The Yamartino method (introduced by Robert J. Yamartino in 1984) is an algorithm for calculating an approximation to the standard deviation σθ of wind direction θ during a single pass through the incoming data. The standard deviation of wind direction is a measure of lateral turbulence, and is used in a method for estimating the Pasquill stability category.
The typical method for calculating standard deviation requires two passes through the list of values. The first pass determines the average of those values; the second pass determines the sum of the squares of the differences between the values and the average. This double-pass method requires access to all values. A single-pass method can be used for normal data but is unsuitable for angular data such as wind direction where the 0°/360° (or +180°/-180°) discontinuity forces special consideration. For example, the directions 1°, 0°, and 359° (or -1°) should not average to the direction 120°!
The Yamartino method solves both problems. The United States Environmental Protection Agency (EPA) has chosen it as the preferred way to compute the standard deviation of wind direction. A further discussion of the Yamartino method, along with other methods of estimating the standard deviation of wind direction can be found in Farrugia & Micallef.
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