THE COLD FRONT 29 
of the cold tongue is generally about 
500 meters above the surface. ‘The 
form of the advanced portion of the 
cold wedge depends upon the rough- 
ness of the surface over which the 
front is traveling, the surface coid 
air being the more retarded the 
rougher the terrain. In hilly or moun- 
tainous sections changes from the 
warm to the cold air are frequentiy 
less abrupt than over flat country. As 
the cold air tongue moves in at some 
upper level there is certain to be 
some mixing with the underlying 
warmer air. The existence of a ccid 
current aloft implies a steep lapse 
rate, which facilitates convective stir- 
ring, and even possibly, for a brief 
interval of time, a superadiabatic 
lapse rate. It also seems logical that 
the cold air aloft may at times break 
off from the main body, much the 
same as the crest of a water wave. 
It is difficult to determine how far 
the cold air may run ahead of tke 
surface boundary of the front. Per- 
haps twenty-five miles is an average, 
one hundred miles a maximum value. 
The rain which occurs before the 
arrival of the cold air at the surface 
(prefrontal rain) is mainly the result 
of vertical lifting of the warm eir 
by the mechanical action of the ad- 
vancing wedge. The cloudiness and 
character of the precipitation deperd 
upon the vertical structure of the 
warm air which is being displaced. 
If the warm current is fairly dry and 
stable, e.g., a current of old Polar 
Pacifie air. the cold front may arrive 
accompanied only by a broken cloud 
deck and no precipitation. On the 
other hand, if the warm air is Tropical 
Gulf air, moisture-laden and condi- 
tionally unstable, the clouds are apt 
to be of the Cunb type with the char- 
acteristic frontal thunderstorms. It 
is evident, then, that an upper air 
sounding within the warm air is of 
importance to the forecaster. 
The ceilings in advance of a well- 
defined cold front decrease compeér- 
atively rapidly. The foremost vwor- 
tion of the frontal cloud deck in the 
warm air may be a Ci or Cist tyre 
which rapidly lowers to Acu and Ast, 
and finally to Nbst. This transition 
may take place in a period of two 
hours. At times rain may be seen 
falling from the clouds and evapor- 
ating before it reaches the earth. 
The frequently observed temperature 
fall in advance of the cold front 
is sometimes associated with evapor- 
ation of rain from the clouds, the 
cooling extending down to the surface 
through turbulent mixing. When this 
is the case the specific humidity at 
the surface increases as the tempera- 
ture falls. This phenomenon im- 
mediately precedes the rain. 
There are times when the precipita- 
tion is entirely confined to the regien 
ahead of the front. Here the struc- 
ture of the warm air is usually of a 
conditionally unstable character at 
intermediate levels (say, 1000 to 3600 
m), and the initial upward impulse 
some distance ahead of the surface 
cold front is sufficient to release aud 
transform the potential energy into 
convective showers. Behind the cold 
front the vertical forces may be in- 
sufficient, or the structure of the 
warm air at higher elevations un- 
favorable for precipitation. 
In other cases the rain zone is 
concentrated behind the front, i.e., 
within the cold air. In this case the 
component of wind flow normal to 
the front is appreciably greater in 
the cold air than that in the warm 
air. Sometimes a cold front becomes 
almost stationary; precipitation and 
cloudiness begin to spread far behind 
it. Pilot balloon observations well 
behind the front then show the warm 
current flowing in opposite direction 
