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. Sigma-theta is used as a proxy to characterize the dispersion capabilities of the lower troposphere with different stability categories. The Yarmartino 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.

Term edited 8 May 2018.

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