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
THE TREND CHART AS AN
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
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
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,
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