Message posted by JB737 on May 25, 2007 at 18:58:37 PST:
In the "more info & photos" section of DLR, under "Radio Beacons", there is a "Groom Lake VOR" recording made on 8/7/2005 around 7pm from Tikaboo Peak. It states that the Morse Code identifier is "MCY for Mercury?" and the weather report only says "local" for the location. I listened to the recording, then input the weather parameters into a density altitude calculator, after appropriate conversions of units (some are English, some Metric, etc.) I used the calculator at http://wahiduddin.net/calc/calc_da.htm It shows that the weather report is for a location at approximately 4282 feet elevation. A parametric study using the density altitude calculator shows that if the given density altitude of 6900 was simply a precise rounding to the nearest hundred, a field elevation between 4241 and 4323 would be implied. And if they used truncation rather than rounding, then an elevation up to 4363 would also give 6900 density altitude. But there are other uncertainties, including that the weather station might not be at the same elevation as the runway, plus there is roundoff or truncation on temperature, dewpoint, and altimeter settings...so I'd add another 50 or 100 feet of tolerance, and say it might have been a forecast for any location between 4150 and 4450 feet elevation. The Groom Lake runways, according to Google Earth, are between 4438 and 4482 ft elevation. So it's possible that it is Groom Lake, but it's far from enough from the 4282 nominal calculation, that we need to check if there are other sites which are a better match. I'll also examine whether it might be intentionally mis-stated for deception, or might have a simple/mundane explanation for the discrepancy. It pretty well rules out Mercury, NV, whose elevation is generally given as 3310ft (e.g., per noaa.gov), while Wikipedia shows Desert Rock Airport serving Mercury as being at 3314 ft. And to make matters even nicer, I couldn't find any other airport in NV much closer than Reno, with an elevation in the right ballpark. As for possible deception, I took a quick look at the implications if the discrepancy was an intentional error. A field elevation of 4446 (true for Groom) would give a density altitude of exactly 7100. Understating the density altitude by 200 feet, from 7100 to 6900, would tend to decrease the safety margins related to runway length. Runway length is not an issue at Groom, so despite the appearance of a safety problem, there actually is none, even if a pilot did not know about, or forgot his training (e.g., "we add 200ft to the density altitude, so that different field elevations are calculated depending on weather, if anyone is spying"). And even if the numbers are intentionally wrong, it is entirely possible that the density altitude is dead-on accurate for safety/liability reasons, and some other parameter (temp, dewpoint, temp-altitude profile assumed in the calculation, etc.) is fiddled with for deception. More likely, there is a mundane explanation. Many are possible, including that the temperature might be updated more often than the density-altitude calculation is. Unfortunately, this simplest explanation would have required the temerature to rise by 3.3 degrees since the density altitude calculation was done....something not likely to be happening at 7pm. Oh well, it was a try. Mundane explanations are still possible, just not that one. Nothing is ever easy to precisely figure out about this place! But still.... Congrats to whoever recorded this weather report. It appears to have been the real deal, an automated weather report for Groom Lake....albeit with enough of a discrepancy to warrant further investigation. JB737
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