The lowering of ceiling with continuous rain or snow in warm frontal and upper trough situations is a familiar problem to the forecaster in many regions. In very short-range forecasting, the question as to whether or not it will rain or snow, and when the rain or snow will begin, is not so often the critical question. Rather, the problem is more likely to be (assuming the rain/snow has started) how much will the ceiling lower in 1,2, and 3 hours, or will the ceiling go below a certain minimum in 3 hours. The visibility in these situations generally does not reach an operational minimum as soon as the ceiling. It has been shown that without sufficient convergence, advection, or turbulence, evaporation of rain into a layer does not lead to saturation, and causes no more than haze or light fog.

Figure 4-12.-Isochrones of beginning of precipitation, an early winter situation.

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.

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.

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