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Weather Glossary: X's

Abbreviation for expendable bathythermograph.
Very energetic electromagnetic radiation with wavelengths intermediate between 0.01 and 10 nanometers (0.1-100 Angstroms) or between gamma rays and ultraviolet radiation. Essentially all X-Rays from space are absorbed in the Earth's upper atmosphere.

Weather Glossary: Y's

1. The period during which the Earth completes one revolution around the sun. This has several interpretations.
  • Sidereal Year - Time of true revolution around the sun; that is, the time it takes the Earth (as seen from the sun) to reappear at the same fixed star, equal to 365.2564 mean solar days or 365 days, 6 hours, 9 minutes, 10 seconds.
  • Tropical Year - The time measured from one vernal equinox to the next; that is, the apparent revolution of the sun through the zodiac, equal to 365.2422 mean solar days or 365 days, 5 hours, 48 minutes, 46 seconds. This is not a constant, but only decreases by 5 seconds in every one thousand years. This is also called mean solar year or ordinary year.
  • Calendar Year - Fixed by the Gregorian calendar of 365 days in an ordinary year and 366 days in leap year.
2. Any arbitrary 12-month period selected for a special purpose, such as the water year.
Yellow Snow 
Snow given a golden or yellow appearance by the presence in it of pine, cypress pollen, or anthropogenic material or animal-produced material. Compare with Brown Snow.

Weather Glossary: Z's

Z/R Relationship
An empirical relationship between radar reflectivity factor z (mm6/m3) and rain rate (mm/hr), usually expressed as Z = A Rb; 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 Flow
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 Mean Time. The word "Zulu" is notation in the phonetic alphabet corresponding to the letter "Z" assigned to the time zone on the Greenwich Prime Meridian.