592 
streamlines in the col over the central United States 
on November 7, 1500Z, fulfill that condition fairly 
well. The mentioned geostrophic departure towards low 
pressure on the warm side of the col also favors the 
transportation of the isotherms towards the axis of 
dilatation. As a result of these processes a surface front 
has formed on November 8, 1500Z, over the region 
previously occupied by the col, while frontogenesis is 
in progress over the Great Lakes region where the col 
has now arrived. A frontal wave has already formed 
over the state of Missouri and is represented in the 
pressure field by a small elongated low. During the 
GGW RAP LBF opc OKC FTW 
RAPID NORTH DODGE OKLA. FORT 
a Gavae: is ike CITY mz WORTH 
MECHANICS OF PRESSURE SYSTEMS 
the foehn air over Dodge City. The foehn on the warm 
side of the col gives the frontogenesis a good start in 
the layers below the level of the continental divide, but 
the process also takes place higher up, as shown by the 
isothermaley between 600 and 560 mb at Rapid City. 
The thermodynamics of upglidmg im the sloping 
frontal zone can be tested through an inspection of the 
293° dry-isentrope which has been inserted in Fig. 11. 
It shows that a particle could be brought dry-adia- 
batically along the profile from near the ground in 
Oklahoma to the tropopause at 350 mb over Montana 
without being subject to stabilizing gravity effects. If 
300 
400 
500 
600 
700 
ive ree 
[he 900 
HIP SSS 
FA EC IS 
NOV. 7, 1948 
NOV.7, 1948 , 
1500 z 
500 mb 
1500zZ 
00° 
Fre. 11.—Profile of early Maenene se, November 7, 1948, 1500Z, and part of the 500-mb map for the same time. Sample eval- 
uations of 20: — dv,/dn below diagram. Upper w inds: Half barb 5 m sec~ 1, full barb 10 m sec™, triangular barb 50 m sec™!. 
following twenty-four hours the new cyclone deepens 
and moves along the front northeastward. A new field 
of deformation is active on November 9 along the cold 
front of that cyclone and helps maintain the frontal 
temperature contrast. 
The described frontogenetical development near the 
ground conforms with the advective rules set forth by 
Bergeron [1] and Petterssen [25]. We shall here add a 
study of the dynamical conditions for isentropic up- 
gliding in the free atmosphere, which is an important 
part of the process of frontogenesis. 
The rate of frontogenesis near the ground is increased 
considerably if the air of the lower part of the frontal 
zone is removed by upgliding. The dynamical possibili- 
ties for that process are considered in Fig. 11, which 
contains a profile across the zone of frontogenesis dur- 
ing its early stage on November 7, 1500Z. At that time 
no clear-cut front was yet discernible on the surface 
map, but on the 850-mb map there is great crowding of 
isotherms between the cold air over North Platte and 
we take into account that condensation would begin 
in such a particle at 700 mb, the ascent from there on 
would follow the saturation isentrope of 282°, which 
climbs more steeply than the dry-isentrope and like- 
wise reaches the tropopause. Actually no single particle 
undergoes such far-reaching isentropic displacements 
inside the profile; but the isentropes can still be used as 
indicators of the direction of the component of stable 
upgliding (see p. 585), which the particles may have in 
addition to their much stronger geostrophic component 
of motion normal to the profile. 
A study of the values of the isentropic upgliding 
(equation (11)), 
Og , Og 
Swot past ihe? 
vu) 
20, — oe” 
On 
along the different sections of the inserted isentropes, 
will reveal the dynamical possibilities for frontogenesis. 
