From Glossary of Meteorology
Revision as of 05:21, 16 May 2018 by imported>Rbrandt (→‎sigma-theta)


(Used in physical oceanography, symbol σθ.) The density ρ (<1000 kg m−3) that a parcel of water of salinity S and temperature T would have if it were raised adiabatically to the sea surface, where its temperature would change from T to potential temperature θ due to adiabatic decompression. In symbols,


(Used in meteorology, symbol σθ.) The standard deviation of horizontal wind direction used primarily in air pollution applications. Sigma-theta can either be computed directly from measured wind direction or with the Yamartino method (Yamartino 1984). Sigma-theta is used as a proxy to characterize the dispersion capabilities of the lower troposphere with different stability categories. The Yamartino method is given by:

where sa = (1/nisinθi and ca = (1/nicosθi are computed from a sequence of n wind angle (θi) measurements.

Yamartino, R. J., 1984: A comparison of several "single-pass" estimators of the standard deviation of wind direction. J. Climate Appl. Meteor., 23, 1362–1366, doi:10.1175/1520-0450(1984)023<1362:ACOSPE>2.0.CO;2.

Term edited 16 May 2018.