Marton G. Hidas, Michael G. Burton, Matthew A. Chamberlain, John W.V. Storey, PASA, 17 (3), 260.
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Title/Abstract Page: Infrared and Sub-millimetre Observing
Previous Section: The effect of temperature
The effect of water content
The starting point for the following analysis is the model shown in Fig.á 1, obtained from radiosonde data taken on a clear day. The total precipitable water vapour for this model is 164m, which is close to the lowest value measured at the South Pole during winter (Van Allen et al. 1996). This model did not include any aerosols.
The spectrum was calculated for the original model, as well as for two variations, one with the relative humidity at each layer of the atmosphere in the model doubled, and another with the relative humidity halved. The resulting emission and transmission spectra are shown in Figuresá 3-6, for the wavelength ranges 2-6m, 5-15m, 15-60m and 50-500m, respectively. Tableá 2 and Tableá 3 give some numerical values for the sky flux and transmission, averaged over several wavelength regions of interest.
Wavelength | Precipitable Water Vapour / Aerosol Visibility | |||||
Range (m) | (m) / (km) | |||||
á | 164 / | 82 / | 324 / | 164 / 100 | 164 / 10 | 164 / 1 |
3.0-3.1 |
2.2 x 10-3 |
1.3 x 10-3 |
3.6 x 10-3 |
3.1 x 10-3 |
1.4 x 10-2 |
3.3 x 10-2 |
3.6-3.8 |
7.0 x 10-3 |
6.5 x 10-3 |
8.0 x 10-3 |
2.9 x 10-2 |
3.0 x 10-1 |
6.5 x 10-1 |
3.0-4.0 |
2.1 x 10-2 |
2.0 x 10-2 |
2.2 x 10-2 |
3.3 x 10-2 |
2.0 x 10-1 |
4.2 x 10-1 |
4.9-5.1 |
1.2 x 100 |
8.3 x 10-1 |
1.8 x 100 |
1.7 x 100 |
8.1 x 100 |
2.2 x 101 |
8.2-9.2 |
2.5 x 101 |
2.4 x 101 |
2.7 x 101 |
3.3 x 101 |
1.3 x 102 |
6.2 x 102 |
10.2-11.2 |
1.3 x 101 |
1.3 x 101 |
1.3 x 101 |
2.2 x 101 |
1.6 x 102 |
9.5 x 102 |
10.0-13.0 |
2.5 x 101 |
2.4 x 101 |
2.7 x 101 |
4.3 x 101 |
2.5 x 102 |
1.4 x 103 |
18.0-22.0 |
5.5 x 102 |
3.8 x 102 |
8.0 x 102 |
6.0 x 102 |
1.2 x 103 |
3.8 x 103 |
20.0-20.2 |
8.1 x 101 |
4.1 x 101 |
1.6 x 102 |
1.3 x 102 |
7.7 x 102 |
3.7 x 103 |
24.1-24.9 |
4.3 x 102 |
2.2 x 102 |
8.0 x 102 |
4.6 x 102 |
9.1 x 102 |
3.3 x 103 |
32.0-32.5 |
2.4 x 103 |
1.5 x 103 |
3.5 x 103 |
2.4 x 103 |
2.6 x 103 |
3.4 x 103 |
220-230 |
2.0 x 102 |
1.5 x 102 |
2.4 x 102 |
2.0 x 102 |
2.0 x 102 |
2.0 x 102 |
330-370 |
4.9 x 101 |
3.8 x 101 |
6.6 x 101 |
4.9 x 101 |
4.9 x 101 |
5.1 x 101 |
430-470 |
2.7 x 101 |
2.0 x 101 |
3.8 x 101 |
2.7 x 101 |
2.7 x 101 |
2.8 x 101 |
Wavelength | Precipitable Water Vapour / Aerosol Visibility | |||||
Range (m) | (m) / (km) | |||||
á | 164 / | 82 / | 324 / | 164 / 100 | 164 / 10 | 164 / 1 |
3.0-3.1 | 0.94 | 0.96 | 0.90 | 0.92 | 0.67 | 0.05 |
3.6-3.8 | 0.98 | 0.98 | 0.98 | 0.96 | 0.64 | 0.03 |
3.0-4.0 | 0.91 | 0.92 | 0.89 | 0.88 | 0.61 | 0.03 |
4.9-5.1 | 0.95 | 0.96 | 0.93 | 0.93 | 0.70 | 0.07 |
8.2-9.2 | 0.96 | 0.96 | 0.96 | 0.95 | 0.86 | 0.38 |
10.2-11.2 | 0.99 | 0.99 | 0.99 | 0.99 | 0.93 | 0.56 |
10.0-13.0 | 0.99 | 0.99 | 0.98 | 0.98 | 0.90 | 0.47 |
18.0-22.0 | 0.89 | 0.93 | 0.85 | 0.89 | 0.77 | 0.27 |
20.0-20.2 | 0.98 | 0.99 | 0.97 | 0.98 | 0.85 | 0.30 |
24.1-24.9 | 0.92 | 0.96 | 0.85 | 0.91 | 0.83 | 0.38 |
32.0-32.5 | 0.48 | 0.67 | 0.24 | 0.47 | 0.44 | 0.27 |
220-230 | 0.21 | 0.39 | 0.07 | 0.21 | 0.21 | 0.20 |
330-370 | 0.56 | 0.66 | 0.41 | 0.56 | 0.56 | 0.54 |
430-470 | 0.61 | 0.71 | 0.46 | 0.61 | 0.61 | 0.60 |
MODTRAN calculates the total vapour column of absorption, h, for a species such as water, as
(1) |
(2) |
Note that part of the reason for the low water vapour content of the atmosphere above Antarctica is that, at these low temperatures, the saturation vapour pressure of water is low. For instance, while rw is 8g/kg at 10C and 1,000mbars it is only 0.2g/kg at C and 600mbars. Therefore, even when the relative humidity is close to 100%, the actual amount of water vapour is still small. When the relative humidity profile of the original model was doubled, it actually exceeded 100% at one level, and had to be reset to 100%. This is why the ``wet'' case does not have exactly twice the precipitable water. In fact, even scaling the relative humidity up by an unphysical factor of 10 only resulted in 540m of precipitable water.
Halving and doubling the humidity this way encompasses the range of precipitable water vapour content encountered at the South Pole. For instance, Chamberlin, Lane & Stark (1997) show that in winter there is less than 190m of precipitable water vapour for 25% of the time, and less than 320m for 75% of the time. Even in summer the column is less than 470m for half the time. Furthermore, the precise determination of water vapour content from radio-sonde data is difficult below C (eg. see Walden, Warren & Murcray, 1998), and so encompassing the likely range this way allows more robust conclusions to be drawn than by detailed modelling of several sets of radio-sonde data.
From 2-15m there is relatively little variation in either transmission or flux as the water vapour content is varied (except within the water absorption band itself around 6m). It seems, therefore, that at a very dry site such as the Antarctic plateau, the exact value of the water content is not an important factor in determining the observing conditions in the dark regions of the near- and mid-IR spectrum.
For wavelengths beyond 20m, however, it is a different story. The opening of the mid-IR window at 30m is critically dependent on low levels of water vapour, being effectively closed for 324m of precipitable water vapour. While the background fluxes do not vary significantly with water vapour content, the transmission, and thus ability to detect a signal, does. In particular, the new far-IR windows at 200m and 230m do require the driest days for successful observing. However the only terrestrial locations from which these windows are accessible at all are probably those on the Antarctic plateau.
Next Section: The effect of aerosols
Title/Abstract Page: Infrared and Sub-millimetre Observing
Previous Section: The effect of temperature
á© Copyright Astronomical Society of Australia 1997