LOWERING OF CEILING IN CONTINUOUS RAIN AREAS

made from them if reasonably regular past motions are in evidence. A separate isochrone chart (or acetate overlay) may be easier to use. Lines for the beginning of continuous precipitation are illustrated in figure 4-12. The isochrones for showery or intermittent precipitation usually give more uncertain and irregular patterns, which result in less satisfactory forecasts. When large-scale section surface weather maps are regularly drawn, it maybe sufficient and more convenient to make all precipitation area analyses and isochrones on these maps. Forecasting the Movement of Precipitation by Using a Distance versus Time (x-t) Diagram The idea of plotting observations taken at different times on a diagram that has horizontal or vertical distance in the atmosphere as one coordinate and time as the other has been used in various forms by forecasters for years. The time cross section that was discussed in the AG2 TRAMAN, volume 1, unit 9, lesson 2. is a special case of this aid, where successive LOWERING OF CEILING IN CONTINUOUS RAIN AREAS One of the many obstacles the forecaster faces in preparing forecasts is the problem of determining “when” ceilings heights will lower in areas expecting rain. In the following paragraphs, we will discuss this dilemma. Frontal Situations 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 . information at only one station is plotted. no more than haze or light fog. Figure 4-12.-Isochrones of beginning of precipitation, an early winter situation. 4-10