# Sigma-theta

From Glossary of Meteorology

Revision as of 09:39, 9 April 2018 by imported>Rbrandt (→sigma-theta)

## sigma-theta

(Used in physical oceanography, symbol

*σ*.) The density_{θ}*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,*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 wit the Yamartino method. Sigma-theta is used as a proxy to characterize the dispersion capabilities of the lower troposphere in the Pasquill-Gifford categories. The Yarmartino method is given by:

_{σ}

where *s _{a}* = (1/

*n*)Σ

_{i}sin

*θ*and

_{i}*c*= (1/

_{a}*n*)Σ

_{i}cos

*θ*are computed from a sequence of

_{i}*n*wind angle (

*θ*) measurements.

_{i}*Term edited 9 April 2018.*