In the Atmospheric Sciences[?], this is the x direction. The zonal wind speed is called u.
See: meridional
... from 18 to 24, 20.6% from 25 to 44, 26.3% from 45 to 64, and 30.2% who are 65 years of age or older. The median age is 49 years. For every 100 females there are 96.9 ...