From: Subject: Tuesday Weather Topic in Greater Depth Date: Sun, 20 Sep 2009 10:56:15 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/dstreme/last_week/t_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3350 Tuesday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Tuesday, 1 = September 2009=20

TIME KEEPING SCHEMES, WEATHER OBSERVATIONS AND MAPS =

Weather is dynamic with weather systems continually forming and = dissipating;=20 weather systems move from place to place and can affect broad areas of = the=20 country. To track the location and movement of these rapidly developing = systems,=20 we must measure weather elements at many locations simultaneously and = make maps=20 th at reflect the weather conditions (the state of the atmosphere) at a = given=20 time. The concept of time becomes crucial. Because of the size and = duration of=20 these weather systems, observers around the world have to coordinate = their=20 efforts in the enterprise. At what time should we make our observations? = What is=20 meant by the time anywhere in the world? The current weather maps that = you will=20 use in this course are identified in "Z" time.

DEVELOPMENT OF A SYNOPTIC OBSERVATION SYSTEM

While various weather instruments such as the barometer, thermometer = and rain=20 gauge were available to some 18th century weather observers, = no=20 large-scale weather observing networks existed for a variety of reasons. = Such=20 notables as Thomas Jefferson, George Washington, and Ben Franklin kept = weather=20 diaries, but their observations along with others are of more value for=20 climatological purposes, where reporting into a network at a fixed time = is not=20 crucial. One important reason for th e absence of a weather network was = the lack=20 of rapid communications that would permit the collection of weather = information=20 from a large area within a short time span. For example, several days = elapsed as=20 weather information sent by ship from Ben Franklin in Philadelphia could = reach=20 his brother in Boston in the 1740s.

In order to get a meaningful snapshot of today's weather requires the = ability=20 to obtain weather observational data from across the country, the = continent, and=20 ultimately the globe. Events during the mid-19th century = permitted=20 the development of what is called a synoptic weather network. The word=20 "synoptic" is derived from the Greek words syn meaning = together=20 and optic meaning seen. Hence, synoptic literally means "seen = together,"=20 or a broad overview. The first critical event that permitted the = development of=20 synoptic meteorology was the advent of the electric telegraph in the = 1840s.=20 Joseph Henry, the first secretary of the Smithsonian Institution, saw = the=20 potential for the rapid transmission of weather data by telegraph. By = 1849, he=20 had persuaded the telegraph companies to transmit weather data from his = network=20 of more than 150 weather observers that had been established across the = eastern=20 half of the nation.

The loss of ships and lives as a result of storms during the Crimean = War in=20 Russia and on the Great Lakes of North America persuaded the governments = of=20 France, England, and the United States to establish professional = national=20 weather services during the 1860s and 1870s. The argument for formation = of such=20 a network came from the realization that many lives could have been = saved if=20 adequate weather forecasts had been made available based upon tracking = of storm=20 systems well before disaster struck.

EARLY WEATHER MAPS AND SYNOPTIC WEATHER ANALYSIS

Heinrich Wilhelm Brandes (1777-1834), a professor at the University = of=20 Leipzig, drew a weather map in 1819 depicting an intense storm over th e = English=20 Channel on 6 March 1783; because of the 36-year time delay, his analysis = could=20 hardly be useful for a weather forecast! He used data from an European = network,=20 particularly that of the Meteorological Society of the Palatinate in = Mannheim,=20 (1781-1795). J. Henry produced the first current weather maps for the = United=20 States in 1850 using weather information obtained by telegraph from his=20 observation network.

To make construction of weather maps meaningful, a common observation = time=20 was needed. The timing of synchronous observations requires the use of = the same=20 time-keeping system everywhere. In the United States, observations were=20 initially based upon the time in Washington, DC. Another 19th = century=20 factor that aided in the development of synoptic weather analysis was = the=20 development of civil time zones. With more rapid long distance = transportation=20 and communication made available by the railroads and telegraphy after = the=20 American Civil War, the railroad and telegraph companies pushed for the=20 simplified standardized time keeping scheme we currently know. = Consequently,=20 civil time zones were initially instituted in the U.S. and Canada in = November=20 1883 to standardize time keeping.

METEOROLOGICAL TIME KEEPING

Since the collection and exchange of weather information are of = international=20 concern, use of a single worldwide time system is needed so all weather=20 observers around the world can take measurements at the same time, = providing a=20 "snapshot" of the weather. The concept of international time zones was=20 officially adopted in November 1884 at the International Meridian = Conference in=20 Washington DC. For more than 50 years, the times for essentially all=20 meteorological reports have been given according to Greenwich Mean Time = (GMT), a=20 time scale based upon the daily rotation of Earth with respect to a = "mean sun".=20 Often, a single letter, Z, phonetically pronounced "Zulu" has been used = because=20 this letter is used to identify the Greenwich time zone (centered on the = Greenwich Prime Meridian or 0 degree longitude). Currently, standard = practice is=20 to use the more precise Coordinated Universal Time or Temps Universel = Coordinn=E9=20 system (UTC), which is now based upon an atomic clock and time reckoned=20 according to the stars (known as mean sidereal time). For practical = purposes,=20 UTC and GMT systems are essentially equivalent.

By international agreement, surface weather observation times are = minimally=20 0000 Z, 0600 Z, 1200 Z and 1800 Z, with upper air measurements at 0000 Z = and=20 1200 Z each day. In the U.S., surface observations are taken hourly (at = the top=20 of the hour). Radar summary charts are also hourly at 35 minutes past = the hour.=20 Fronts are analyzed on maps every three hours, 0000 Z, 0300 Z, and so = forth. A=20 table of time conversions in the U.S. is listed in the DataStreme=20 Atmosphere website User's Guide.

UNIVERSAL TIME CONVERSION

Because Earth rotates on its axis with respect to the sun once every = 24=20 hours, ideally we should have 24 major civil time zones of equal width. = The 360=20 degrees of rotation divided by 24 give 15 degrees of width to each zone. = The=20 central meridian of the zone is then defined as a longitude evenly = divisible by=20 15 on the system based from the Greenwich Prime Meridian as 0 degrees. = If I were=20 located in the U.S. Central Standard Time Zone, I would be near 90 = degrees west=20 longitude (6 times 15). At any place in this zone, the local time would = be 6=20 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=20 Greenwich (1200) is 6 AM CST. To reduce confusion, all times should be = expressed=20 in the 24-hour time format, such that, 8:45 A.M.=3D0845 and 1:15 = P.M.=3D1315.

Modifications of the boundaries between time zones have been made to=20 accommodate political boundaries in the various countries. Some = countries adhere=20 to a local civil time that may differ by one half hour from that of the = central=20 meridian. A world=20 map available from the US Naval Observatory that shows the current=20 boundaries of all these 24 time zones plus ten additional zones that = have been=20 added to conform to local practice. For the precise location of the four = time=20 zones in the continental United States, consult a recent atlas or = almanac. While=20 most of the United States observes Daylight Saving Time during the = summer (mid=20 March through early November), UTC remains fixed and does not adhere to = a=20 "summer schedule". Therefore, you will have to adjust the time by one = hour=20 during summer. As an example, during the summer, the residents in the = U.S.=20 Eastern Time zone will lag Greenwich by only 4 hours, with 0800 = EDT=3D0700=20 EST=3D1200 Z. An informative web site http://webexhib= its.org/daylightsaving/index.html=20 explains the history involved with the institution of Daylight Saving = Time not=20 only in the United States but in other countries. This site has the = potential=20 for use in the classroom since it also has links to other items of = interest,=20 including the history of calendars and time.

Note: The Energy Policy Act of 2005 has extended Daylight = Saving Time=20 until the first Sunday in November (1 November 2009).

THE TIME IS CURRENTLY...

Suppose that you would like to know the current time as maintained by = the=20 Master Clock at the U.S. Naval Observatory in Washington, DC. Get your = clocks or=20 watches ready and then access the current time from the Time Service = Department=20 at http://www.usno.navy.mil/USNO= /time.=20 The site http://www.time.gov/ also = provides=20 an accurate time check.

Return to the Tuesd= ay Daily=20 Summary

Prepared by Edward J. Hopkins, Ph.D., email mailto:hopkins@meteor.wisc.edu
=A9=20 Copyright, 2009, The American Meteorological Society. =