1. David Paynter, Igor Ptashnik, Keith Shine Department of Meteorology University of Reading Kevin Smith (RAL) June 2006 Pure water vapor continuum measurements between 3100 and 4400 cm -1 : Evidence for water dimer absorption in near atmospheric conditions

2. Key Facts: Used a FT-IR spectrometer at MSF RAL to measure the absorption of a known amount of pure water vapour through a multi-pass cell. Measured spectrum between 1200 cm -1 and 8000 cm -1 – Near infrared Resolution of 0.03cm -1 -No Apodization Temperature 296-351K Pressure 20mb -350mb

3. Deriving the Continuum Theory calculated Using Lorentzian lines out to 25cm -1. Line data from HITRAN 2004

11. Conclusions Evidence for water dimer absorption between 3000 and 4200 (1/cm ) Temperature dependence of features consistent with that of the S&K dimer. In other regions the continuum absorption agrees ‘well’ with the MT_CKD continuum Considerable uncertainty of continuum absorption in the wings and of Continuum absorption under 300 K.