What is Sigma Theta?
[TOC] Sigma Theta is measured by our wind vane and is related to horizontal turbulence. Mathematically it is the standard deviation of the vane position. This calculation is tricky because the vane can swing in a circle (it uses polar coordinates). At LLNL we take a reading of the wind vane every second. Every 15 minutes the data logger takes the 900 readings to calculate an average wind direction and sigma theta. The dynamics of our Met One 020 wind vanes are well known and within specifications set down by EPA for damping ratio, starting threshold, and distance constant. We use Sigma Theta to estimate the potential for the atmosphere to spread a plume. The EPA provides guidance to calculate a common plume dispersion index (Pasquill Stability Class) from measurements of wind speed and sigma theta.
