Stability Indices

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Pasquill Stability Indices are used to calculate a stability category ranging from A to F that can help predict how well pollution will disperse, and are available for our Synoptic Stations

The Pasquill-Turner Stability Index

A Extremely Unstable
B Unstable
C Slightly Unstable
D Neutral Stability
E Slightly Stable
F Stable
G Extremely Stable

In addition to the six classes A to F defined by Pasquill a seventh class 'G' has been included for night conditions of clear skies and winds less than 2m/sec. In these cases it is not possible in general to estimate the distribution of airborne pollutants and they can give rise to the highest levels of concentration of pollution.

The Pasquill Index is always neutral (D) for overcast conditions with a cloud ceiling that is less than 7000 feet. During "anticyclonic gloom", an inversion at a lower level could persist for days with a D Pasquill Index.

Input Parameters
(1) Surface wind speed (in knots)
(2) Total cloud amount N (in oktas, i.e. N=8 means overcast).
(3) Cloud ceiling (in hundreds of feet). This is the lowest level above the ground at which the cumulative amounts of lower cloud total 4 okta. If N<4, there is no ceiling (ceiling = 999).
In the printout for individual days, the data is given in the form: Year, month, day. The surface wind direction (in tens of degrees) is included also.

Stability categories

For A-B take the average of figures for A and B etc.

In this table strong insolation corresponds to sunny midday conditions in midsummer and slight insolation to similar conditions in midwinter. Night refers to the period from one hour before sunset to one hour after dawn. Neutral category 'D' is assumed for overcast conditions day or night irrespective of wind speed.

Turner defined strong, moderate and slight insolations as used by Pasquill in terms of the sun's altitude and the amount and height of cloud present, thus making it possible to compute stability classes objectively.