This Supplemental Information File complements the Daily
Summary of 10 September 1996

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".

CIVIL TIME ZONES
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. 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 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.

METEOROLOGICAL 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 the letter 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 DataStreme Homepage User's Guide,
section X., Miscellaneous, subsection C (pg. 27).

UNIVERSAL TIME CONVERSION
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.

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Prepared by Edward J. Hopkins, Ph.D.
(c) Copyright, 1996, The American Meteorological Society.