METEOROLOGICAL ANALYSIS IN MIDDLE LATITUDES 
Detailed analysis and study is likewise a requisite for 
short-period local forecasts. This whole question of the 
selection of a meteorological base map is not one which 
can be solved theoretically once and for all time; it is a 
recurrent practical problem, whose solution is con- 
tingent upon the specific conditions under which the 
map will be used. This specificity, which is characteristic 
of so many meteorological perplexities, renders the 
problem by no means unimportant. 
Moisture Charts 
Another equally specific, equally significant, difficulty 
is that of devising a chart, or charts, which will portray 
graphically those factors which are conducive to showers 
other than those associated with fronts or pressure 
jumps. Such a chart (or charts) would be the sine qua 
non otf local forecasting, particularly in regions of high 
summertime precipitation. In an attempt to solve this 
problem, sundry types of energy diagrams have been 
developed along with techniques for computing the like- 
lihood of showers. Empirical results have been disap- 
pointing to such an extent that it appears probable that 
our present methods for evaluating the energy due to 
thermal instability fail to take into account all the 
factors involved in the release of energy as shower 
activity. This is corroborated by the recent studies on 
entrainment of air into the sides of inerescent cumuli- 
form clouds. The findings of these investigations suggest 
that the occurrence of showers is not often due to pure 
thermal instability, pure air-mass showers being very 
rare. Indeed, evidence has been disclosed that a wide- 
spread convergent wind flow, a pressure jump, a moun- 
tain range, or some other type of trigger action is 
necessary to give well-developed shower activity. The 
recent studies of the thundershower activity in Florida 
and its relation to the convergent effect of the sea 
breezes from two edges of the peninsula afford a good 
illustration of how important such effects are on air- 
mass shower activity [7]. Certainly, this is a subject on 
which more research is sorely needed. 
Although our knowledge of the relationship between 
the vertical energy distribution and subsequent shower 
activity is imperfect at present, there appears to be a 
more readily applicable relationship between the hori- 
zontal moisture distribution and the resultant showers. 
In the days before there were reliable upper-air sound- 
ings, an analysis of ciouds and precipitation was the 
main clue to the distribution of moisture. But after the 
establishment of the network of suitably accurate 
radiosondes, many experiments were made to determine 
the type of chart (or charts) which would be best 
suited for portraying the horizontal distribution of 
moisture in the atmosphere and for following its succes- 
Sive variations. 
At the present time the horizontal distribution of 
atmospheric moisture is usually studied by means of 
isopleths of dew point on either the 850-mb or 700-mb 
chart, or on both. The basic disadvantage of this pro- 
cedure is that air particles frequently undergo vertical 
motions rendering it impossible to be sure that one is 
following from moment to moment the same parcels of 
725 
air. This, of course, makes various invalid inferences 
very tempting. But even with the assumption of quasi- 
horizontal motion, this method for the analysis of 
atmospheric moisture is far from being completely satis- 
factory. Its value is impaired by the fact that the 
patterns produced by the dew-point isopleths tend to be 
indistinct and fragmentary on both the 850- and 700-mb 
charts. Through the study, however, of these particular 
isopleths, considerable valuable information may be 
obtained. 
There is evidence that it is in the plateau and moun- 
tain regions of western North America that the most 
fruitful application of this procedure for moisture anal- 
ysis can be made, a somewhat surprising disclosure 
since most analytical methods give their best results 
over oceans or over plains of low elevation. In the areas 
of these western plateaus and mountains we find that 
the 700-mb configuration of isopleths of dew point seem 
closely correlated to shower precipitation all during the 
warm season. In fact, many meteorologists feel that the 
wind and moisture patterns at 700 mb may be used 
alone to give satisfactory forecasts for showers in these 
mountain areas. Over relatively flat areas where the 
trigger action of mountains is lacking, more information 
is needed for the prediction of showers. In particular, it 
would be of benefit to know the location of areas where 
upslope-or downslope motion of moist parcels of air is 
occurring. 
Isentropic Analysis 
This need for a more inclusive chart, designed to 
depict not only the distribution of atmospheric motion 
but also proximity of each air parcel to saturation, the 
location of areas where there are upslope or downslope 
movements, and the major flow patterns of the air, is 
met by the isentropic chart [11]. Besides its important 
property of inclusiveness, the isentropic chart is the 
chart best suited for following the motion of air particles 
since the particles of an isentropic surface are conserva- 
tive for all adiabatic changes. The patterns formed by 
lines of equal condensation height, or pressure, on an 
isentropic chart are therefore less fragmentary and 
more persistent than the configurations formed by the 
isopleths of dew point on a surface of constant pressure. 
In short, on an isentropic chart the distribution of 
moisture is simply and vividly apparent and the analyst 
can, in general, validly follow a particular air particle 
in its day-to-day convolutions. Thus the isentropic 
chart is a potent addition to our tools for preparing 
optimum analyses. 
Shortly after the upper-air network had become well 
established, the isentropic chart was introduced and 
tried out in all the meteorological centers of the United 
States. At that time it was the practice to transmit to 
the field stations, along with the other upper-air data, 
the data for three different constant potential temper- 
ature surfaces. The purpose of transmitting the data for 
three isentropic surfaces was to allow the local mete- 
orologist to select for analysis the one isentropic surface 
most useful to his own locale. 
During this period of experimentation the most im- 
