1. Applied to winds blowing in a mainly west-to-east or east-to-west direction, particularly in describing the main, broad airstreams of the general or large-scale atmospheric circulation, e.g. the zonal westerly winds of middle latitudes. The zonal (or circulation) index is a conventional measurement indicating the strength of the west-to-east airflow over middle latitudes, e.g. in latitudes 35–55° N. Strong westerlies with the pressure systems also in a west-to-east orientation accompany high values of the zonal index. Cellular or meridional airflow (or sometimes weak and chaotic patterns of circulation) accompany low values of the zonal index.

2. Applied to features (e.g. soils and vegetation) characteristic of a particular region that is approximately bounded by lines of latitude (i.e. a region lying parallel to the equator).