NORWEGIAN WAVE-THEORY 45 
Nevertheless it will be necessary 
to solve directly the problem of a 
wave motion at a frontal surface in- 
clined to the surface of the earth, 
especially in order to obtain reli- 
able quantitative criteria to decide 
whether an observed wave is stable 
or unstable. 
Finally, cyclonic waves have thus 
far been investigated only on a ro- 
tating plane. Considering their di- 
mension, however, it is to be ex- 
pected that the curvature of the earth 
has a certain influence. In this re- 
spect also much remains yet to be 
done. But the possibility of cyclonic 
waves in our atmosphere can be re- 
garded as proven in spite of these 
gaps in the theory, and it can safely 
be said that objections to the wave 
theory of cyclones result from in- 
sufficient knowledge of the theoreti- 
cal investigations. 
Frontal Waves 
Fic. 12. Four TYPES oF FRONTAL WAVES.—It follows from the wave-the- 
ory (see article by Haurwitz, above) that the propagation of a frontal wave 
can be divided into a dynamical part, which always is directed eastward with a 
normal temperature distribution (colder to the north) and westward with a 
reversed one, and an advective part, being approximately equal to the mean 
of the velocities of the two air masses surrounding the front. Hence one gets 
4 types of frontal waves: (1) rapid waves, forming in a Wly current with 
colder air polewards (Ja). or in a general Ely current with an inverted tem- 
perature distribution (Jb).; and (2) quasi-stationary waves, forming in a 
general Ely current with normal temperature distribution (J/a), or in a gen- 
eral Wly current with inverted temperature distribution (J/b).These are all 
frequently observed on synoptic maps.—From: Bergeron, “Physics of Fronts,” 
Bull. Amer. Met. Soc., Sept., 1937, pp. 269-70. 
Sources cf Energy for Extratropical Cyclones 
Dr. Bergeron of the “Norwegian (2) The potential energy of the 
School” recently stated..... “that hozizontal distribution of mass 
the process of cyclogenesis derives its 
energy mainly from three sources: 
(1) The kinetic energy of the pre- 
existing air currents on both 
sides of the cyclogenetic front 
(“current energy”) — insuffi- 
cient, however, alone to ex- 
plain the display of energy in 
intense cyclones. 
(“frontal energy”), which is 
transformed into kinetic en- 
ergy according to the Mar- 
gules’ principles of energy— 
probably the main source of 
energy. 
(3) The potential energy of a 
moist-labile [i.e., unstable for 
saturated air] stratification, 
