652 
lines. A sudden rise of pressure is normally observed 
upon arrival of the wedge of rain-cooled air. However, 
Tepper further theorizes that the “‘pressure Jump” is 
associated with a gravitational wave aloft (following a 
suggestion by Freeman [3]), and he suggests that the 
wave originates from an acceleration of the cold front. 
No convincing evidence has been presented in support 
of this theory. It seems extremely unlikely that there 
can be any close association between the leading edge 
of outflowing cold air at low levels and a gravitational 
wave aloft, as would be required. 
The most substantial work to date is that based on 
observations of the recent Thunderstorm Project. Wil- 
liams [15] made a microanalysis of selected squall lines 
passing through the Wilmington net. The official report 
of the Thunderstorm Project [14] further discusses 
some of the factors observed in connection with squall 
lines, and a still more recent paper by Newton [8] 
attempts to give a more detailed picture of their physi- 
cal structure and mechanics, based on Thunderstorm 
Project data. The latter two studies are too recent to 
have been taken into account in the preceding discus- 
sion. Newton’s examples probably do not embody the 
salient characteristics of all types of squall lines but 
shed light on the type included in his studies. He 
suggests that squall lines in some cases appear to form 
first over the cold-front surface and subsequently to 
move into the warm sector, and he confirms by obser- 
vation the existence of a pseudo-cold front along the 
warm-sector squalls. He further suggests that squall- 
line activity can be accounted for partly as a result of 
this ‘front”’ and partly by the continuous generation 
of new thunderstorms as a result of convergence-di- 
vergence patterns produced by the vertical transfer 
of horizontal momentum in pre-existent thunderstorms 
and augmented by solenoidal circulations. He sug- 
gests that kinetic energy brought down from higher 
levels is an essential source of energy for maintaming 
squall-line activity. 
Future Research 
Future progress toward a better understanding of the 
physical structure and mechanics of the instability line 
will require both the collection of more adequate de- 
tailed observational data and the analysis of existing 
and future data. Undoubtedly there is much to be 
gained by further analysis of the Thunderstorm Project 
data, though this project was not aimed so much at the 
specific problems of the instability line as of the thunder- 
storm. More observational data for upper levels in the 
immediate vicinity of tornadoes is especially needed; 
this is difficult to obtain but will be very important 
because of the frequent association of tornado condi- 
tions with the instability line. A more adequate theory 
of the mechanics of the tornado would be an important 
step toward understanding the instability line. 
From the practical standpoint of the synoptic meteor- 
ologist, whose primary problem is to forecast develop- 
ment of the instability line before it occurs, much 
remains to be done. His problem is somewhat different 
from that of basic research in that he must develop 
MECHANICS OF PRESSURE SYSTEMS 
better forecasting methods based only on the use of 
data which are available synoptically. Synoptic data are 
at best very incomplete, so that he must interpolate to 
a great extent; better basic knowledge of the phenom- 
enon would aid in this interpolation. There is a large 
and relatively untouched field for synoptic studies, par- 
ticularly from a statistical standpoint, m which only 
synoptically available data are used. There are im- 
portant geographical differences involved so that similar 
types of studies need to be made for different geograph- 
ical areas. While forecasting development of the line is 
the main synoptic problem, it is also important to 
develop criteria for determining the individual char- 
acteristics of each instability line (turbulence, intensity 
of rainfall, surface winds, icing, likelihood of tornadoes, 
etc.) and means of forecasting the dissipation of the 
line. 
REFERENCES 
1. Bsrrxnes, J., ‘Exploration de quelques perturbations 
atmosphériques 4 ]’aide de sondages rapprochés dans le 
temps.” Geofys. Publ., Vol. 9, No. 9 (1930). 
2. Durst, C.S., and Surciirre, R. C., ‘The Importance of 
Vertical Motion in the Development of Tropical Re- 
volving Storms.”? Quart. J. R. meteor. Soc., 64:75-84 
(1938). 
3. Freeman, J. C., Jr., “An Analogy between the Equatorial 
Easterlies and Supersonic Gas Flows.” J. Meteor., 5:138- 
146 (1948). 
4, Fuuxs, J. R., Some Aspects of Non-gradient Wind Flow, 
unpublished. Paper presented before Joint Meeting of 
Amer. Meteor. Soc. and Amer. Inst. of Aero. Sciences, 
New York, January, 1946. 
5. Harrison, H. T., and OrEnDorrr, W. K., ‘“‘Pre-coldfrontal 
Squall Lines.” United Air Lines Meteor. Dept. Cire. 
No. 16 (1941). 
6. Litoyp, J. R., “The Development and Trajectories of 
Tornadoes.’’ Mon. Wea. Rev. Wash., 70:65-75 (1942). 
7. Means, L. L., “‘The Nocturnal Maximum Occurrence of 
Thunderstorms in the Midwestern States.’’ Dept. Meteor. 
Univ. Chicago Misc. Rep., No. 16 (1944). 
8. Newton, C. W., ‘‘Structure and Mechanism of the Pre- 
frontal Squall Line.” J. Meteor., 7:210-222 (1950). 
9. Ottver, V. J., and Oxiver, M. B., ‘Forecasting the 
Weather with the Aid of Upper-Air Data” in Handbook 
of Meteorology, F. A. Berry, Jr., E. Bouuay, and N. R. 
Beers, ed., pp. 818-857. New York, McGraw, 1945. 
(See pp. 815-817) 
10. Rosspy, C.-G., ‘“Kinematic and Hydrostatic Properties of 
Certain Long Waves in the Westerlies.’”’? Dept. Meteor. 
Univ. Chicago Misc. Rep., No. 5 (1942). (See pp. 32-37) 
11. SHowatter, A. K., and Furxs, J. R., Preliminary Report 
on Tornadoes. U.S. Weather Bureau, Washington, D. C., 
1943. (See pp. 20-28) 
12. Sonperc, H., ‘Le mouvement d’inertie de l’atmosphére 
stable et son réle dans la théorie des cyclones.”’ P. V. 
Météor. Un. géod. géophys. int., Edimbourg, 1936. II, 
pp. 66-82 (1939). 
13. Tepper, M., ‘‘A Proposed Mechanism of Squall Lines: 
The Pressure Jump Line.” J. Meteor., 7:21-29 (1950). 
14. U. S. WearHer Bureau, The , Thunderstorm, 287 pp. 
Washington, D. C., U. S. Govt. Printing Office, 1949. 
(See pp. 122-130) 
15. Wruuiams, D. T., ‘‘A Surface Micro-study of Squall-Line 
Thunderstorms.’’ Mon. Wea. Rev. Wash., 76:239-246 
(1948). 
