| Parameter | Definition | Interpretation | ||||||
|---|---|---|---|---|---|---|---|---|
| AvgUTH (%) | Mean relative humidity of the middle and upper Troposphere (~ 3000 m – 9500 m) | Increasingly negative (positive) departures from 50% indicate higher probability of photometric (spectroscopic) conditions | ||||||
| Probability of: | UTH < 30% | UTH: 30 – 50% | UTH: 50 – 80% | UTH: 80 – 100% | ||||
| Photometric | 0.98 | 0.8 | 0.56 | 0.1 | ||||
| Non-photometric | 0.02 | 0.2 | 0.44 | 0.9 | ||||
| SigUTH (%) | Standard deviation of the UTH | Higher values indicate greater moisture variability during the period | ||||||
| PWV (mm) | The Precipitable Water Vapour in the atmosphere above the site | Larger values indicate more atmospheric attenuation at Infra-red and radio wavelengths | ||||||
| Clear conditions (8/99 – 9/01) | 10%-ile | Median | ||||||
| Paranal | 1.6 | 3.0 | ||||||
| La Silla | 4.0 | 6.5 | ||||||
| Opaque Cloud (%) | Percentage of sky covered by opaque cloud, on average, during the period | The complement of this parameter is the fraction of the sky suitable for astronomy (photometry and/or spectroscopy) | ||||||
| Transparent Cirrus (%) | Percentage of the sky covered by transparent cirrus cloud, on average, during the period | The fraction of the sky suitable for spectroscopic observations (see Transparency Index) | ||||||
| Total Cloud (%) | The sum of % opaque and % transparent cirrus cloud cover | The complement of this parameter is the fraction of the sky suitable for photometry | ||||||
| Transparency Index | TI = 1.0: All pixels clear TI = n/a: 10 ≤ %opaque pixels < 100 TI computed (0 < TI ≤ 1): %opaque pixels < 10% |
Photometric Unusable to marginally spectroscopic Marginally spectroscopic to marginally photometric (see also, AvgUTH) |
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| CldAdj | Upward or downward adjustment in cloud cover forecast for the forecast period | La Silla: indicates probability of in-place
(orographic) cloud formation Paranal: indicates dissipation of cloud moving east across the Andes “0”: indicates that no adjustment in the cloud cover forecasts is expected “+” (“−”): indicates a moderate (25 – 50%) increase (decrease) “+ +” (“− −”): indicates a significant (> 50%) increase (decrease) |
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