Greenwich Mean Time

Weather balloon system Since World War II, meteorologists have used weather balloons to provide profiles of temperature, pressure, relative humidity, and wind velocity in the upper atmosphere. Besides the 70 launching sites in this country, there are more than 700 other sites around the world. The balloons are released at the same time in all countries—at 6 00 and 18 00 hours Greenwich Mean Time (GMT). Data from weather stations all over the world reach the United States via a central computer in Maryland. From there, the data are sent to regional weather service stations throughout the country for release to newspapers and television and radio networks. 

GEOREF— World Geographic Reference System GMT— Greenwich mean time GN— grid north GZ— ground zero... 

Julian date (JD) - The number of days elapsed since noon Greenwich Mean Time on January 1, 4713 B.C. Thus January 1, 2000, Oh (midnight) will be JD 2,451,543.5. This dating system was introduced by Joseph Scaliger in 1582. 

Universal time (t j, UT) - Mean solar time counted from midnight at the Greenwich meridian. Also called Greenwich mean time (GMT). The interval of mean solar time is based on the average, over one year, of the time between successive transits of the sun across the observer s meridian. 

Coordinated Universal Time (UTC) follows International Atomic Time (TAl) exactly except for an integral number of seconds, presently 32. These leap seconds are inserted on the advice of the International Earth Rotation Service (lERS) to ensure that, on average over the years, the sun is overhead within 0.9 s of 12 00 00 UTC on the meridian of Greenwich. UTC is thus the modern successor of Greenwich Mean Time, GMT, which was used when the unit of time was the mean solar day. 

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