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MMA Memo #158

Correlation Between Opacity and Surface Water Vapor Pressure Measurements at Rio Frio

M.A. Holdaway [1], Masato Ishiguro [2], Naomasa Nakai [2], Satoki Matsushita [2]

August 1, 1996

Keywords: water vapor pressure, water vapor scale height, weather stations, solar flux, opacity, Chile, Rio Frio, site testing,

We use the surface water vapor pressure measured by weather stations at 4060m and 4200m elevation at Rio Frio to estimate the water vapor scale height. We also estimate the water vapor column above the array and attempt to correlate it with the water vapor column inferred from the 220 GHz opacity.

The surface water vapor pressure is inverted some 20% of the time at night and some 35% of the time during the day, making scale height measurements impossible. Of the non-inverted conditions, the night time water vapor scale height is typically 1km, and the day time water vapor scale height is about 2km. The scale height during summer days is about 2.5 times larger than during winter days, while the night time scale height shows no seasonal variation. The integrated solar flux, averaged over the summer half of the data, is about 1.8 times higher than during the winter, indicating that the input of solar energy is related to the diurnal and seasonal variations of the water vapor scale height. The water vapor column estimated from the water vapor pressure measurements show a very rough agreement with the opacity measurements. The nature of the correlation is different between day and night time conditions.

Some very complicated events are seen in which either the opacity or the surface water vapor pressure changes dramatically while the other remains constant. Strategically placing more weather stations at higher elevations on nearby mountains would provide crucial information on inversion layers which would help in the interpretation of the water vapor and opacity data and would provide important insights into the atmosphere. Any new weather stations should add atmospheric pressure to the meteorological parameters already sampled by the existing stations.

[2] NRO

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Last modified: 09 December, 1999