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THE TREND CHART AS AN EXTRAPOLATION AID | Up
Aerographers Mate 1 & C | Next
CLOUD ANALYSIS AND FORECASTING |
station may suffice. The trend chart format, figure 4-13,
is but one suggested way of portraying the weather
record. Experimentation and improvisation are
encouraged to find the best form for any particular
location or problem.
TIME-LINER AS AN EXTRAPOLATION AID
In the preceding sections of this chapter, several
methods have been described for “keeping track of the
weather” on a short-term basis. Explanations of
time-distance charts, isochrone aids, trend charts, etc.,
have been presented. It is usually not necessary to use
all, or even most of these aids simultaneously. The aid
described in this section is designed for use in
combination with one or several of the methods
25NP0029
Figure 4-13.-Trend chart suggested format.
previously described. Time-liners are especially useful
for isochrone analysis and follow-on extrapolation.
Inasmuch as a majority of incorrect short-range
forecasts result from poor timing of weather already
upstream, an aid, such as described below, may improve
this timing.
Construction of the Time-Liner
The time-liner is simply a local area map that is
covered with transparent plastic and constructed as
follows:
1. Using a large-scale map of the local area,
construct a series of concentric circles centered on your
station, and equally spaced from 10 to 20 miles apart.
This distance from the center to the outer circle depends
on your location, but in most cases, 100 to 150 miles is
sufficient.
2. Make small numbered or lettered station circles
for stations located at varying distances and direction
from your terminal. Stations likely to experience your
future weather should be selected. In addition to the
station circle indicators, significant topographical
features, such as rivers or mountains, maybe indicated
on the base diagram. (Aeronautical charts include these
features.)
3. Cover and bind the map with transparent plastic.
Plotting and Analysis of the Time-Liner
By inspection of the latest surface chart, and other
information, you can determine a quadrant, semicircle,
or section of the diagram and the panmeters to be
plotted. This should be comprised of stations in the
direction from which the weather is approaching your
station. Then, plot the hourly weather SPECIALS for
those stations of interest. Make sure to plot the time of
each special observation.
Overlay the circular diagram with another piece of
transparent plastic, and construct isochrones of the
parameter being forecast; for example, the time of
arrival of the leading or trailing edge of a cloud or
precipitation shield. The spacing between isochrones
can then be extrapolated to construct “forecast
isochrones” for predicting the time of arrival of
occurrence of the parameter at your terminal. Refer to
figure 4-14 for an example.
4-12
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