Weather constantly changes, with weather systems forming and dying; they move and affect many areas of the country in succession. To keep track of the location and movement of these systems, we must measure the weather elements simultaneously and make maps and charts that reflect the weather conditions at a given time. Furthermore, because of the size of these systems, observers around the world have to coordinate their efforts in the enterprise. At what time should we make our observations? What do we even mean by the time anywhere in the world? The current weather maps that you will use in this course are identified in "Z time".
For centuries, humans measured their activities by the daily motions of the sun. Local noon was a convenient reference, marking the time when the sun would be overhead locally. As a result, locations even several miles displaced in the east-west direction could have different local times. With more rapid long distance transportation and communication made available by the railroads and telegraphy after the American Civil War, travel east or west meant that a person's local time kept constantly changing. To reduce the large number of locally observed times, the railroad companies pushed for the simplified standardized time keeping scheme we currently know. As a result, civil time zones were initially instituted in the U.S. and Canada in November 1883 to standardize time keeping. The concept of international time zones was officially adopted in November 1884 at the International Meridian Conference in Washington DC. Because The Old Royal Observatory in Greenwich, England had instituted the best early astronomical time determinations, the meridian of longitude passing through this observatory became the Prime Meridian and serves as the world-wide standard for time keeping.
Since the collection and exchange of weather information are of international concern, use of a single worldwide time system is needed so all weather observers around the world can take measurements at the same time, providing a "snapshot" of the weather. The times for essentially all meteorological reports are given according to the Universal Time Coordinated system (UTC), the current name for Greenwich Mean Time (GMT). Often, a single letter, Z, phonetically called "Zulu" is used because this letter is used to identify the Greenwich time zone (centered on the Greenwich Prime Meridian).
By international agreement, surface weather observation times are minimally 0000 Z, 0600 Z, 1200 Z and 1800 Z, with upper air measurements at 0000 Z and 1200 Z each day. In the U.S., surface observations are taken hourly (at the top of the hour). Radar summary charts are also hourly at 35 minutes past the hour. Fronts are analyzed on maps every three hours, 0000 Z, 0300 Z, and so forth. A table of time conversions in the U.S. is listed in the Online Weather Homepage User's Guide.
Because the earth rotates on its axis with respect to the sun once every 24 hours, ideally we should have 24 major civil time zones of equal width. The 360 degrees of rotation divided by 24 gives 15 degrees of width to each zone. The central meridian of the zone is then defined as a longitude evenly divisible by 15 on the system based from the Greenwich Prime Meridian as 0 degrees. If I were located in the U.S. Central Standard Time Zone, I would be near 90 degrees west longitude (6 times 15). At any place in this zone, I would be 6 hours different from time in Greenwich, England, UK. With the Earth rotating eastward, Greenwich is ahead of CST then by 6 hours. For example, noon in Greenwich (1200) is only 6 AM CST. To reduce confusion, all times should be expressed in the 24 hour time format, such that, 8:45 A.M. = 0845 and 1:15 P.M. = 1315.
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. For the precise location of the four time zones in the continental United States, consult a recent atlas or almanac. While most of the United States observes Daylight Saving Time during the summer (April through October), UTC remains fixed and does not adhere to a "summer schedule". Therefore, you will have to adjust the time by one hour during summer. As an example, during the summer, the residents in the U.S. Eastern Time zone will lag Greenwich by only 4 hours, with 0800 EDT = 0700 EST = 1200 Z.
Suppose that you would like to know the current time as maintained by the Master Clock at the U.S. Naval Observatory in Washington, DC. So get your clocks or watches ready and then access the current time from the Time Service Department at http://tycho.usno.navy.mil/.
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Prepared by Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu