the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Apr 2026
Upper Air Humidity from Automatic Aircraft Surveillance Data
Abstract. Upper air humidity information is under sampled in the current operational meteorological observing network. Radiosondes observations form the backbone, but radiosondes balloons are typically launched only once or twice per day to limit the costs. The number of aircraft humidity observations are low in Europe, because in Europe only a few aircraft are equipped with water vapour sensors.
In this paper a novel technique is presented to derive humidity information from aircraft Automatic Dependent Surveillance Broadcast (ADS-B) data, whenever an aircraft is descending or ascending. The retrieved virtual temperatures observations, averaged over a vertical layer of 500 m, have an accuracy between 0.5 K and 0.75 K when compared to European Centre for Medium Range Forecast (ECMWF). Using additional external temperature information, estimates of the specific humidity can be calculated with an accuracy of 3–4 g kg-1 and in some cases between 2–3 g kg-1 (that is, when more than 20 estimates are available at the same reference height within 20 minutes). Applying the method to measurements from the Falcon F20 French research aircraft SAFIRE shows that even a single aircraft can be used to derive high-quality virtual temperature information (observation error ≈ 0.5 K). Comparison with Aircraft Meteorological Data Relay (AMDAR) and radiosonde humidity showed similar statistics.
Since ADS-B data is received from all ascending or descending aircraft in the vicinity of an airport, a vast amount of upper air virtual temperatures could be made available, when ADS-B information is gathered by ADS-B receivers installed at, or nearby airports.
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CC1: 'Comment on egusphere-2026-717', Gert-Jan Marseille, 10 Apr 2026
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AC2: 'Reply on CC1', Siebren de Haan, 16 May 2026
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Dear Gert-Jan,
Thank you for your comments. Below you find my replies.
- " I could not find a definition for \delta_T in the text."
The virtual temperature is defined as T(1+ δ q) , with δ =0.608 and to obtain the right hand side of eq requires the calculation of the derivative to temperature/virtual temperature, and assume that both are close.
- 8 g/kg follows from eq line 127:
(2 /(288)2 / 0.608 2 )1/2 = 0.008 kg/kg
" I guess you can provide estimates of the error of derived specific humidity based on the equations in section 3? Or are these the blue lines in Figure 4?"
Yes the error is described in section 3. And no these are not the blue lines in fig. 4
" In figure 6, the lower right panel shows very good agreement for Tv, but much less so for q. Can you add some lines in the text to explain this discrepency, e.g. by refering to (the potential weaknesses of) the equations in section 3."
The weakness of deriving q from Tv lies in the fact that the error in temperature needs to be small to have some skill. This is explained in Section 3.
Kind regards,
Siebren de Haan
Citation: https://doi.org/10.5194/egusphere-2026-717-AC2
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AC2: 'Reply on CC1', Siebren de Haan, 16 May 2026
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RC1: 'Comment on egusphere-2026-717', Anonymous Referee #2, 10 Apr 2026
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The manuscript presents a novel technique of extracting real-time in-situ humidity observations from any ADS-B equipped aircraft during ascent and descent. In-situ humidity data is indeed very limited, since there are only 9 WVSS-II equipped aircraft currently in Europe and a bit more than a hundred in the USA, which makes this a technique of interest for potentially any met office, since it would provide a new source of global humidity data.
Going through the script, I have some questions regarding the error estimates for humidity and virtual temperature, and only minor issues elsewhere:
line 29: Enhanced Surveillance (EHS)
line 82: with a temperature of T_0 = 288.15 K
line 85: also define g_0
line 126: "using Taylor" is a bit misleading here, since Gaussian error propagation is used citing Taylor as a source, while line 139 says "applying again Taylors approximation". Even though Gaussian error propagation technically is a Taylor approximation, it wold be more clear to say "can be approximated via Gauß, according to Taylor (1997)" in line 126 and "and an estimate of the error can again be obtained via Gauß (Taylor 1997)" in line 139.
line 127: Explain here, how the \sigma^2_{T_\nu} term disappears. It seems like \sigma^2_{T_\nu} \approx \sigma^2_T was used here, which has to be stated in that case. Also the error should scale with 1/\delta^2 then, i.e.: \sigma^2_q \approx \frac{2}{\delta^2 T^2} \sigma^2_T
line 140: "(neglecting the last term)" actually all terms containing \beta_0 seem to be neglected. Please also elaborate on why those terms can be dropped.
Figure 2: "Two solutions are show: in blue denotes the solution" --> "Two solutions are shown: The blue lines denote the solution"
Figure 2: "The bottom two lines expresses the average difference " --> "The bottom two lines express the average difference"
line 213: "The resulting statistics are show" --> "The resulting statistics are shown"
Figure 6: This can be enlarged to a full page, since the individual Figures are quite small.
line 285: "MetoeFrance" --> "MeteoFrance"
Citation: https://doi.org/10.5194/egusphere-2026-717-RC1 -
AC1: 'Reply on RC1', Siebren de Haan, 11 Apr 2026
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-717/egusphere-2026-717-AC1-supplement.pdf
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AC1: 'Reply on RC1', Siebren de Haan, 11 Apr 2026
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Dear author,
Thanks a lot for this very interesting work. I have a question related to the equation on line 127. How do you arrive at the right hand-side of this equation? I could not find a definition for \delta_T in the text. In addition, how do you arrive at an error "around 8 g/kg" in the calculation example below the equation?
In line 189 you state: "The quality of derived specific humidity (i.e. retrieved from temperature and virtual temperature, both with an error of 1 to 1.5 K) is not good enough to estimate the relative humidity with some kind of skill."
I guess you can provide estimates of the error of derived specific humidity based on the equations in section 3? Or are these the blue lines in Figure 4?
In figure 6, the lower right panel shows very good agreement for Tv, but much less so for q. Can you add some lines in the text to explain this discrepency, e.g. by refering to (the potential weaknesses of) the equations in section 3.