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Z

Z dr:  See differential reflectivity.

Z/R Relationship:  An empirical relationship between radar reflectivity factor z (in mm^6 / m^3 ) and rain rate ( in mm / hr ), usually expressed as Z = A R^b; A and b are empirical constants.

Zero Datum:  A reference "zero" elevation for a stream or river gage. This "zero" can be referenced (usually within ten feet of the bottom of the channel) to mean sea level, or to any other recognized datum.

Zone Forecast Product (ZFP):  This National Weather Service product will provide the general public with a clear statement of the expected weather conditions within a given county or set of counties.  The forecast will include:  sky condition, temperature, type of precipitation and its probability, and wind direction and speed (this is normally given only during the first 2 periods of the forecast).

Zone of Aeration:  The locus of points just above the water table where soil pores may either contain air or water. This is also called the vadose zone.
 
Zone of Saturation:  The locus of points below the water table where soil pores are filled with water. This is also called the phreatic zone.
 
Zonal Wind: Large-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant. The accompanying meridional (north-south) component often is weaker than normal. Compare with meridional flow.

Zoned Embankment Dam:  An embankment dam which is comprised of zones of selected materials having different degrees of porosity, permeability and density.

"Zulu" (Z) Time:  For practical purposes, the same as Coordinated Universal Time (UTC).  The notation formerly used to identify time Greenwich MeanTime.  The word "Zulu" is notation in the phonetic alphabet corresponding to the letter "Z" assigned to the time zone on the Greenwich Prime Meridian.  


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