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
