776 
2. From the foregoing rule, as well as from synoptic 
experience, Rodewald [66] had observed (1937) that a 
rise (fall) is weakened as it approaches the position of 
the trough (ridge) in the upper absolute hypsography. 
Y 
CYCLONIC 
fed 
ANTICYCLONIC 
SHEAR 
a 
(a) SURFACE LOW DEEPENING (b) FILLING LOW 
\ CYCLONIC 
SHEAR 
ANTICYCLONIC 
SHEAR Y 
(c) SURFAGE HIGH 
STRENGTHENING 
Fic. 4.—The deepening and filling of disturbances at sea level 
and the shear in the flow aloft. 
(d) WEAKENING HIGH 
3. A sea-level pressure trough or ridge will weaken 
if the surface and upper-air maps show that its axis 
is vertical or nearly so. This hypothesis, which follows 
PRESSURE 
PRESSURE 
HIGH 
Fra. 5.—The compensating effects of the curvature and 
spreading of the absolute isohypses of a quasi-stationary upper 
trough. (After Scherhag [72].) 
from the two foregoing rules, is correct if the fric- 
tional effect of the earth upon the surface flow is not 
compensated by other diabatic effects or by an ap- 
preciable transfer of angular momentum. 
WEATHER FORECASTING 
Finally, some physical reasoning should be involved 
in estimating accelerations in the deepening (filling) 
of the pressure system. If, for instance, the surface 
map shows a wave in its early stage of development, 
the forecaster would advisably use a greater average 
deepening per 3 hr for the coming 12 hr or 24 hr than 
the rate found either by formulas or from the map 
sequence at the past 3 hr. In other words, a “deepening 
acceleration” should be estimated according to the 
particular stage in the life history of the chosen tropo- 
spheric model and applied qualitatively as a correc- 
tion to the simple first-order extrapolation of the actual 
pressure systems. 
The Movement of Sea-Level Pressure Systems. A 
young cyclone generally propagates in the direction of 
its warm-sector isobars. An older low (high) with 
strongly diverging, noncircular isobars tends to have a 
straight path in a direction between its longer axis of 
symmetry and the isallobaric gradient (ascendent); the 
more the isobars diverge, the closer is the path im the 
direction of this long axis, according to Philipps [63]. 
A low (high) with approximately circular isobars pro- 
gresses in the direction of the isallobaric gradient (as- 
cendent), that is, normal to the isallobar through its 
center or, according to synoptic experience, sometimes 
a little to the left; it may also be extrapolated toward 
the domain of subnormal (supernormal) winds. Tem- 
perature, as well as pressure and pressure tendency, 
should be considered. For example, a stable wave pro- 
gresses along a front in the direction of the warm-air 
flow, while a cyclone with sharply defined zones of 
temperature maximum and minimum moves normal 
to the temperature gradient. The center of an anti- 
cyclone shifts in the direction of most rapid decrease 
of temperature. In the usual case of cold air poleward 
of a front along which warm air is flowing eastward, 
the eastward paths of the successive members of a 
cyclone family—the duration of which is roughly 5-6 
days—have a progressive translation equatorward. 
Anticyclones and ridges of high pressure which lie 
between cyclones move generally in the same direc- 
tion and with the same speed as the frontal cyclones. 
But anticyclones which are cut off from a series have 
an equatorward component of motion. Moreover, a 
cyclone seeks to revolve in a clockwise sense around a 
neighboring stationary anticyclone, while two cyclonic 
disturbances of the same intensity tend to revolve 
about each other in a counterclockwise sense. 
Centers with a linear pressure profile or a strong 
pressure gradient may be extrapolated with a constant 
propagation speed which, for a circular center, is di- 
rectly proportional to the isallobaric gradient and in- 
versely proportional to the curvature of the pressure 
profile. The speed of stable waves and of cyclones with 
weak pressure and isallobaric gradients is greater than 
that of a deepening cyclone, which in turn moves more 
rapidly than a filling cyclone. Centers with a strong 
pressure gradient move even more slowly; a center is 
quasi-stationary if it is concentric with a symmetrical 
isallobarie center. The cyclone should be extrapolated 
with an increasing (decreasing) speed of propagation 
