790 
traversed by it. Hail starts rather suddenly and usually 
lasts from a few minutes to less than half an hour; 
it is mostly followed by rain, these two phases being 
partly superimposed. It falls along narrow streaks a 
few kilometers wide. The foregoing considerations show 
H799 (FT) —> 
9400 9700 10,000 10,300 
APsro-LAX 
(mb) 
15) 
Pseo-(mb) ~~ 
1004 J010 1016 1022 
APL ax-PHX 
(mb) 
(d) 
WEATHER FORECASTING 
for predicting the amount of winter rainfall occurring 
in Los Angeles during the 30-hr period beginning at 
to + 6. 
Figure 10 shows the results of the complete graphical 
integration process using the six variables listed in 
Dsps 
+12 
+6 
(e) 
Fic. 10.—A nomogram for making an objective forecast of rainfall at Los Angeles. The rectangular coordinates of the scatter 
diagrams represent f-values of the six predictors listed in Table II. The solid curves are isograms of rainfall amount, adjusted to 
a scale of 0 to 10, during the 30-hr period following t) + 6%. In part (c), the variable Dspp loses its sensitivity as an indicator of 
the true wind field at low wind speeds, and thus was not used for speeds less than seven knots. These cases were plotted against 
T7 along the vertical axis to the right of the scatter diagram and analyzed separately to determine X3. The solid curves in 
part (e) define a variable W,, which is used in computing rainfall probabilities as given in Table III. 
that the individual shower prediction will, at best, 
be possible only some few hours in advance for very 
small districts and with the assistance of rareps from 
specially tramed observers, who reside within these 
districts in places with free horizon. 
Amount of Precipitation. The application of objective 
techniques introduced in the first subsection can now 
be illustrated by Thompson’s objective method [79] 
Table II. The analysis of each part of this figure was 
carried out by first constructing isograms of rainfall 
amounts on parts a-c of Fig. 10 and then adjusting 
the isogram values to a scale 0 to 10. This latter device 
provides a uniform system of coordinates on all suc- 
ceeding scatter diagrams d-e of Fig. 10, an operation 
which somewhat simplifies their use by the forecaster. 
The order in which the variables were combined is 
