770 
extrapolation methods to the sea-level pressure maps for 
to and t + 3". After the completed prognostic surface 
map for ¢t) + 3° has been used as an intermediate step 
for finding the sea-level prognostic surface map for f) ++ 
24" (by reapplying the foregoing methods), it is laid 
aside to be used later as the next surface-map analysis. 
The Physical Extrapolation of the Surface Map. As an 
analyst, the meteorologist 1s concerned with selecting 
models which can be fitted to the observations. As a 
forecaster, he considers how well the development of the 
chosen model fits the geometrical extrapolation de- 
scribed previously, and to what extent adjustments are 
necessary, either to this extrapolation or to the idealized 
development of the models during the forecast period. 
In both their ideal and deformed states, these tropo- 
spheric models are formulated from experience (in daily 
weather analysis) combined with special scientific im- 
vestigations. Synthesizing, roughly, our increasing 
knowledge about atmospheric dynamics and thermo- 
dynamics, these models should not only be continually 
readjusted and improved but should also be supple- 
mented by new models. Since no rigid set of tropospheric 
models exists, weather-map prognosis should not stiffen 
into a series of routine mechanical procedures. 
The prognosis of the surface map can be facilitated 
by the following general considerations concerning the 
physical consistency of the tropospheric models. Air 
masses, fronts, cyclones, and anticyclones extend over 
large areas. In other words, the models must be geo- 
metrically consistent, a condition which leads to simplifi- 
cations for the prognosis. Further, the displacement and 
deformation of the models during the prognostic period 
must be kinematically probable. Equipollently, the ex- 
trapolated position of a model must be kinematically 
consistent with the model at the present time and with 
the average field of motion during the prognostic period, 
both in the region covered or passed by the model and 
within the model itself. For example, a shallow, lentic- 
ular-shaped depression must be connected with) an 
initial wave at a frontal zone, a deeper and rather 
circular one with an occlusion. Moreover, the structure 
and the stage of development of the chosen model 
must also be in agreement with the field of forces acting 
upon it during the prognostic period; that is, the model 
must be dynamically consistent. For example, a shallow 
and lenticular depression, implying small vorticity and 
a comparatively small supply of kinetic energy, will 
correspond to the initial stage of a frontal disturbance 
with a maximum of potential energy. On the other hand, 
the deep and circular depression, implying maximum 
vorticity and kinetic energy, could as a rule result only 
from the occluding process of a frontal disturbance. 
Finally, the chosen model must harmonize with the 
observed processes of radiation, heat conduction, con- 
densation, and precipitation, that is, the model must 
be thermodynamically consistent. 
The tropospheric models possess certain physical 
characteristics, found by theory and synoptic experi- 
ence, which must be borne in mind by the meteorologist 
in making the map prognosis and the weather forecast. 
We shall now recapitulate these characteristics in the 
WEATHER FORECASTING 
same order as the irreversible, cyclic development of the 
models, namely, (1) frontogenesis, frontal cyclogenesis, 
and anticyclogenesis, (2) the movement: of sea-level 
pressure systems, (3) frontal occlusion, (4) degenera- 
tion and regeneration of cyclones and anticyclones, 
(5) local modifications of air currents, air masses, and 
fronts, and (6) frontolysis, cyclolysis, and anticyclolysis. 
Frontogenesis, Frontal Cyclogenesis, and Anticyclo- 
genesis. In an extensive quasi-stationary pressure col 
on the surface map, the forecaster should probably 
expect kinematic frontogenesis if the isotherms form a 
smaller angle with the axis of stretching than with the 
axis of shrinking (the part of the front marked “‘active”’ 
in Fig. 2). At least in the lowest layer, the general pole- 
ward decrease of temperature may sometimes be 
exceeded, especially along coastal regions, by the longi- 
tudinal temperature differences connected with the dis- 
tribution of land and sea. Then, in these coastal regions, 
the cols with a north-south (west-east) axis of stretching 
give rise to frontogenesis (frontolysis). 
Aloft, an accelerating jet stream in the direction of 
the frontogenetically effective axis of stretching aids 
in the upward extension of the frontogenetic field of 
deformation. Experience has indicated that a weak 
frontal surface will change in intensity (either by fronto- 
lyzing or by frontogenizing) wherever on a constant- 
pressure map one or more of the following conditions 
are observed: The relative isohypses*® within the frontal 
strip’ (1) have essentially the same spacing as they 
have outside the frontal strip, (2) are not parallel to the 
frontal strip, or (8) are not strongly curved in crossing 
the fronts. 
For the quasi-stationary front thus formed, friction 
tends to increase the slope up to a certain height if 
the colder air is situated on the low-pressure side of 
the current—as is mostly the case with fronts in the 
westerlies and within the outskirts of an old, cold cy- 
clone. A corresponding increase in sharpness and shear 
must be expected. This is one probable reason why 
such quasi-stationary fronts show a maximum (kine- 
matic) frontogenetic tendency. 
A front kinematically created along some thermal 
discontinuity at the surface of the earth may be 
thermodynamically conserved or imtensified by pro- 
longed temperature differences between, say, land and 
sea, snow-free and snow-covered land, or adjacent sea 
currents. In fact, it is quite probable that the at- 
mospheric thermal discontinuity at the ice limit may 
in itself create a hyperbolic flow with a slanting surface 
of stretching. A cold front of limited extent may form 
within the lower layer of an air mass crossing a moun- 
tain range, and be intensified through cooling by its 
forced ascent and by the evaporation of the falling, 
orographically released precipitation. The forecaster 
should also expect the intensification of a quasi-sta- 
tionary front when the isallobaric gradient is directed 
3. For a definition of the aerological terms used in this 
text, see Table I. 
4. The frontal strip is defined as the area of the map bounded 
by the intersections of a frontal surface with the boundary 
surfaces of a mandatory layer, as shown for instance in Fig. 3. 
