The westerly index, appears in a CPL syllabus but I can't find much info on it, I assume it is a measure of how systems move from west to east in the southern hemisphere.

How exactly is it calculated, and what is it's purpose and significance for aviation?

  • $\begingroup$ Related question on Reddit $\endgroup$ – Mast Apr 13 at 9:56
  • $\begingroup$ I'm not sure, but I'd hazard a guess and say it's related to the Westerlies, prevailing winds from west to east between 30 and 60 degrees lat. $\endgroup$ – Mast Apr 13 at 9:58
  • $\begingroup$ Weather systems move from West to East in the Northern hemisphere too. $\endgroup$ – CatchAsCatchCan Apr 14 at 0:04
  • $\begingroup$ "The Westerly Index (WI) is defined as the proportion of days per month with prevailing wind coming from the west in a four point quadrant (wind blowing between 225 and 315 degrees)". Used to study the variability of Westerlies. It is related to a defined area, e.g. south hemisphere, Asia, etc. Here for the English Channel. WI data set example for the same area. $\endgroup$ – mins Apr 27 at 1:29

The only mention of it I could find in relation to a CPL is from a New Zealander CPL syllabus (govt.nz; PDF).

Including NZ in the search leads to this definition:

The westerly index is the Trenberth Z1 index (Trenberth 1976), which is the departure of the monthly mean pressure difference between Auckland and Christchurch (near 44°S) from its mean over the 1961–90 period.


From that definition, a pilot needs to be aware of the pressure difference between Auckland and Christchurch.

Others here may be able to expand on its usage (feel free to reuse, anyone).

From an Earth Science perspective, I asked on the sister site: What does a change in the mean pressure across 750 km indicate/signify?


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