152 
time of maximum temperature. This 
type represents conditional instability 
of a sort which may easily become 
realized during the warmer part of 
the day. Type 3 is very stable for 
convective impulses near the surface, 
and normally only negative areas will 
be observed. 
From the above remarks it might 
be supposed that showers and con- 
vective thunderstorms rarely occur 
with types 1 and 8, while they are 
common with type 2. While this sim- 
ple rule would have considerable suc- 
cess in its application to forecasting, 
it would fail in certain cases. Before 
we discuss these cases, it is possible 
to comment further on such vertical 
distributions of temperature and 
moisture as are represented in fig. 
11. In summer the normal tempera- 
ture distribution over a large part of 
the United States is almost baro- 
tropic, and the main concentration of 
solenoids appears to be found along 
the northern border of the continent*. 
Consequently, strong westerlies are 
DETROIT DAY TON 
23rd 24th 23rd 24th 
7 
= 
371.8) 
45 (2.6) 
2301.5) 
MONTGOMERY 
AIR MASS ANALYSIS 
observed over this portion, and we 
may look upon the zonal distribution 
of velocity as being similar to that 
pictured in the lower half of fig. 2. 
South of such a westerly current an- 
ticyclonic eddies form which create 
distinct patterns of moisture. This 
phenomenon occurs so _ frequently 
over certain areas of the United 
States that mean isentropic charts 
constructed for a month, season, 
or group of the same seasons of 
different years, reveal the eddies 
through the moisture lines (ef. fig. 
14, eg.). The normal flow pattern 
shows that dry air from the north 
curls anticyclonically southward while 
moist air from the south converges 
in a spiral fashion with this dry 
air into the anticyclonic eddy. The 
axis of the dry tongue normally runs 
through the Mississippi Valley, while 
the moist tongue generally makes its 
appearance over northern Mexico and 
then curves eastward. 
*See para. 9 infra for a more detailed pic- 
ture of the normal state. 
SAN ANTONIO 
26th 27th 28th 
\ 
— \6 
181.6) 
48 (4.0) 
= 
x 
' 
z 
9 
& 
> 
us 
a) 
Ww 
49 (7.0) 2 a (5.4) 
7 
Ne ) 44 (6.3) 
76 (10.4) 
30|¢3. ( 38 (41) 
! I 1 
6105.4), B64) 660104) BE C10. 2) 84(17.0) 80015. 8914.4) 
[fe} 10 10 fe} fo) 10 20 20 
TEMPERATURE —°C 
Fic. 12. DESTRUCTION OF DRY-TYPE STABLE ZONES (SHOWN BY ARROWS) 
AFTER THE ADVECTION OF A Moist TONGUE ALOFT. These soundings were made 
from June 23rd to 28th, 1987. Numbers to the right of the soundings are the 
relative and (in parentheses) specific humidities. Clouds are indicated by 
international symbols. 
