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Qnh from a TAF
Flyingpom created the topic: Qnh from a TAF
I haven a question re QNH from a TAF. In your met book it states that the QNH values are valid for 90 min intervals so if we have a forecast from 0200 the TAF QNH values reading 1010 1011 1012 1013
Would read as
0200 - 0330 is 1010
0330 - 0630 is 1011
0630 - 0930 is 1012
0930 - 1230 is 1013
Now there is a question in the perf book ( Page 3.28 ) that states the QNH at time 0705 is 1009. From the logic described above shouldn't it be 1008??
It's not a typo actually. The AIP used to say that the temperature and QNH information was presented as four values, each valid for 90 minutes before and after the commencement of the TAF validity period and at three hourly intervals after that. That presentation was changed with the last revision of the AIP.
The temperature and QNH information is now presented as four 'spot' values starting at the commencement of the validity period and then at three hourly intervals. To get a value at any other time, you should interpolate between the 'spot' values.
I have amended the text books to reflect the new presentation, but unfortunately I can't do anything about the books that are already in circulation.
Funnily enough, I actually prefer the other method. At least it gave you the values you need and there was no educated guess work involved.
And thanks for that tip! Guess I'm still learning how to fully use the forum!
Oh, and sorry for the hasty 'it's a typo' response, Bob. I was under the impression that it was still being worked out the old way! And I guess that's true for flying pom as well. I literally got a box full of updates the other day, haven't had time to look at them yet!
Aviatordan wrote: Funnily enough, I actually prefer the other method.
Dan, you are officially mental
Seriously though, we've been shaking our heads about the +/- 90 minutes rule and breathed a great sigh of "Thank god for that" when the AIP changed. To me at least it feels more intuitive to interpolate between spot values. It's something you can do in your head rather than grabbing a bit of paper and checking +/- 90 minute ranges in the old format.
The linear interpolation may seem to be a pain but really it isn't something likely to cause a problem. No one is going to interpolate to decimal places and in nearly all cases it is immediately obvious what the intermediate values are going to be.
The funny thing is, this whole thing revolves around guesstimates made from a forecast based on best guesses
I guess I like the other method because it plagued me when I was first taught how to use it when I was doing my PPL a couple of years back. I had just finally gotten familiar with it and they take it away!
I guess you're right about it being easier to calculate in your head, though! It'll make things easier in the cockpit, that's for sure.
And you're right, the term forecast pretty much by definition is the weatherman's best guess.