36 
AIR MASS ANALYSIS 
layers. When this phenomenon oc- 
curs the occluded front may rightfully 
be called a cold front. (f) illustrates 
the formation of this type of discon- 
tinuity, where the occlusion has been 
bent south in the rear of the cyclone 
(in agreement with the general air 
flow). The section of the front from 
C to D gradually assumes the char- 
acteristics of a cold front, while the 
section from D to E becomes a warm 
front. When the occluded front bends 
around as in (f) it is called a “loop- 
back” or a “bent-back” occlusicn. 
The forecaster must be on the look- 
out for the development of these 
fronts; for a time they may appear 
quite harmless, but later on may 
become important. In summer they 
may be the deciding factor (the 
“trigger action’) in the formation 
of thunderstorms. 
(g) represents a vertical cross- 
section through the line of AB in 
(d). 
The following outline gives the 
general behavior of the meteorolo- 
gical elements at the surface with 
the passage of the different fronts. 
These are, of course, average con- 
ditions which are frequently observed 
over the eastern section of the United 
States. Individual cases may show 
quite different characteristics. 
Element 
(at surface) 
f TABLE I 
AVERAGE BEHAVIOR OF THE COMMONLY OBSERVED METEOROLOGICAL ELEMENTS WITH 
Front PASSAGES OVER THE EASTERN UNITED STATES IN WINTER. 
Type of Front Passage: 
Warm Front 
Cold Front 
(Cold) Oceluded Front 
change, 
inereas- 
fairly 
increasing to 
Pressure Falling, then steady | Falling then slightly | Falling slowly, then 
or falling less rapidly | rising, then abrupt| rising slowly 
rapid rise 
Temperature Steady, then rising at| Rising slowly, then |Not much 
an increasing rate, | falling slightly, tcen | falling slightly aiter 
then steady dropping abruptly at | front 
a rapid rate 
Relative Fairly constant, then | Increasing gradually | Gradually 
Humidity increasing to nearly |to almost saturation, | ing to almost satu- 
saturation, then fail-| then falling ration, then falling 
ing off 
Specific Steady, then increas- Fairly constant, | Slight increase then 
Humidity ing at an increasing | slightly increasing, | general decrease 
rate, then constant then falling abrupt- 
ly and rapidly 
Clouds Ci to Cist to Ast to|Cist rapidly chang-| Ast to Nbst to Cu 
Nbst to Frst ing to Cunb, then to 
Freu 
Precipitation None, then gradual/None, then heavy|Gradual, 
steady rain, then 1a-|but brief shower, |steady, but not iong 
pidly falling off to|perhaps thund er-/| lasting 
none shower, clearing! 
rapidly 
Visibility Fair, becoming poor,| Poor, then rapidly | Decreasing to poor, 
and remaining poo1 becoming good to’ then 
very good fair or good 
Wind 'SE or S to SW S or SW to NW W to NW 
The physical interpretation of these changes in terms of what has been said 
about fronts and air masses is left to the reader. 
