A PROCEDURE OF SHORT-RANGE WEATHER FORECASTING 
against it, and of a moving front as it approaches a 
deep pressure trough. 
It has been known for some time that the formation 
of frontal waves is somehow partly initiated by a 
lessening of dynamic stability in the adjacent air 
masses, a decrease that has taken place through the 
preceding frontogenesis. For example, as observed al- 
ready in 1909 by Guilbert [86] and subsequently 
affirmed by Hesselberg [40] in 1915, a cyclonic, sub- 
geostrophic, and divergent flow with mdraft is fol- 
lowed locally by a stronger pressure gradient and so 
by cyclonic deepening. A more complete and up-to- 
date theory of frontal wave formation has been pre- 
sented in ‘“Extratropical Cyclones” by J. Bjerknes, 
pp. 577-598 in this Compendium. The main points of 
his theory are as follows: The first formation of the 
frontal wave depends on the intensity that can be 
reached by organized upgliding and downgliding on 
the quasi-stationary frontal slope which, as a result of 
the preceding frontogenesis, has just acquired ‘a tilt 
almost the same as the saturation isentropes. Organized 
upgliding of saturated air is dynamically restricted to 
those parts of the frontal slope where the geostrophic 
wind increases along the warm air streamline. It will 
be stronger the more nearly the isentropic anticyclonic 
shear of the geostrophic wind approaches 20., where 
@, is the vertical component of the earth’s rotation. 
The latter condition is identical with the lessening of 
dynamic stability. That condition is satisfied along the 
eastward half of the axis of stretching of a col (the 
front portion marked “active” in Fig. 2), where the 
first active upgliding of the beginning cyclogenesis is 
most frequently found. The first organized downglid- 
ing will occur where the progressive frontogenesis has 
produced a large local decrease in the geostrophic wind 
on the cold side of the front. Such downgliding, fol- 
lowing a dry isentrope of much flatter slope than that 
of the frontal zone, will form the first cold bulge on the 
quasi-stationary front. The warm sector of the nascent 
cyclone forms immediately east of that cold bulge. 
Wave formation thus is the combined product of warm- 
air upgliding and cold-air downgliding along neigh- 
boring front sectors, which, during the process, be- 
come the warm front and the cold front of the new 
wave. 
_ The first cold push must be a shallow one. The first 
formation of the frontal wave therefore depends on 
trigger action from low levels and should be studied 
carefully on the surface map. With the colder air 
situated on the low-pressure side of the current, the 
frictionally steepened and sharpened quasi-stationary 
fronts in the westerlies and in the rear of large polar- 
front lows have attained the slope of a saturation 
isentropic surface and therefore present initial condi- 
tions favorable for wave formation (see Fig. 2). With 
the opposite temperature distribution (such as in high- 
latitude easterlies, in westerlies with poleward increase 
of temperature, and within the outskirts of anticy- 
clones) the quasi-stationary fronts have a diminished 
slope and shear and therefore no cyclogenetic tend- 
eney. Waves are therefore to be expected along a 
771 
quasi-stationary polar front between principal air 
masses, especially if the front has a west-east orienta- 
tion with cold air poleward. Although a medium- or 
high-speed warm front is an anafront (for definition, 
see Fig. 2), the friction will tend markedly to diminish 
its slope, at least in the lowest kilometer of the at- 
mosphere. Within strongly cyclonically curving easterly 
flow near a cold low, the low-layer convergence and 
lifting of polar air contributes to a steepening of the 
cold-front surface; here, however, cyclogenesis is gen- 
erally prevented because near the center of the low 
the cold front becomes a katafront (defined in Fig. 2). 
Since wave formation depends on the degree of les- 
sening of dynamic stability that has taken place 
through the preceding frontogenesis, the parameter 
20, — 0V</dy should preferably be checked on low- 
level aerological profiles constructed across the front 
part that is expected to produce a frontal wave. (On 
an isentropic plane the horizontal direction of its 
steepest slope is the y-direction, which is directed north- 
ward.) The increase of geostrophic wind in the warm 
air from the surface front to the front on the 500-mb 
map can also be used in a rapid estimate of 
22, = 0Vo/dy. 
The determination of exactly where and when a 
frontal wave will appear is very difficult, because it is 
difficult to assess the surface maps for probable future 
lessening of dynamic stability in the vicinity of the 
front. So, the forecaster must usually be satisfied with 
just being able to detect, as soon as possible, an al- 
ready existing frontal wave on his surface map. In 
this connection, he therefore should carefully assay 
the analyzed maps for indirect indications of even the 
slightest wavelike formation, which sometimes can- 
not be found directly, as the maccuracies in the ana- 
lyzed position of the front far exceed its initial wave 
amplitude. For example, he should (1) examine the 
analyzed limits of the altostratus-nimbostratus cloud 
systems for upglide clouds on the cold side of the 
front, (2) look for protuberance into the cold air of 
the isallobar which hitherto most nearly coincided with 
a slowly moving and almost rectilinear cold front, and 
(3) observe especially the almost rectilinear warm 
fronts slowly approaching mountainous areas where a 
forced wave flow may form as a result of orographic 
drag. On the other hand, he should (4) be mindful that 
slightly unstable short waves (200-km wave length), 
already formed, are susceptible to turbulent and oro- 
graphic influences tending to break them up into 
smaller and dynamically stable, transient waves. Even 
waves less than 1000 km in length are generally stable. 
Only the longer, more unstable waves (1500-2500-km 
wave length), immune to small-scale local effects, will 
develop into cyclones. 
When a frontal wave has been identified on the 
surface map, the forecaster must then decide whether 
or not it will develop into a cyclone. The meteorologist 
should first bear in mind that, according to synoptic- 
climatological experience, certain geographical regions 
are more favored than others for the occurrence of 
cyclones. For the 1899-1939 series of daily surface 
