A PROCEDURE OF SHORT-RANGE WEATHER FORECASTING 
back occlusion or by the influence of the convergence 
at the front which causes ascent and the release of 
new potential lability. In EHurope during summer and 
in the eastern United States during winter, these fac- 
tors are especially effective at the cold-front occlusion. 
As a result of a “false”? warm sector forming in its 
bent-back trough, the warm-front occluded cyclone 
acquires not only additional potential energy of hori- 
zontally adjacent air masses but also a mechanism 
(viz., the bent-back occluding process) which can trans- 
form this potential energy into kinetic energy in the 
form of increasing cyclonic vorticity. The consequent 
conditional instability of the air mass within the false 
warm sector and the existence there of labile energy 
favors the regeneration of the cyclone. In the eastern 
United States this kind of regeneration is presumably 
more frequent in summer than in winter. A dying cy- 
clone also deepens anew when it comes under the 
influence of a fresh cold air mass or when there is a 
general sharpening of its fronts. An apparent regenera- 
tion of the occluded cyclone often takes place when a 
new wave of the main front overtakes the occlusion. 
If an occluded cyclone regenerates, its speed of prop- 
agation is increased. 
As the anticyclone develops, both its movement and 
its anticyclogenesis decrease, so that the anticyclone 
and cyclone cease to resemble each other in the manner 
of their development. In particular, the duration of 
the anticyclone is highly variable, from a week or so to 
a month or even longer. The anticyclone, now stable 
both physically and dynamically, assumes a semifixed 
geographical location. In the stationary anticyclone, 
the subsidence inversion intensifies and lowers, hinder- 
ing convection. The transformation in properties of 
its air mass has reached a quasi-equilibrium state as 
the air mass, now thoroughly homogenized, is adapted 
to the various steady-state processes operative in its 
region. The anticyclone imposes decisive restrictions on 
the movement, development, and position of adjacent 
cyclone systems. Certain types of anticyclones exhibit 
a remarkable permanence, in time as well as in space. 
In mature anticyclones, the importance of this stability 
in regulating the remote weather is paramount. Per- 
sistent abnormal weather is usually an indirect conse- 
quence of the permanent nature of the mature anti-_ 
cyclone. 
The essential difference between the cyclone and 
the anticyclone is the fact that the latter is usually 
conterminous with a single air mass. One important 
characteristic of the anticyclone, found in classifying 
the air masses according to their source regions, is the 
homogenizing influence of the quasi-permanent sub- 
tropical anticyclones upon the migratory cold anticy- 
clones, which are absorbed into these warm subtropical 
highs. The success of weather prognosis depends, to a 
large extent, upon the somewhat orderly behavior of 
the cyclone. The maximum extent of time in which 
detailed weather prognosis is possible is determined 
partly by the time of evolution of the cyclone. The 
weather immediately associated with the anticyclone 
is not so hazardous as the cyclonic weather; and, partly 
779 
for this reason, anticyclones are not known so well as 
cyclones. Anticyclones with divergent motion are never- 
theless primarily instrumental in the transformation of 
the solar radiative energy received by the atmosphere 
into the kinetic energy of its motion relative to the 
earth. They are sources, then, for the conversion of 
geopotential energy or internal heat energy into hori- 
zontal kinetic energy [75]. From them the processes of 
advection and work by pressure forces are continually 
and rapidly transferring this kinetic energy to cyclonic 
areas, that is, to the centers of major weather activity. 
Local Modifications of Air Currents, Air Masses, 
and Fronts. Whenever possible, the forecaster should 
apply the foregoing models while working out the 
prognostic maps. Sometimes, however, their applica- 
tion must be introduced between the preparation of 
the prognostic map and the formulation of the fore- 
cast. This introduction may be particularly necessary 
because of important (thermal and orographic) local 
modifications to the idealized, normal evolution of the 
lower-tropospheric models. These disturbances are par- 
ticularly important to the area forecaster; but they 
often operate on a scale so large that, if ignored by the 
regional forecaster, they would introduce grave errors 
into the general forecast. 
When an air mass moves over a colder (warmer) 
surface, its stability (mstability) is increased. An air 
mass becomes stable in winter (unstable in summer) by 
motion from sea to land; for motion from land to sea 
the reverse is true. The stability is increased (de- 
creased) for an air mass moving poleward (equator- 
ward) over an isothermal portion of the surface of the 
earth. The air-mass stability (instability) of a sta- 
tionary continental anticyclone increases from day to 
day in fall and winter (spring and summer). Especially 
near the ground, the properties of warm and cold air 
masses show a marked diurnal variation, because they 
depend mainly on the vertical lapse rate of tempera- 
ture. At night (in the afternoon), a warm (cold) air 
mass will have all its properties accentuated and a 
cold (warm) air mass will, in the lowest layer, tem- 
porarily acquire the properties of a warm (cold) air 
mass: uniform wind, poor visibility, stratiform clouds, 
even drizzle occasionally in the case of a warm air 
mass; gusty wind, improved visibility, cumuliform 
clouds and showers in the case of a cold air mass. Over 
sea the effects will be the opposite, although much less 
marked. 
Moreover, periodic wind systems are often created 
owing to differences in the diurnal temperature varia- 
tion over land and sea, and over mountain and plain 
these wind systems may become both intensive and 
extensive enough to influence even the general forecast. 
The more stable a moving air mass, the more it tends 
to converge horizontally about an obstacle such as 
an elevated part of a continent bordering a plain or a 
lowland bordering the sea. To the low-pressure side of a 
stable warm air-mass current, the forecaster should 
thus predict an extraordinary intensification of the 
wind at the ‘‘corner”’ of the obstacle—the corner effect— 
and of the “coastal”? convergence and precipitation at 
