Since the collection and exchange of weather information are of international concern, use of a single, systematic time keeping scheme is a necessity. By international agreement, the reported times for essentially all meteorological reports are given according to Universal Coordinated Time (UTC), which has replaced the previously used Greenwich Mean Time (GMT). GMT time is also referred to as "Z" or, phonetically, "Zulu" time, for the letter identifying that time zone centered on the Greenwich Prime Meridian (See the time zone labels along the top of the accompanying World Map of Time Zones). Essentially all the maps that you will encounter in this course will be identified in Z time.
This document focuses upon how to convert from your local civil time to Z time, and vice versa. Before we can compare the local time that we normally use in our everyday lives with UTC times, we will need to look at the concept of civil time zones.
Ideally, the world would be divided into 24 major civil time zones of equal width. Each zone would have an east-west dimension of 15 degrees of longitude centered upon a central meridian . This central meridian for a zone is defined in terms of its position relative to a universal reference, the Greenwich Prime Meridian. In other words, the central median of each zone has a longitude divisible by 15 degrees. When the sun is directly above this central meridian, local time at all points within that zone would be noon.
The International Date Line is an imaginary line located near the 180 degree meridian of longitude that traverses the sparsely populated portions of the Pacific Ocean. When the line is crossed going west, the calendar date is advanced one day. However, when the line is crossed going east, the locally observed date becomes one day earlier than that to the west of the Dateline.
Modifications of the boundaries between time zones have been made to accommodate political boundaries in the various countries. Some countries adhere to a local civil time that may differ by one half hour from that of the central meridian.
Source: U.S. Naval Observatory: Astronomical phenomena for the year 1992. U.S. Govt. Printing Office. Washington, DC.
A newer version of the World Time Zone Map is available from the U.S. Naval Observatory
Local civil time converts to Z time or UTC in the following manner. First, local civil time must be expressed in the 24 hour time format (e.g., 8:45 A.M. = 0845 and 1:15 P.M. = 1315). Then, depending upon your longitude, subtract or add1 hour of time to this local standard time for each 15 degree increment of longitude (or time zone) that your location is west or east from the Prime Meridian to determine the equivalent UTC time. Inspect the World Map of Time Zones for the location of civil time zones.
For the exact location of the boundaries between the four time zones in the continental United States, consult an atlas or almanac. For example, Madison, Wisconsin is at approximately 90 degrees West Longitude and therefore is near the center of the Central Time Zone. As a result, Central Standard Time lags UTC by 6 hours {since 90 degrees/(15 degrees per hr) = 6 hr}. For example, 3:10 PM CST = 1510 CST = 2110 UTC. During the summer, Central Daylight Saving Time is 5 hours behind UTC because UTC does not adhere to a "summer schedule"; thus, 3:10 PM CDT = 2010 UTC.
Other Systems of Time documentation
Time Zone Page with explanation
Last revision 4 September 2001
© Copyright, 2001 Edward J. Hopkins, Ph.D. hopkins@meteor.wisc.edu|
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