784 
crease due to advection. In most instances of saturated 
ascent, the initial lapse rate departs only slightly from 
the saturated imdifferent one. In such cases even large 
upward vertical velocities have negligible effects upon 
the relative-height change. 
The Local Changes in the Relative Height Asso- 
ciated with the Diabatic Processes. The local change 
in the relative height can be regarded as due in part 
also to a variation in the entropy of the air particles 
within the isobaric layer. Mainly because of evapora- 
tion, the latent heat being supplied by the air, and 
also by direct cooling, falling rain produces an ap- 
preciable diabatic decrease of the relative height in 
the lower layers. For example, the 850/1000-mb height 
is reduced about 20 gpm (geopotential meters) during 
the first four hours of a typical warm-front rainfall; 
after that (as the layer approaches a saturated state) 
no appreciable reduction takes place. Of course, in 
the adjacent lower layer of a temperature inversion 
the direct warming from falling precipitation partly 
reduces the larger relative-height decrease due to the 
evaporation. (In Fig. 8a, for example, it can be seen 
that the nonadvective decrease in the 500/1000-mb 
height corresponds roughly to the areas of precipitation 
and of the altostratus-nimbostratus cloud system.) 
Diabatic changes in the relative height may also be 
brought about by the contact of the air mass of the 
total mandatory layers with warmer or colder ground 
or sea surface. Owing to increased turbulence by heat- 
ing from below, a warm earth surface affects the rela- 
tive height of a deeper layer than does a cold surface. 
The increase in the relative height is also particularly 
large for the 1000—700-mb layer in the cold air mass of 
polar outbreaks. (In Fig. 8a, for example, where the 
northwesterly flow of the cold air passes from the 
snow-covered Great Plains to the snow-free south- 
central states, a crescent-shaped center of nonadvective 
increase is found in the 500/1000-mb hypsography; 
the opposite considerations apply to the southerly flow 
of warm air as it passes from snow-free southeastern 
United States to snow-covered New England and 
Quebec.) Yet, even the ordinary turbulence in winds 
around 15 knots produces a decrease in the relative ' 
height for as much as the lowest hundred meters of a 
warm air mass. Even so, this decrease is still relatively 
large compared with the decrease by radiative cooling 
of that layer, the last atmospheric process of impor- 
tance to be considered for relative-height change. 
The relative-height decrease due to the net radiative 
cooling of the cloudless atmosphere is greater in sum- 
mer, at low latitudes with warm humid air, and for 
low altitudes than im winter, at high latitudes with 
cold dry air, and for high levels. For example, the 
summertime radiative decrease in the 700/1000-mb 
height of a cloudless atmosphere is very roughly 5—10 
gpm per day in a typical tropical air mass. Although 
according to estimates of Hlsasser [24] the net rate of 
radiative cooling decreases steadily from 2 km to 5 km, 
later investigations by Penner [57] in an arctic region 
show that this rate is a maximum in the 600-mb to 
500-mb layer. 
WEATHER FORECASTING 
The Prognosis of the Relative Hypsography by Non- 
advective and Diabatic Extrapolation. These changes 
occurring in the quasi-conservative aspects of relative 
height are described by the nonadvective relative al- 
lohypsography, the allo-entropic centers of which are 
mainly due to (1) transisobaric lifting or sinking and 
to (2) condensation or evaporation (the latter espe- 
cially in connection with precipitation) if the period 
is 24 hr. For periods shorter than 24 hr they will often 
be due mainly to (3) radiative heating or cooling of 
the air, which appreciably modifies this pattern, espe- 
cially over high plateau areas such as western North 
America. 
Further subdivision of the change in the relative 
height according to its component processes is not 
feasibly incorporated into the daily prognosis. In the 
practical method for calculating the transisobarie dis- 
placement of the air, the necessary assumption that 
the entire nonadvective change in relative height is 
adiabatic prevents us from separating the nonadvective 
relative allohypsography into, say, its “convective,” 
“precipitative,” and “radiative” relative allohypsog- 
raphies. (Godske et al [33] have shown how the com- 
parative intensities in these diabatic processes can be 
determined.) The results of Godske’s method can then 
be introduced into stage five (see below). 
The foregoing considerations of the nonadvective 
changes in the relative height, substantiated by synop- 
tic experience, can be generalized into a synoptic wpper- 
air model, which relates the nonadvective relative 
allohypsography to the corresponding relative hypsog- 
raphy. For example, the center R of nonadvective in- 
crease in Fig. 8a is just behind the equatorward portions 
of the troughs in the 500/1000-mb hypsography in 
Fig. 8b. The ridges in the relative hypsography (see 
Fig. 8b) and its centers of nonadvective decrease, F 
in Fig. 8a, are similarly related. Generally speaking, 
these centers of nonadvective increase and decrease in the 
relative hypsography for the period ty) — (to — 24") appear 
respectively at, or just behind, the equatorward portion 
of the trough and the poleward part of the crest in the 
relate hypsography for to . 
Important, then, in preparing the way for the prog- 
nosis is the analysis of the nonadvective relative 
allohypsography for the past 24-hr period, the zero iso- 
pleth of which, when traced onto the relative hypsog- 
raphy for t), can be applied as the starting place of 
prognosis by advective-adiabatic extrapolation. At the 
position of this isopleth, the relative isohypses have 
a past 24-hr displacement due just to their isobaric 
advective transport according to the past 24-hr flow 
of their layer. As the starting point for the fourth prog- 
nostic stage, already introduced, we may proceed, then, 
to advect adiabatically, during the 24-hr prognostic 
period, those portions of the relative isohypses for 
to which are in the vicinity of this isopleth. The prog- 
nostic positions so obtained constitute a tentative, 
fixed framework for the prognosis by nonadvective 
and diabatic extrapolation. 
Thus, as stage five in the order of prognostic opera- 
tions, we now complete the layer prognosis by supple- 
