Significant Weather and Wind Charts — Page 505, Lesson 485

I want to walk you through the UK and NW Europe significant weather and wind charts, specifically the forms 215 and 415, and how they present weather information for flight planning. First, let's talk about the validity period. These charts are produced for a 9-hour period of validity. That period starts 4 hours before the forecast time and extends to 5 hours after it. So if the forecast time is, say, 12:00 UTC, the chart is valid from 08:00 UTC to 17:00 UTC. Form 215 is produced for the UK, and form 415 is produced for NW Europe. There are also spot wind and temperature charts. These are valid for 6-hour periods centred on the same times. Form 214 covers the UK, and form 414 covers NW Europe for those spot wind and temperature charts. Now, let's focus on forms 215 and 415. The presentation on these charts is somewhat different to the WAFS significant weather charts you might have seen before. However, the same symbology is used, along with an extended set of abbreviations. So the symbols for things like thunderstorms, fog, or fronts will look familiar, but there are additional abbreviations specific to these charts. The chart is divided into lettered areas. These areas are delineated with wavy lines on the chart. For each area, the surface visibility, weather, cloud information, and the altitude of the 0°C isotherm are tabulated on the right-hand side of the chart. The 0°C isotherm is the altitude where the temperature is freezing, which is critical for icing considerations. Areas can be subdivided. For example, area B on this chart is subdivided when there are minor changes that don't justify creating a completely separate area. This keeps the chart manageable while still showing local variations. Let me walk you through the conditions described for area A as an example of how to read these charts. For visibility and weather in area A: Generally, visibility will be 15 kilometres with nil weather or light rain. But there are variations across the area. In less than 50% of the area, visibility will be 7 kilometres in moderate rain or light rain and drizzle. In less than 25% of the area, visibility will be 3000 metres in moderate rain and drizzle and mist. And in less than 25% of the area, visibility will be 800 metres in fog or drizzle and hill fog on some hills. Also, less than 50% of the area will experience hill fog. The phrase "ISOL" and similar abbreviations are used here, and we'll interpret those shortly. Now for cloud in area A: There are 5 to 8 oktas of Altocumulus and Altostratus. Oktas are eighths of the sky covered. So 5 to 8 oktas means more than half to fully covered. These clouds have moderate icing and turbulence, extending from a base at 8000 feet to tops that are above 10,000 feet. The chart shows "xxx" where the exact top might be omitted or variable. Additionally, there are 3 to 7 oktas of Cumulus and Stratocumulus. Locally, at the front, this can be 8 oktas. These clouds have moderate turbulence from a base of 1500 feet to 3000 feet, and also from 4000 feet to 10,000 feet. So there are two turbulent layers. In less than 50% of the area, there are 3 to 7 oktas of Stratus with a base at 400 feet. However, in fog, the stratus base is on the surface. The tops of this stratus are 1000 to 1500 feet. The freezing level in area A varies from 8000 feet to above 10,000 feet. This is important because any cloud above the freezing level can contain supercooled water droplets, which cause icing. A critical note: All vertical positions given—cloud bases, tops, freezing level—are above mean sea level, not above ground level. So if you're flying over high terrain, you need to account for that. Finally, the arrows on the chart indicate the direction and speed of movement of the fronts and weather areas. If an arrow says "Slow," that means the speed is less than 5 knots. When you plan a flight, you must account for the movement of these weather systems. A front moving at 15 knots could shift the weather significantly during your flight time.