Significant Weather and Wind Charts — Page 505, Lesson 487

I want to walk you through the definitions and abbreviations used on significant weather charts, and then we'll look at how to extract wind and temperature data from the Forms 214/414. Let's start with the definitions that describe how much of an area is affected by weather. These come from the F215/415 forms and are adopted in the UK by the Aviation Met Authority in the CAA, used by the Met Office. There are four key terms. First, Widespread, abbreviated WDSPR. This implies conditions affecting many places that will be difficult to avoid. It means greater than 50% of the area is affected, and it's used for both non-convective and convective types of weather. Second, Frequent, abbreviated FRQ. This is used only for convective types. It means within a particular area there is little separation between phenomena, and the spatial coverage is greater than 50% of the area forecast to be affected by that phenomenon. These features will be difficult to avoid. Third, Occasional, abbreviated OCNL. This is used when an area consists of well-separated features forecast to affect an area with a maximum spatial coverage between 25% and 50% of the area concerned. These features can be avoided by users. Fourth, Isolated, abbreviated ISOL. This is used when an area consists of individual features forecast to affect an area with a maximum spatial coverage of less than 25% of the area concerned. These features can be easily avoided. Now, there are other abbreviations you may encounter on these charts. Let me run through them: - BLW — Below - BTN — Between - CIT — Near or over large towns - CLD — Cloud - COR — Correction - COT — At the coast - LAN — Inland, or over land - LCA — Locally - LSQ — Line squall - LV — Light and variable, relating to wind - SEV — Severe - SFC — Surface - VAL — In valleys - VRB — Variable - VSP — Vertical speed - WRNG — Warning - WS — Windshear - WSPD — Wind speed Now let's move to the Forms 214/414. These forms give spot wind and temperature information at selected locations. As stated on the form, the heights are in thousands of feet above mean sea level. To extract winds and temperatures for a route, you will need to interpolate between the heights and the appropriate data boxes. Let me give you an example. Say you're flying a route at Flight Level 070 over Bristol, which is EGGD. Unless there is exceptionally high or exceptionally low pressure, there will be no need to adjust the height for the difference between QNH and SPS — that's the standard pressure setting. Now, if you interpolate between the data from the boxes for 5230 North 0230 West and 5230 North 0500 West for FL140, you get a temperature of minus 7 degrees Celsius, to the nearest integer, and a wind velocity of 260 degrees true at 35 knots, to the nearest 5 degrees and 5 knots. Another question that may arise is determining the stability between two levels. For example, at 50 North 0230 West, what is the stability state between FL020 and FL050? Here the temperature reduction is 6 degrees over 3000 feet, giving a lapse rate of 2 degrees Celsius per 1000 feet. So at that location between those levels, the atmosphere is conditionally unstable. In a situation where pressure patterns are changing rapidly — for example, when a fast-moving polar front depression is crossing the region — it would be prudent to interpolate between consecutive Forms 214/414, particularly towards the limits of the forecast periods. As we've seen, the speed of movement of systems is found on the Forms 215/415.