ATPL · Meteorology · Meteorology Atpl Ground Training Series 2014 lesson — Page 497, Lesson 475

I want to walk you through the start of Chapter 27, which is all about Significant Weather and Wind Charts. This is a core part of your meteorology training as a professional pilot, because these charts are how we get a big-picture, pre-flight view of the weather that will affect our route. Let's begin with the very first thing you see on this page, which is actually a set of examples showing how weather information is formatted. These are not full charts yet; they are examples of the text that might appear in a weather report or forecast. I'll read them to you as they are written, and then we'll break down exactly what each part means. The first example reads: "variable in direction and speed; averaging 8 kt; with a visibility of 10 km or more, and a cloud base of 2500 ft above aerodrome level". So, this is describing a surface wind that is variable—meaning its direction is not steady, and its speed is also not steady. But it gives us an average speed of 8 knots. The visibility is reported as 10 kilometres or more, which is good, and the cloud base—the lowest height at which you see cloud—is at 2500 feet above the aerodrome level, which is the height above that specific airport's elevation. Now, the second example is a bit different. It says: "Valid from the 13th at 0600 UTC to the 13th at 1500 UTC; surface wind will be variable at 8 kt, with a visibility 10 km or more; 3-4 oktas of cloud with a base of 2500 ft above aerodrome level". Notice the key difference here: this is a forecast, not an observation. It starts with a validity period: "Valid from the 13th at 0600 UTC to the 13th at 1500 UTC". That means this forecast is good for that 9-hour window. UTC stands for Coordinated Universal Time, the standard time used in aviation worldwide. The surface wind is forecast to be variable at 8 knots, visibility 10 kilometres or more, and now we get a cloud amount: 3 to 4 oktas. An okta is a unit of cloud cover, where the sky is divided into eighths. So 3 to 4 oktas means about three-eighths to half the sky is covered by cloud, with the base at 2500 feet above aerodrome level. The third example is an observation. It reads: "Observed at 0600 UTC; the surface wind was variable in direction and speed; with a visibility of 10 km and a cloud base of 2500 ft above ground level". This is a report of what was actually measured at a specific time: 0600 UTC. The wind was variable, visibility exactly 10 kilometres, and the cloud base is given as 2500 feet above ground level, or AGL. Note the subtle difference from the forecast: the forecast said "above aerodrome level", and the observation says "above ground level". In practice, for an aerodrome, these are essentially the same, but the terminology is precise. So, to summarise what we have here: you are seeing three different ways weather information is presented. One is a general description of conditions, one is a forecast with a validity time, and one is an observation at a specific time. Each one gives you the wind (direction and speed, or variability), visibility, and cloud base. The forecast also adds cloud amount in oktas. Now, after these examples, you see the chapter title: "Significant Weather and Wind Charts". This is the main topic we are about to dive into. The table of contents shows the sections we will cover: an introduction, symbols for significant weather, tropopause, freezing level, fronts and convergence zones, low level forecast information, and then specific forms like Forms 215/415 and 214/414. You also see terms like Widespread (WDSPR), Frequent (FRQ), Occasional (OCNL), and Isolated (ISOL). These are abbreviations used on the charts to describe how widespread or frequent certain weather phenomena, like thunderstorms, are. We will get to each of those in detail as we move through the chapter. For now, I want you to focus on the structure of those text examples. As a pilot, you will see this kind of information in METARs (the observations) and TAFs (the forecasts). Understanding that "variable" means the wind direction is not fixed, that "kt" is knots, that visibility is given in kilometres or metres, and that cloud base is in feet above either aerodrome level or ground level—these are the building blocks. And the concept of "oktas" for cloud cover is fundamental. Let's move on to the first proper section of the chapter.