Clouds at high altitude airports

[Petraeus 118]

I know that there was a problem with this with the earlier products, but it looks as though it is still with us.At airports like Denver or Salt Lake City, that are situated around 4000 or 5000 feet altitude, when the METAR says there are clouds at 5000 or 6000 feet, the clouds appear to be sitting on the ground. The METAR height means, of course, height above the ground, not above sea level, but it looks as though ASA is interpreting the number as a height above sea level. Is my assumption correct?

[JSkorna 137]

Hi,You'll also see this with the weather in FS from time to time. It is a limitation of FS at higher altitudes.Thanks,

[Petraeus 118]

Hi,You'll also see this with the weather in FS from time to time. It is a limitation of FS at higher altitudes.Thanks,

Why is it a limitation of FS? Surely, ASA is setting the cloud heights - therefore it could set them based on the station altitude?

[JSkorna 137]

Hi,It is a bug in FSX and though we try to work around as many bugs as we can, there are times when we just cannot do so. This happens to be one of those times.Thanks,

[Petraeus 118]

Hi,It is a bug in FSX and though we try to work around as many bugs as we can, there are times when we just cannot do so. This happens to be one of those times.Thanks,

I'm not sure where the FSX bug comes in.Perhaps my understanding is incorrect here, but doesn't ASA tell FSX at what height to generate clouds, at each station? If that is so, and clouds are reported as 4500 feet at KSLC, isn't it just a matter of telling FSX to generate clouds at 4200 (station height) + 4500 = 8700 feet?

[JSkorna 137]

Hi,We supply the data to FSX just as Real Weather does, and then FSX uses the data. We don't tell FSX to add anything to the data.Thanks,

[Petraeus 118]

Hi,We supply the data to FSX just as Real Weather does, and then FSX uses the data. We don't tell FSX to add anything to the data.Thanks,

But if you did add the station height to the data, wouldn't that correct the problem? If in the above example you supply FSX with a cloud height of 4500 feet, then that's exactly what it will draw - a cloud at 4500 feet above sea level, which will appear to be ground level at Salt Lake City. On the other hand, if you ask FSX for a cloud, as per the above example, at 8700 feet, then it will appear 4500 feet above the KSLC ground, which is what the METAR is reporting.Essentially, the problem seems to be that FSX draws its clouds at a specific height above sea level, whereas the METAR, and the data used by ASA, refers to height above the level of the reporting station. So by simply adding the station height to the METAR height, ASA could tell FSX the correct height above sea level, and the clouds would be shown where they are in real life. Or am I missing something?I keep quoting Salt Lake City, because that's where the problem is most exaggerated*, but the above logic should apply to all reporting stations, whatever their altitude.I don't know what logic FSX Real Weather uses, but if it treats METAR height as height above sea level then it is equally wrong, and there is no need to perpetuate its error.* (with the exception of Mexico City at 7400 feet, or other high-altitude airports I can't remember at the moment :()

[JSkorna 137]

Federal Meteorological Handbook No. 1 CHAPTER 9 SKY CONDITION"At mountain stations, clouds below the level of the station may be observed."

[Petraeus 118]

"At mountain stations, clouds below the level of the station may be observed."

Yes that's true, at the top of a mountain, you can often see clouds below you. In the hypothetical case of a mountain top airport, then presumable the METAR would report cloud height as negative. But I don't see how that is relevant to the present discussion.Denver International and Salt Lake City International are not on the tops of mountains, and when they report a cloud height of 4000 feet, then according to the rules for decoding a METAR, that means that the clouds are 4000 feet above the airport, not above sea level. If there are clouds on the deck at KDEN or KSLC, then they will be reported as something like 100 or 200, which means 100 or 200 feet above the ground. ASA is misinterpreting the METAR height above station level as an altitude above sea level, and therefore incorrectly showing clouds lower than they should be for any station above sea level. The simple solution would seem to be to tell FSX to draw the clouds at an altitude that is the sum of the cloud height and station height. Is there anything incorrect in what I'm saying?

[aua668 5]

Hi,Cloud heights are reported as height over/in near vicinity of airfield in feet. That's fact. Below the point of measurement there is typically ground. Therefore the minimum value for cloud height is 000 - not negative, as there are no clouds below/near the airfield :( More details on METARs can be found here

FEW008 SCT012 BKN020 .. amounts and height-of-base of clouds over/in near vicinity of airfield. SKC (some use CLR) = no cloud; FEW = 1 or 2 eighths cover; SCT = 3 or 4 eighths cover; BKN = 5, 6 or 7 eighths cover & OVC = 8/8 cover. Heights are given in 100's of feet above airfield level, thus 008 = 800ft, 012 = 1200ft, 020 = 2000ft etc. [ When there is more than one layer of cloud, the convention for inclusion of cloud groups is .... 1): the lowest layer2): the next highest layer, covering 3 oktas or more of the sky (SCT or more) 3): the next highest layer, covering 5 oktas or more of the sky (BKN or more) 4): any CB not already included by these rules - the group being inserted in 'natural' height order.]When fog or heavy snow is occurring, and it is not possible to determine cloud structure, then these groups are replaced by VVhhh or VV///, where either the vertical visibility can be determined (hhh) in hundreds of feet, or impossible to determine (///)

But if you have to specify absolute or relative height values to the FlightSim, should be defined in the documentation. Anyone, who knows this ?RgdsReinhard

[Petraeus 118]

Hi,Cloud heights are reported as height over/in near vicinity of airfield in feet. That's fact. Below the point of measurement there is typically ground. Therefore the minimum value for cloud height is 000 - not negative, as there are no clouds below/near the airfield :(

I was talking about reports for some (probably fictitious) airfield near the top of a mountain, where there could be clouds in the viciniity that are lower than the airfield itself. Perhaps that skiing place in the French Alps?However that is very much a side issue. You do in fact quote the important fact regarding METARs.Heights are given in 100's of feet above airfield levelTherefore clouds reported as being at 4000 feet above a 5000 foot high airfield should be shown as being 9000 feet above sea level, not 1000 feet below the ground.

[JSkorna 137]

Hi All,1. Cloud height is reported in AGL so there is no reason to add anything to the ASA data.2. It is possible for low level clouds at high altitude airports. None of the data would have negative values.3. If you are seeing low level clouds where you think there should not be any, please copy and paste the ASA data into a post here.Thanks,

[Petraeus 118]

I am sorry to have to say this, but I am most unhappy with the response to the problem that I am reporting. I have attempted three or four times to explain the problem, and its solution, in very clear and deliberate language. All I am getting in return is debating points. I see no serious attempt to engage with the case I am outlining.I will explain the problem, and what I see as the solution, once more, step by step. If you think that my assumptions are incorrect at any point, or my logic, then please correct me at that specific point.I will start with the assumption that we are both agreed upon.

1. Cloud height is reported in AGL

(To be absolutely clear, the METAR report shows the cloud base as feet above ground level, and not above sea level.)2. ASA supplies this METAR figure to FSX, but FSX interprets this figure as an altitude above sea level and so it generates the clouds at that height above sea level, not above ground level.3. For any airport above sea level, this will result in the cloud appearing to be lower, relative to the ground, than it is reported by the METAR.4. For very high altitude airports, this error will be very pronounced. To take KSLC as an example, at 4200 feet above sea level, if the METAR reports clouds at 5000 feet, then that means 5000 feet above the ground at Salt Lake City. But if ASA gives that figure to FSX, FSX will generate clouds at 5000 feet above sea level, which is only 800 feet above the ground at Salt Lake City.5. The solution to the problem in this instance would be for ASA to add the KSLC height to the cloud height of 5000 feet, which FSX would then use to generate clouds at 9200 feet, which is where they are in real life, and in the METAR. The generalised solution for all locations would be to supply FSX with the figure of (cloud height + station height.)And finally, to illustrate the problem graphically, here is a shot from KSLC. The lowest clouds are reported at 3000 feet, then 5000 feet etc. The lowest clouds should therefore be 3000 feet above the ground. The clouds you can see are 100 feet, 200 at most, above the ground. In fact they are below the control tower which is 320 feet above the ground.

[JSkorna 137]

Hi,So what you are purposing is that we display the proper data in ASA, but then before sending that data to FSX we gather the altitudes for every data station and add that altitude value to every cloud layer?I wish for you to try with DWC off and see if that makes a difference.Thanks,

[Petraeus 118]

Hi,So what you are purposing is that we display the proper data in ASA, but then before sending that data to FSX we gather the altitudes for every data station and add that altitude value to every cloud layer?

Yes, if my diagnosis of the problem is correct. But not every data station height added to every cloud layer. Only the station heights and cloud layers within a specific locality. In other words, just the cloud layers above each particular station. You know better than I how ASA works, and what would be the most efficient way of doing that.DWC is off. I don't use it because it slows up Radar Contact.