analysis, the valid time is the synoptic hour of the observations. For a prognosis, the valid time is the moment in the future when the actual conditions should be most like the conditions depicted. Certain products are called time-phased products. These products show boundaries  of  areas  that  will  be  effected  by  moving weather systems over a specified period of time. Time- phased products may either state a valid period of time, such as ACCUM PRECIP PROGNOSIS VT: 2 1 / 1200Z TO 22/1200Z DEC 96, or may indicate the time period in the title, and use a valid time for the end of the period, such as 24HR PRECIP 24 HR FCST VALID 22 DEC 96 1200Z. When a product is used as a briefing aid, it is common practice to enlarge the valid time on the paper copy or video display so that the audience may see the time without eye strain, and to show both the UTC time and the local time. For a briefing in Norfolk, Virginia (time zone "R"), a briefing product valid at 12Z 18 Dec 96 would be identified with an enlarged valid time of VT:   ^1200Z18 DEC 96 0700R See Appendix III for a chart of time zones. GRAPHIC  HISTORY Occasionally, you will be asked to place history on a surface chart or a constant pressure level chart prior to manual   analysis. History is the past positions  of pressure or height centers, fronts, troughs, or ridges. History helps the analyst or forecaster to determine past movements of major chart features.  It is a valuable tool in  both  analysis  of  the  current  situation  and  the prognosis  of  future  positions  of  the  same  features. To place history on a chart, the previous locations of pressure centers or height centers, frontal systems and troughs,  and  major  features,  such  as  jet  stream locations, are marked on the chart. Selected isobars, isoheights, or isotherms may also be required by the forecaster. Normally, two sets of positions are placed on a chart:  either the 6- and 12-hour-old positions or the 12-  and  24-hour-old  positions.  For  instance,  a  chart containing  surface  plots  of  05  December  1200Z observations  may  have  history  marking  the  past positions of fronts and pressure centers of 5 December at 0000Z (VT minus 12 hours) and 4 December at 1200Z (VT minus 24 hours). When two sets of history are used, the most recent history positions are marked in orange, while the oldest positions are marked in yellow. If only one set of history is marked, the positions are marked in yellow. Standard symbols are used for all features. The symbols are discussed in the following section. DATA  DEPICTION  STANDARDS Nearly   every   graphic   product   that   weather observers,  analysts,  and  forecasters  deal  with  is designed  either  to  be  received  by  electrographic methods as ready to use or to be completed by an analyst and sent to another user via electrographic methods. The methods sometimes used to disseminate charted products may be as simple as copying the chart on a photocopier and carrying it to the user. Although some of these methods allow the use of color, many graphics transmission  devices  operate  only  in  black-and-white mode. Because of this, charts produced on color- capable computer displays normally use standard depictions that combine color and patterns. When charts are reproduced in black and white, the shape of the symbol alone identifies the feature depicted. Figure 4-12 shows the standard symbols and colors used on surface and constant pressure level analysis and prognosis  charts.    Figure 4-13 shows the symbols and TERM SYMBOL COLOR Divergence line (asymptote) BLUE Convergence  line  (asymptote) RED Anticyclonic circulation center BLUE Cyclonic  circulation  center RED Neutral  point BLACK Figure 4-13.—Symbols used on surface and constant-pressure level streamline analysis and prognosis charts. 4-15