778 
tend to continue to be steered (but nonsteered lows 
may eventually move parallel to the steering flow), 
and (2) usually veer relative to the steering flow. In 
this study, he has also observed that (8), especially 
for fillmg lows, the angular deviation of their move- 
ment from the steering flow is generally independent 
of their intensity (umber of closed isobars) and less 
for cyclones than for anticyclones, and that (4) this 
angular deviation increases as the upper flow departs 
more and more from straight flow. Longley [49] has 
found evidence to state that strengthening highs move 
toward the left of the steering flow. It is also possible, 
as shown by experience, to predict the deepening and 
filling of the highs and lows. 
The motion of sea-level systems can also be esti- 
mated from the relative hypsography. Term (iw) in 
equation (1) is the so-called thermal steering term. 
This term indicates the tendency for the sea-level sys- 
tems to be steered by the relative isohypses, that is, to 
move im the direction of the thermal wind with a propa- 
gation speed proportional to the thermal wind. Sawyer 
[69, p. 113] has shown empirically that term (iv) be- 
comes increasingly important as the sea-level system 
develops. In practice, the speed of displacement by 
thermal steering can at best be assessed by the fore- 
caster only from experience and statistics. The failure 
in practice of term (iw) to give the correct propaga- 
tion speed is due to the neglected (1) stabilizing control 
of the vertical motion, (2) diabatie processes, and (3) 
variation of the vorticity of the thermal wind (term 
wu). As we have already noted in the previous sub- 
section, the genetic regions of the thermal jet thus 
favor the existence of lows in the confluent warmer 
half of the thermal jet and in the difluent colder half 
and favor the existence of highs in the confluent colder 
half and the difluent warmer half. Consequently cy- 
clones tend to swing out of the thermal jet toward 
lower relative height, anticyclones toward higher rela- 
tive height—and slow down. 
The veering of lows relative to the steering flow, as 
_ cited by Austin [5], may not be in disagreement with 
the above-mentioned backing of lows relative to the 
thermal jet. In the baroclinic region just ahead of a 
low, the thermal jet veers relative to the steering flow 
(i.e., the sense of the horizontal, isosteric-isobaric sole- 
noids is negative). So, by combining Austin’s observa- 
tions with our interpretation of term (72), we should 
expect the lows to move in a direction which is some- 
where between that of the steering flow and the thermal 
flow. In other words, the lows move toward the con- 
centration of horizontal, isosteric-isobaric solenoids. 
This tendency for lows to migrate toward the solenoid 
concentration has been physically confirmed by Pet- 
terssen [62, p. 136, eq. 12]. By applying the horizontal 
del-operator to the vorticity equation and considering 
only horizontal motion, he found an expression for the 
horizontal ascendent of the vorticity tendency. Lows, 
being centers of maximum relative vorticity, must move 
in the direction of this ascendent. According to Pet- 
terssen’s expression for this ascendent, lows have a 
component of motion toward the concentration of hori- 
WEATHER FORECASTING 
zontal, isosteric-isobaric solenoids; opposite considera- 
tions apply to the highs. 
Frontal Occlusion. The frontal wave, even in the case 
of a long wave length, is a fairly long-lived system, 
whereas the so-called ideal cyclone with a narrow but 
unoccluded warm sector exists only for about 6-18 hr. 
Thus, a wave of a quite innocent appearance may well 
occlude into an intensive cyclone during the next prog- 
nostic period (f + 24-36"). 
At the very first stage of occlusion, particularly in 
the lower layers and near the cyclonic core, the con- 
verging and ascending precyclonic air is colder than 
the diverging and subsiding postcyclonic air. These 
dynamic effects should favor the formation of a warm- 
front occlusion. Later, especially at higher levels and 
in the outer region of cyclones which are propagating 
eastward at temperate latitudes, the equatorward cur- 
rent of posteyclonic polar air is usually colder than 
the poleward, precyclonic current, either because it 
originates in higher latitudes or because its recent 
life history is colder. In Europe during the summer 
(winter), the continental precyclonic air of the ordi- 
nary west-east cyclone is warmer (colder) than the 
maritime postcyclonic air, so that the forecaster may 
expect the occlusion to be of the cold-front (warm- 
front) type. In the eastern United States the fore- 
going conditions should, of course, be reversed. For 
cyclones moving in other directions than towards the 
east, the forecaster can still predict the type of oc- 
clusion by systematically using the general temperature 
distribution as shown by the surface and upper-air 
maps. At the last stage of occlusion, the rapidly sink- 
ing tropopause and the marked anticyclonic motion 
within the upper air result in a general tendency to- 
wards subsidence and dynamic warming of the pre- 
cyclonic air, while in the posteyclonic air a tendency 
exists towards ascending motion and dynamic cooling. 
Whereas a cyclone neither oeccludes nor deepens so 
long as the warm-sector tendency is zero, according to 
Petterssen [60, p. 433] the occluding rate of a young 
cyclone is proportional to its deepening and to the 
negative tendency in its warm sector. Moreover, if a 
cold front flows slowly over a mountain range normal 
to its path, an orographic occlusion may occur. Upon 
occlusion the path of the cyclone deviates to the left, 
moving in the direction of the isobars of its warmest 
part (the “false” warm sector). Once the occlusion 
process starts, the propagation speed of the cyclone 
diminishes. If an occluded front approaches a sta- 
tionary continental anticyclone from the west, its move- 
ment is retarded. Finally, the forecaster should watch 
especially for secondary disturbances often arising at 
the point of occlusion. 
Degeneration and Regeneration of Cyclones and 
Anticyclones. The potential energy of adjacent air 
masses is gradually consumed during the occlusion 
process, so that the cyclone normally degenerates dur- 
ing this stage. But the occluded cyclone and its fronts 
may be regenerated either by the same general fronto- 
genetical effects that, according to synoptic experience, 
create the secondary cold front at the end of the bent- 
