Temp's and QNH's in a TAF
Thread Starter
Temp's and QNH's in a TAF
Hi guys,
Can anyone explain why the first forecast Temp and QNH given in a TAF are only valid for 90mins while the other three sets of forecast values are valid for 3 hours?
Can anyone explain why the first forecast Temp and QNH given in a TAF are only valid for 90mins while the other three sets of forecast values are valid for 3 hours?
I struggled hard with getting my head around this one until someone told me to think about it in 90 min blocks. Eg; The 2nd QNH lasts 90 mins before and after the time.
A diagram makes more sense
TAF valid from 0100.
T 24 20 18 24
0100 24 0230 20 0530
|----------|---------!----------|
See how it's 90 mins either side of the middle time? Hopefully this helps.
If you are asking why they do it this way? No idea, convention I guess.
A diagram makes more sense
TAF valid from 0100.
T 24 20 18 24
0100 24 0230 20 0530
|----------|---------!----------|
See how it's 90 mins either side of the middle time? Hopefully this helps.
If you are asking why they do it this way? No idea, convention I guess.
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TAF and METAR
The times on a TAF are hours only, because it is a prediction onto the future. No Met man (unless he has heavenly powers) can "see" that accurately into the future. The temp and QNH forecast therefore covers a block where the start time is the 'earliest' this phenominon will happen (before the published hour) and the end time is the 'latest' it is predicted to happen (slightly after the published time)
Remember: The future is the worlds best kept secret.
A METAR, on the other hand, is recent 'history'. It has happened already. It was physically measured during the 10 min prior to its publication time, on the hour e.g. 0800 UTC and, at International airports also on the 1/2 hour e.g. 0800, 0830, 0900, 0930 etc
A SPECI will give you the exact time of observation, hours and minutes, because it is 'history' also, measured physically or observed.
Read the AIP Chapter 3.5 MET section. You will now understand it better..
Remember: The future is the worlds best kept secret.
A METAR, on the other hand, is recent 'history'. It has happened already. It was physically measured during the 10 min prior to its publication time, on the hour e.g. 0800 UTC and, at International airports also on the 1/2 hour e.g. 0800, 0830, 0900, 0930 etc
A SPECI will give you the exact time of observation, hours and minutes, because it is 'history' also, measured physically or observed.
Read the AIP Chapter 3.5 MET section. You will now understand it better..
Last edited by Nirak; 17th Mar 2011 at 12:02. Reason: Spelling mistake
Thread Starter
not sure what your getting at Nirak? i understand these values are only a "prediction", that's the reason i called them "forecast values"
What i'm asking is why the first of the 4 values given is only valid for 90mins? This would mean that in a 12hour TAF the last 90mins of it's validity period would has no valid "forecast" Temp or QNH.
Seeing as most of the TAF's i see are valid for a 12 hour period I thought this was an topic.
What i'm asking is why the first of the 4 values given is only valid for 90mins? This would mean that in a 12hour TAF the last 90mins of it's validity period would has no valid "forecast" Temp or QNH.
Seeing as most of the TAF's i see are valid for a 12 hour period I thought this was an topic.
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E.g. the 0900 TAF:
The forecasted Temp and QNH can happen from as early as 0730 onwards but can also occur as late as 1030. The prediction covers the three hours bridging / saddling the forecasted time.
With the last Temp and QNH: It can happen from 90 min before the 12 h TAF ends, but it can also happen as late as 90 min after the end (13,5 hrs from the start time of the TAF), but that will then be the updated start time of the new 12 h TAF (TAF's are updated every 6 hrs and therefore overlap, but the futher in the future you go, the more inaccurate and vague they become.
That is why you have to fuel and plan for an alternate that has much better wx and less chance of a mishap. TAF's are for planning.
During your flight you compare the predicted with the actuals (METAR) to determine how accurate the Metman was, and if needed replan, refuel, re-route or divert.
If you look at the TAF:
TAF YXXX 110835 110921 ....
TAF was finalised and published on the 11th of the month at 0835 UTC
covering the period 0900 UTC to 2100 UTC
0835 UTC is the exact time it was published by the Metman (hours and min)
The actual TAF period is just in hours because it can happen either slightly before or slightly after, no Metman can predict right down to the minute, not humanly possible. That is why there is that "bridging / saddling" time covering 90 min before and after.
The forecasted Temp and QNH can happen from as early as 0730 onwards but can also occur as late as 1030. The prediction covers the three hours bridging / saddling the forecasted time.
With the last Temp and QNH: It can happen from 90 min before the 12 h TAF ends, but it can also happen as late as 90 min after the end (13,5 hrs from the start time of the TAF), but that will then be the updated start time of the new 12 h TAF (TAF's are updated every 6 hrs and therefore overlap, but the futher in the future you go, the more inaccurate and vague they become.
That is why you have to fuel and plan for an alternate that has much better wx and less chance of a mishap. TAF's are for planning.
During your flight you compare the predicted with the actuals (METAR) to determine how accurate the Metman was, and if needed replan, refuel, re-route or divert.
If you look at the TAF:
TAF YXXX 110835 110921 ....
TAF was finalised and published on the 11th of the month at 0835 UTC
covering the period 0900 UTC to 2100 UTC
0835 UTC is the exact time it was published by the Metman (hours and min)
The actual TAF period is just in hours because it can happen either slightly before or slightly after, no Metman can predict right down to the minute, not humanly possible. That is why there is that "bridging / saddling" time covering 90 min before and after.
Last edited by Nirak; 17th Mar 2011 at 13:07. Reason: Adding last paragraph
I always believed they were the forecast values for that particular time. So in a 0000 TAF the first set is the forecast values at 00, the next set at 03, 06 and 09. Then you can interpolate the values in between. I couldn't find a reference in AIP or on the BoM website.
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The principle is explained somewhere in that chapter.
I do not have an AIP here with me at the moment, will have to search a bit for the exact page ref.
I do not have an AIP here with me at the moment, will have to search a bit for the exact page ref.
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I'm not sure I understand nirak's point about the forecaster being human. But I will disagree about interpolating between the QNHs. Each forecast QNH is valid for the specified times. There is no interpolating.
Someone probably has more details, but to add something else to the discussion, the temp/QNH section of the TAF format changed a couple of years ago from each temp/QNH being valid for 3 hours starting at the beginning of the TAF's validity, to the present system where the first is valid for 90 minutes and the rest being valid for 3 hours each.
The reasoning, I believe, is that under the previous system, the potential for error was somewhat greater. By way of example, imagine the QNH is decreasing from 1013 at a rate of 1hPa per hour. At the end of each 3 hour block, there is a 3hPa error. Under the new system, the forecast is being made for the central time in each 3 hour block, so the error in that example would be only +/- 1 hPa.
As for the first one only being 90 minutes and the last 90 minutes of 12 hour TAF being unforecast, it's just how it has to be in order to maintain the +/- 1.5 hour system. The final 90 minutes is not an issue for most operations because you need to obtain a forecast within 60 minutes of your departure and few of us are still flying 11 hours later. And if you are, it is likely you could obtain an updated forecast in flight to cover that period.
I disagree with the suggestion about the future being impossible to predict and all that stuff. When it comes to these QNHs, it really is fairly predictable and as a result of that, we are able to fly instrument approaches using only a forecast QNH and the only penalty is adding a 100ft (3hPa) buffer. So with all the risk assessment that goes into instrument procedure design, they have decided they will always be within 3hPa of the correct QNH. Not bad. Even the area forecast QNH is guaranteed within +/- 5.
Someone probably has more details, but to add something else to the discussion, the temp/QNH section of the TAF format changed a couple of years ago from each temp/QNH being valid for 3 hours starting at the beginning of the TAF's validity, to the present system where the first is valid for 90 minutes and the rest being valid for 3 hours each.
The reasoning, I believe, is that under the previous system, the potential for error was somewhat greater. By way of example, imagine the QNH is decreasing from 1013 at a rate of 1hPa per hour. At the end of each 3 hour block, there is a 3hPa error. Under the new system, the forecast is being made for the central time in each 3 hour block, so the error in that example would be only +/- 1 hPa.
As for the first one only being 90 minutes and the last 90 minutes of 12 hour TAF being unforecast, it's just how it has to be in order to maintain the +/- 1.5 hour system. The final 90 minutes is not an issue for most operations because you need to obtain a forecast within 60 minutes of your departure and few of us are still flying 11 hours later. And if you are, it is likely you could obtain an updated forecast in flight to cover that period.
I disagree with the suggestion about the future being impossible to predict and all that stuff. When it comes to these QNHs, it really is fairly predictable and as a result of that, we are able to fly instrument approaches using only a forecast QNH and the only penalty is adding a 100ft (3hPa) buffer. So with all the risk assessment that goes into instrument procedure design, they have decided they will always be within 3hPa of the correct QNH. Not bad. Even the area forecast QNH is guaranteed within +/- 5.
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Now that that's sorted, can anyone explain howthese ALT Correct vs Temp. charts in the front of DAPs work? I remember when the old DAPs had a table in the front - much easier. Then they were taken out, and now we have this new "improved" version.
Does it apply to precision approaches only, or to all approaches?
The example talks about not only adjusting the DA but also the IAF. What about the intermediate ALTs?
How will ATC know what adjustments the pilot has made to the IAF ALT when cleared to the IAF for an approach?
How will other pilots know what ALT adjustments other pilots have made to their IAF ALTs when in Class G?
Does it apply to precision approaches only, or to all approaches?
The example talks about not only adjusting the DA but also the IAF. What about the intermediate ALTs?
How will ATC know what adjustments the pilot has made to the IAF ALT when cleared to the IAF for an approach?
How will other pilots know what ALT adjustments other pilots have made to their IAF ALTs when in Class G?
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Oh, I forgot ! Pilots walk on water, therefore they are not human and can predict the future right down to the minute!!!
I guess the Metman's forecast is always right, pilots complain about inaccuracies purly as a matter of it is a nice chance from complaining about the airline food
I have to deduct then that the Japanese are human too, the fact that they could not predict the earthquake beforehand
Getting an inflight updated forecast will make the data more accurate, because accuracy deteriorate the further you predict into the future.
Getting an actual from the tower or the ground staff during the descent will be even more accurate, leaving you with only temperature, instrument and calibration error on the altimiter only
No heavenly powers needed, because 100' is added as a safety margin for determining your minima. Sorry to make your godly appearance evaporate in the mist
I guess the Metman's forecast is always right, pilots complain about inaccuracies purly as a matter of it is a nice chance from complaining about the airline food
I have to deduct then that the Japanese are human too, the fact that they could not predict the earthquake beforehand
Getting an inflight updated forecast will make the data more accurate, because accuracy deteriorate the further you predict into the future.
Getting an actual from the tower or the ground staff during the descent will be even more accurate, leaving you with only temperature, instrument and calibration error on the altimiter only
No heavenly powers needed, because 100' is added as a safety margin for determining your minima. Sorry to make your godly appearance evaporate in the mist
Last edited by Nirak; 18th Mar 2011 at 00:40.
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Off topic I know, but can somebody answer this:
If planning an IFR flight to a destination that does NOT have a TAF, is there a requirement to plan for an alternate (for the reason that there is no TAF)?
Assume that the destination in question is one of those that NEVER have TAFs issued for it (as opposed to those destinations that do have TAFs, but whose coverage is for less than 24 hours).
If planning an IFR flight to a destination that does NOT have a TAF, is there a requirement to plan for an alternate (for the reason that there is no TAF)?
Assume that the destination in question is one of those that NEVER have TAFs issued for it (as opposed to those destinations that do have TAFs, but whose coverage is for less than 24 hours).
FGD135 ...
AIP GEN 57.1.3 (page 88 of the PDF from the web)
When an aerodrome forecast is not available or is "provisional", the pilot in command must make provision for a suitable alternate that has a firm forecast.
AIP GEN 57.1.3 (page 88 of the PDF from the web)
When an aerodrome forecast is not available or is "provisional", the pilot in command must make provision for a suitable alternate that has a firm forecast.
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anotherwit
You are welcome to do so, mate. The choice is yours, just remember, terra firma is down there somewhere below.......'terain, terain, pull up, pull up'
You are welcome to do so, mate. The choice is yours, just remember, terra firma is down there somewhere below.......'terain, terain, pull up, pull up'
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Nirak
I have to deduct then that the Japanese are human too, the fact that they could not predict the earthquake beforehand
