Abstract
Water vapour modulates energy flows in Earth's climate system through transfer of latent heat by evaporation and condensation and by modifying the flows of radiative energy both in the longwave and shortwave portions of the electromagnetic spectrum. This article summarizes the role of water vapour in Earth's energy flows with particular emphasis on (1) the powerful thermodynamic constraint of the Clausius Clapeyron equation, (2) dynamical controls on humidity above the boundary layer (or free-troposphere), (3) uncertainty in continuum absorption in the relatively transparent "window" regions of the radiative spectrum and (4) implications for changes in the atmospheric hydrological cycle.
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Notes
Latent heat released by condensation is retained in the atmosphere only after the water is removed by precipitation to the surface before re-evaporation in the atmosphere can occur.
The first term is larger than the second term by a factor \(q_{\rm s}p/\epsilon e_{\rm s}=p/(p-e_{\rm s}(1-\epsilon))=1+q_{\rm s}((1/\epsilon)-1) \sim 1+0.6q_s\) assuming fixed pressure, where ε = 0.622 is the ratio of gas constants for dry air and water vapour. For a surface pressure of 1,000 hPa and using the August−Roche–Magnus empirical approximation for e s(T), the first term exceeds the second term by around 0.2% at 273 K and by 1% at 300 K.
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Acknowledgments
Thanks to Paul O’Gorman, Remy Roca, Keith Shine, Brian Soden and two anonymous reviewers for comments and corrections on the original manuscript and for their input, along with B.-J. Sohn, Andrew Dessler and many others, to the International Space Sciences Institute Workshop Session on Observing and modelling Earth’s energy flows. R Allan was supported by the UK Natural Environment Research Council PREPARE project (NE/G015708/1).
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Allan, R.P. The Role of Water Vapour in Earth’s Energy Flows. Surv Geophys 33, 557–564 (2012). https://doi.org/10.1007/s10712-011-9157-8
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DOI: https://doi.org/10.1007/s10712-011-9157-8