Astroclimatology of Paranal
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Precipitable Water Vapour at Paranal during the VLT site survey: ground based measurements

A median value of 2.3 mm H2O was measured at Paranal during the VLT Site Survey, 1983-1989 (VLT Report 62, 1990), using the KPNO Infrared Sky Radiance Monitor (Morse, D and Gillett,F. 1982: AURA Eng. Report 73)

Overall and Seasonal distribution at Paranal (VLT Site Survey, Jul92-Dec94)

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PWV at Paranal during VLT operation: Statistics from Satellite Imagery

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Clear time [Aug 1999-Nov 2005] PWV at Paranal and La Silla from 3-hourly satellite images (data file, format: YYYY MM DD HH PWV)

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Clear time [Aug 1999-Nov 2005] PWV at Paranal and La Silla from 3-hourly satellite images (data file, format: MM PWVPAR PWVSIL)

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Monthly Average Precipitable Water Vapour at Paranal (mm H2O, clear time only)

3-hourly GOES8 Satellite Images	1999	2000	2001	2002	2003	2004	2005
January		8.1	3.2	3.5	3.1	2.7	4.0
February		4.3	4.6	6.2	5.5	3.2	3.1
March		4.8	7.3	6.6	3.9	3.1	3.1
April		4.5	4.9	4.6	2.2	3.0	2.4
May		3.1	3.3	4.2	2.5	1.8	2.2
June		2.8	2.5	2.4	2.0	1.7	2.4
July		1.7	2.4	3.0	2.0	2.3	1.8
August	2.0	1.6	2.2	2.6	1.4	1.4	2.1
September	2.2	1.6	2.7	2.2	1.8	2.0	1.8
October	2.5	1.8	2.6	2.7	1.9	1.8	1.3
November	2.5	1.6	2.9	2.9	2.3	2.4	1.9
December	2.6	2.4	3.5	3.1	2.3	3.2
Year (ave/median)		3.2/2.4	3.5/3.0	3.5/3.0	2.7/2.3	2.4/1.9

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R. J. Mathar; Calculated Refractivity of Water Vapor and Moist Air in the Atmospheric Window at 10 micron, Appl. Opt. 43, 4, 928-932, Feb. 2004 (other publications)

IR Atmospheric Transmission ISAAC User Manual (pdf).

Spectra of the atmospheric transmission above Mauna Kea (Lord, 1992) in the near-IR (0.9-5.5 mu) are available on the Gemini Observatory Observing Conditions page. They show that an increase of 1 to 3 mm H2O is similar to increasing the observation airmass from 1 to 2:

- AM=1.0 and 2.0 at 1.0mm precipitable water vapour: 0.9 - 1.4, 1.4 - 1.9, 1.9 - 2.3, 2.3 - 2.7, 2.7 - 3.3, 3.3 - 4.3, 4.3 - 5.0 and 5.0 - 5.6µm
-1.0 and 3.0mm H₂O at AM=1.0: 0.9 - 1.4, 1.4 -1.9, 1.9 - 2.3, 2.3 - 2.7, 2.7 - 3.3, 3.3 - 4.3, 4.3 - 5.0 and 5.0 - 5.6µm

Send comments to <Marc.Sarazin@eso.org> Last update: September 24, 1999