Background
The authors write that there has been great interest recently "in the widespread declining trends of wind speed measured by terrestrial anemometers at many mid-latitude sites over the last 30-50 years," citing the work of Roderick et al. (2007), McVicar et al. (2008), Pryor et al. (2009) and Jiang et al. (2010); and they say that this stilling, as it has come to be called, is "a key factor in reducing atmospheric evaporative demand," which drives actual evapotranspiration when water availability is not limiting, as in the case of lakes and rivers. In addition, they note that near-surface wind speed (u) nearly always increases as land-surface elevation (z) increases, as demonstrated by the work of McVicar et al. (2007), and that increasing wind speeds lead to increases in atmospheric evaporative demand, while decreasing wind speeds do the opposite, both of which changes can be of great significance for people dependent upon water resources derived from mountainous headwater catchments. Consequently, it would be advantageous to learn how this latter phenomenon (the change in near-surface wind speed with ground elevation) may also have varied over the last few decades of global warming, since the authors write that "over half the global population live in catchments with rivers originating in mountainous regions (Beniston, 2005), with this water supporting about 25% of the global gross domestic product (Barnett et al., 2005)."

What was done
Defining uz as change in wind speed with change in elevation -- uz = Δu/Δz, where Δu = u2-u1, Δz = z2-z1, and z2 > z1 -- McVicar et al. calculated monthly averages of uz based on monthly average u data from low-set (10-meter) anemometers maintained by the Chinese Bureau of Meteorology at 82 sites in central China and by MeteoSwiss at 37 sites in Switzerland from January 1960 through December 2006, which activity, in their words, constituted "the first time that long-term trends in uz in mountainous regions have been calculated."

What was learned
The seven scientists determined that "for both regions uz trend results showed that u has declined more rapidly at higher than lower elevations."

What it means
This double-benefit -- a general decline in wind speed at many mid-latitude sites and a further decline in wind speed at higher elevations -- should act to reduce water loss via evaporation from high altitude catchments in many of the world's mountainous regions, providing more water for people who obtain it from such sources. In addition, McVicar et al. note that the "reductions in wind speed will serve to reduce rates of actual evapotranspiration partially compensating for increases in actual evapogtranspiration due to increasing air temperatures."

References
Barnett, T.P., Adam, J.C. and Lettenmaier, D.P. 2005. Potential impacts of a warming climate on water availability in snow-dominated regions. Nature 438: 303-309.

Beniston, M. 2005. Mountain climates and climatic change: An overview of processes focusing on the European Alps. Pure and Applied Geophysics 162: 1587-1606.

Jiang, Y., Luo, Y., Zhao, Z. and Tao, S. 2010. Changes in wind speed over China during 1956-2004. Theoretical and Applied Climatology 99: 421-430.

McVicar, T.R., Van Niel, T.G., Li, L.T., Hutchinson, M.F., Mu, X.-M. and Liu, Z.-H. 2007. Spatially distributing monthly reference evapotranspiration and pan evaporation considering topographic influences. Journal of Hydrology 338: 196-220.

McVicar, T.R., Van Niel, T.G., Li, L.T., Roderick, M.L., Rayner, D.P., Ricciardulli, L. and Donohue, R.G. 2008. Capturing the stilling phenomenon and comparison with near-surface reanalysis output. Geophysical Research Letters 35: 10.1029/2008GL035627.

Pryor, S.C., Barthelmie, R.J., Young, D.T., Takle, E.S., Arritt, R.W., Flory, D., Gutowski Jr., W.J., Nunes, A. and Roads, J. 2009. Wind speed trends over the contiguous United States. Journal of Geophysical Research 114: 10.1029/2008JD011416.

Roderick, M.L., Rotstayn, L.D., Farquhar, G.D. and Hobbins, M.T. 2007. On the attribution of changing pan evaporation. Geophysical Research Letters 34: 10.1029/2007GL031166.