ISENTROPIC ANALYSIS 
151 
eee ——————— 
tal precipitation occurs the gradient 
of contour lines is steep and a source 
of moist air is not far removed. The 
precipitation band is normally ori- 
entated to the left of moist tongues so 
that there is probably considerable 
upslope motion of the moist air in 
this region. 
In the warmer seasons much of the 
precipitation in continental areas is 
non-frontal in character. This pre- 
cipitation is chiefly of the convective 
type, and occurs as local showers and 
and thundershowers. In Articles VIII 
and IX we discussed the detailed use 
of energy diagrams for forecasting 
these showers. However, the use of 
energy diagrams becomes even more 
effective when one takes into consi- 
deration not only the state rep- 
resented by the energy diagram at 
the time of the sounding but also the 
probable changes with time caused by 
the advection of moist and dry 
tongues at various levels. The isen- 
tropic analysis offers by far the most 
satisfactory method of doing this. 
We shall first discuss a few of the 
characteristic vertical distributions 
of temperature and moisture normally 
observed over continental United 
States in summer. The first type 
(shown in fig. 11) has an extensive 
2.0 5.0 10 ib) 10 15 
290 300 
NNN 
YS 
Wa a |] 
Fic. 11. CHARACTERISTIC VERTICAL DISTRIBUTIONS OF TEMPERATURE AND 
MoIsturRE OBSERVED OVER CONTINENTAL UNITED STATES IN SUMMER. NUMER- 
ALS PLOTTED ALONG THE ASCENT CURVES INDICATE RELATIVE Humipity. (Plot- 
ted on pseudo-adiabatic charts, »°* vs T, with the dry adiabats (isentropes) 
of @ = 290°, 300°A, only.) 
dry layer overlying a relatively moist 
stratum of about 2 km thickness. The 
transition zone between the lower 
moist and the overlying dry air is nor- 
mally a very stable layer, often a 
marked temperature inversion. The 
second type has no discontinuities in 
temperature and moisture content. 
Furthermore, the air column is not 
far from saturation. Type 3 repre- 
sents a transition between the types 
1 and 2, and here there is a 2-km 
layer of moist air next to the surface, 
with dry air sandwiched in between 
this stratum and another layer of 
high moisture content aloft. As in 
type 1 there is a stable layer between 
the dry and moist air, although less 
stable, and as in type 2 the lapse rate 
aloft is fairly uniform and normally 
slightly steeper than the saturated 
adiabat, while in type 1 it is almost 
equal to the dry adiabat. 
cient to overcome the negative area 
below and positive areas above for 
convective impulses from below. Nor- 
mally such impulses (even at the time 
of maximum temperature) are insuffi- 
cient to frustrate the negative area 
and cause overturning of the whole 
air column. Type 2 normally gives 
large amounts of available energy for 
upward impulses that occur at the 
