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