covered by cloud. There are four standard reporting codes. FEW means 1 to 2 oktas. SCT — scattered — means 3 to 4 oktas. BKN — broken — means 5 to 7 oktas. OVC — overcast — means 8 oktas, the entire sky covered. Next, cloud base. The formal definition is: "That lowest zone in which the type of obscuration perceptibly changes from that corresponding to clear air haze to that corresponding to water droplets or ice crystals." In simpler terms, the cloud base is the height of the bottom of the cloud above ground — specifically, above official aerodrome level. Cloud ceiling is a related but distinct term. It is defined as "the height above aerodrome level of the lowest layer of cloud of more than 4 oktas." So if the lowest layer of cloud covers more than half the sky — 5 oktas or more — that height is the cloud ceiling. Cloud ceiling is also referred to as the main cloud base. Now, how do we measure cloud base? There are several methods. By day, we can release a balloon with a known rate of ascent. We note the time between release and the moment the balloon disappears into the cloud. From that time and the known ascent rate, we calculate the cloud base height. By night, we use an alidade. The alidade is positioned a known distance from a searchlight. The searchlight shines upward, and the alidade measures the angle above the horizontal of the glow of the searchlight on the base of the cloud. Using trigonometry — specifically, the known distance and the measured angle — we calculate the height of the cloud base. Then there's the cloud base recorder, also called a ceilometer. This is a device that uses a laser or other light source to determine the height of a cloud base. There are several types, depending on whether a normal light source or a laser light source is used. The first type uses a normal light source, and there are several versions. The optical drum ceilometer consists essentially of three components: a projector, a detector, and a recorder. The projector emits an intense beam of light into the sky. The detector is located at a fixed distance from the projector and uses a photoelectric cell to detect the projected light when it is reflected from clouds. There are two configurations of this type. In the fixed-beam ceilometer, the light is beamed vertically into the sky by the projector, and the detector is aligned at various angles to intercept the reflected light. In the rotating-beam ceilometer, the detector is positioned vertically, and the light is projected at various angles. In either case, trigonometry is used to determine the altitude of the clouds reflecting the light — using the angle at which the light is detected and the known distance between the projector and detector. The recorder is calibrated to indicate cloud height directly. So to summarise: we have three main methods for measuring cloud base — the balloon method by day, the alidade and searchlight method by night, and the ceilometer, which can use either a normal light source or a laser. Each gives us the height of the cloud base, which is critical information for your flight planning and operations.