To complement the Daily Weather Summary for Tuesday, 6 September 2011


NOTE: This document is a repeat of that appearing as a Supplemental Information File for 30 August 2011.

Weather is dynamic with weather systems continually forming and dissipating; weather systems move from place to place and can affect broad areas of the country. To track the location and movement of these rapidly developing systems, we must measure weather elements at many locations simultaneously and make maps that reflect the weather conditions (the state of the atmosphere) at a given time. The concept of time becomes crucial. Because of the size and duration of these weather systems, observers around the world have to coordinate their efforts in the enterprise. At what time should we make our observations? What is meant by the time anywhere in the world? The current weather maps that you will use in this course are identified in "Z" time.


While various weather instruments such as the barometer, thermometer and rain gauge were available to some 18th century weather observers, no large-scale weather observing networks existed for a variety of reasons. Such notables as Thomas Jefferson, George Washington, and Ben Franklin kept weather diaries, but their observations along with others are of more value for climatological purposes, where reporting into a network at a fixed time is not crucial. One important reason for the absence of a weather network was the lack of rapid communications that would permit the collection of weather information from a large area within a short time span. For example, several days elapsed as weather information sent by ship from Ben Franklin in Philadelphia could reach his brother in Boston in the 1740s.

In order to get a meaningful snapshot of today's weather requires the ability to obtain weather observational data from across the country, the continent, and ultimately the globe. Events during the mid-19th century permitted the development of what is called a synoptic weather network. The word "synoptic" is derived from the Greek words syn meaning together and optic meaning seen. Hence, synoptic literally means "seen together," or a broad overview. The first critical event that permitted the development of synoptic meteorology was the advent of the electric telegraph in the 1840s. Joseph Henry, the first secretary of the Smithsonian Institution, saw the potential for the rapid transmission of weather data by telegraph. By 1849, he had persuaded the telegraph companies to transmit weather data from his network of more than 150 weather observers that had been established across the eastern half of the nation.

The loss of ships and lives as a result of storms during the Crimean War in Russia and on the Great Lakes of North America persuaded the governments of France, England, and the United States to establish professional national weather services during the 1860s and 1870s. The argument for formation of such a network came from the realization that many lives could have been saved if adequate weather forecasts had been made available based upon tracking of storm systems well before disaster struck.


Heinrich Wilhelm Brandes (1777-1834), a professor at the University of Leipzig, drew a weather map in 1819 depicting an intense storm over the English Channel on 6 March 1783; because of the 36-year time delay, his analysis could hardly be useful for a weather forecast! He used data from an European network, particularly that of the Meteorological Society of the Palatinate in Mannheim, (1781-1795). J. Henry produced the first current weather maps for the United States in 1850 using weather information obtained by telegraph from his observation network.

To make construction of weather maps meaningful, a common observation time was needed. The timing of synchronous observations requires the use of the same time-keeping system everywhere. In the United States, observations were initially based upon the time in Washington, DC. Another 19th century factor that aided in the development of synoptic weather analysis was the development of civil time zones. With more rapid long distance transportation and communication made available by the railroads and telegraphy after the American Civil War, the railroad and telegraph companies pushed for the simplified standardized time keeping scheme we currently know. Consequently, civil time zones were initially instituted in the U.S. and Canada in November 1883 to standardize 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 concept of international time zones was officially adopted in November 1884 at the International Meridian Conference in Washington DC. For more than 50 years, the times for essentially all meteorological reports have been given according to Greenwich Mean Time (GMT), a time scale based upon the daily rotation of Earth with respect to a "mean sun". Often, a single letter, Z, phonetically pronounced "Zulu" has been used because this letter is used to identify the Greenwich time zone (centered on the Greenwich Prime Meridian or 0 degree longitude). Currently, standard practice is to use the more precise Coordinated Universal Time or Temps Universel Coordinné system (UTC), which is now based upon an atomic clock and time reckoned according to the stars (known as mean sidereal time). For practical purposes, UTC and GMT systems are essentially equivalent.

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 AMS Weather Studies Homepage User's Guide.


Because 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 give 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, the local time would be 6 hours different from time in Greenwich, England. With the Earth rotating eastward, Greenwich is ahead of CST then by 6 hours. For example, noon in Greenwich (1200) is 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. A world map available from the US Naval Observatory that shows the current boundaries of all these 24 time zones plus ten additional zones that have been added to conform to local practice. 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 (mid March through early November), 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. An informative web site explains the history involved with the institution of Daylight Saving Time not only in the United States but in other countries. This site has the potential for use in the classroom since it also has links to other items of interest, including the history of calendars and time.

Note: The Energy Policy Act of 2005 has extended Daylight Saving Time until the first Sunday in November (7 November 2010); Standard Time will run until the second Sunday in March (13 March 2011).


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. Get your clocks or watches ready and then access the current time from the Time Service Department at The site also provides an accurate time check.

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Prepared by Edward J. Hopkins, Ph.D., email
© Copyright, 2011, The American Meteorological Society.