Sigma-theta

sigma-theta

(Used in physical oceanography, symbol σ_θ.) 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 σ_θ.) 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 s_a = (1/n)Σ_isinθ_i and c_a = (1/n)Σ_icosθ_i are computed from a sequence of n wind angle (θ_i) 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.

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