Z
ZENITH
The point which is elevated 90 degrees from all points on a given observer's astronomical horizon. The point on any given observer's celestial sphere that lies directly above him. The opposite of nadir.
ZODIAC
The position of the sun during the course of the year as it appears to move though successive constellations. Also, the band where the ecliptic runs centrally through the celestial sphere and contains the sun, the moon, and all the planets except Venus and Pluto.
ZONAL FLOW
The flow of air along a latitudinal component of existing flow, normally from west to east.
ZONAL INDEX
The measure of the strength of the westerly winds of the middle latitudes. It is expressed as the horizontal pressure difference between 35 degrees and 55 degrees latitude, or as the corresponding geostrophic wind.
ZULU TIME
One of several names for the twenty-four hour time which is used throughout the scientific and military communities.