THE TREND CHART AS AN EXTRAPOLATION AID

It is important to recognize the difference between the behavior of the actual cloud base height and the variation of the ceiling height, as defined in airway reports. The ceiling usually drops rapidly, especially during the first few hours after the rain or snow begins. However, if the rain or snow is continuous, the true base of the cloud layer descends gradually or steadily. The reason for this is that below the precipitation frontal cloud layer there are usually shallow layers in which the relative humidity is relatively high and which soon become saturated by the rain. The cloud base itself has small, random fluctuations in height superimposed on the general trend. Time of Lowering of Ceiling Forecasting the time when a given ceiling height will be reached during rain is a separate problem, Nomograms, tables, air trajectories, and the time air will become saturated can all be resolved into an objective technique tempered with empirical knowledge and subjective considerations. This forecast can be developed for your individual station. Extrapolation of Ceiling Trend by Means of the x-t Diagram The x-t diagram, as mentioned previously in this chapter, can be used to extrapolate the trend of the ceiling height in rain. The hourly observations should be plotted for stations near a line parallel to the probable movement of the general rain sea, originating at your terminal and directed toward the oncoming rain area. Ceiling-time curves for given ceiling heights may be drawn and extrapolated. There may be systematic geographical differences in the ceiling between stations due to local (topographic) influences. Such differences sometimes can be anticipated from climatological studies, or experience. In addition, there may be a diurnal ceiling fluctuation, which will become evident in the curve. Rapid and erratic up-and-down fluctuations also must be dealt with. In this case, a smoothing of the curves may be necessary before extrapolation can be made. A slightly less accurate forecast may result from this process. In view of the previous discussion of the precipitation ceiling problem, it is not expected that mere extrapolation can be wholly satisfactory at a station when the ceiling lowers rapidly during the first hours of rain, as new cloud layers form beneath the front. However, by following the ceiling trend at surrounding stations, patterns of abrupt ceiling changes may be noted. These changes at nearby stations where rain started earlier may give a clue to a likely sequence at your terminal. THE TREND CHART AS AN EXTRAPOLATION AID The trend chart can be a valuable forecasting tool when it is used as a chronological portrayal of a group of related factors. It has the added advantage of helping the forecaster to become “current” when coming on duty. At a glance, the relieving duty forecaster is able to get the picture of what has been occurring. Also, the forecaster is able to see the progressive effect of the synoptic situation on the weather when the trend chart is used in conjunction with the current surface chart. The format of a trend chart should be a function of what is desired; consequently, it may vary in form from situation to situation. It should, however, contain those elements that are predictive in nature. The trend chart is a method for graphically portaying those factors that the forecasters generally attempt to store in their memory. Included in this trend chart is a list of key predictor stations. The forecaster uses the hourly and special reports from these stations as aids in making short forecasts for his/her station. Usually, the sequences from these predictor stations are scanned and committed to memory. The method is as follows: 1. Determine the direction from which the weather will be arriving; i.e., upstream. 2. Select a predictor station(s) upstream and watch for the onset of the critical factor; for example, rain. 3. Note the effect of this factor on ceiling and visibility at predictor station(s). 4. Extrapolate the approach of the factor to determine its onset at your station. 5. Consider the effect of the factor at predictor station(s) in forecasting its effect at your station. The chief weakness of this procedure is its subjectivity. The forecaster is required to mentally evaluate all of the information available, both for their station and the predictor station(s). A question posed, “How many trend charts do I need”? The answer depends on the synoptic situation. There are times when keeping a graphic record is unnecessary; and other times, the trend for the local 4-11