Let's start at the top of the chart, where you'll find the WAFC reference — Page 497, Lesson 480

I want to walk you through the details of a significant weather chart — specifically the one shown in Figure 27.1 of your meteorology manual. Let's start at the top of the chart, where you'll find the WAFC reference. WAFC stands for World Area Forecast Centre. The reference given here is PGDE14, and right next to it is the date and time of issue: 280000, with the Z indicating Zulu time, which is the same as UTC. So this chart was issued on the 28th of the month at 0000 Zulu. Below that, you'll see the originating agency and the issuing agency, and in this case both are WAFC London. Then there's a reminder printed on the chart: these are fixed time charts, valid only for the time stated. That stated time here is 0000 UTC on 29 APR 2013. So the chart was issued at midnight on the 28th, and it's valid for midnight on the 29th — a 24-hour forecast. Now, an important point: unlike other SIGWX charts — that's significant weather charts — this particular chart covers levels from FL100 up to FL450. FL means flight level, which is altitude in hundreds of feet measured on the standard pressure setting of 1013.25 hPa. So FL100 is 10,000 feet, and FL450 is 45,000 feet. That's the vertical coverage of this chart. Let's look at the thick black lines on the chart. Those are jet streams. For example, the jet stream to the west of the UK has a speed of 100 knots southwest of Iceland, then it rises to 120 knots to the northwest of Ireland, and then drops back to 100 knots. The core of that jet stream is at FL310, with speeds exceeding 80 knots — which is 40 metres per second — extending from FL250 up to FL370. The start and end of the jet stream lines on the chart occur where the wind speed exceeds 80 knots. And by convention, the wind speed triangles and feathers on the jet stream lines point towards the low temperature — that's the cold side of the jet. Now, to the south and southwest of the UK, you'll see jet streams that have an enclosed area surrounded by a dashed line, and inside that dashed line there's the number 3 in a square box. That symbol indicates an area of clear air turbulence, abbreviated CAT. This area is amplified in the legend on the left side of the chart, which advises moderate CAT extending from FL240 to FL360. Next, extending from southeast England to the Northern Baltic, there's an area designated by a wavy line. That wavy line marks an area of significant weather. Over the Southern Baltic, you'll see a box with an arrow attaching it to this area. That box contains the details: moderate turbulence extending from below FL100 — which is the lowest level of this chart — up to FL140, and moderate icing extending over the same altitude band, from below FL100 to FL140. Between Sardinia, Spain, and France, there's another area of significant weather attached to a box. That box tells us: within the area we have individual cumulonimbus embedded in other cloud, with the cloud base below FL100 and tops up to FL320. No further information is given in that box, but the middle box of the legend on the chart advises what you should expect from CB — cumulonimbus clouds. Let me also cover the abbreviations associated with CB and TS — that's thunderstorm. Here they are: ISOL means isolated, referring to individual cumulonimbus clouds. OCNL means occasional, which is well separated. FRQ means frequent, indicating little or no separation between CB. And SQL means squall, which is a line of CB with little or no separation between them. Over the UK, there's a rectangular box with the number 350. That shows the height of the tropopause at that location is FL350. West of northern Spain, there's a similar box but with an upward-pointing arrow, indicating a tropopause high — meaning the tropopause is higher than usual in that area. Finally, you'll recognize the letters on the chart as the locations of major cities or aviation locations. The full symbology for significant weather charts is given in Figure 27.2 and Figure 27.3 of your manual.