94 SECTIONAL ADDRESSES. 
the troposphere into the stratosphere are sufficient to account for the 
observed slope. This is an important result, for if it is true it would 
appear that the slope of the stratosphere from equator to pole is due to 
the dynamics of the atmosphere rather than to differences in radiation. 
On the other hand, the interdependence of the various factors in the 
stability of the atmosphere is so complicated that it may not be possible 
ever to assign the relationship of cause and effect to any of them. 
The second surface of discontinuity discussed by Bjerknes is between 
the trade winds and the anti-trade winds above them. That there is a 
difference of temperature between the relatively cold trade winds blowing 
towards the equator and the anti-trade winds blowing away from the 
equator goes without saying. Also the trades have a wind component 
towards the west and the anti-trades a component towards the east; that 
is, there is relative motion parallel to the boundary. With these condi- 
tions the surface of discontinuity should slope downwards from high to 
low latitudes; that is, the depth of the trade winds should decrease as they 
approach the equator. From the few observations we have this would 
appear to be the case, and the theory appears to receive support in this 
case also. 
Bjerknes’ third surface of discontinuity, which has received the name 
of the ‘ polar front,’ is a very important one in modern meteorological 
theory. On the whole there is very little air motion in polar regions, and 
the cap of air over each pole is losing heat by radiation and so tending to 
subside and flow away from the pole. As the air from the polar cap flows 
radially outwards it is deflected to the west on account of the earth’s 
rotation, On the other hand, in middle latitudes, from near latitude 30 
to the polar circle, the air is moving in an almost unbroken stream from 
west to east. Relative to the air in the polar cap this air is very warm. 
We therefore have a cold cap of westerly-moving air embedded in a 
warmer mass of air moving towards the east, and between the two there 
must be a pronounced surface of discontinuity. In such conditions the 
surface should slope upwards toward the pole. V. Bjerknes considers 
that there are such surfaces of discontinuity associated with each pole 
and that they are very stable. These ‘ polar fronts ’ play a large part, as 
we shall see later, in Bjerknes’ theory of the formation of cyclones. 
It will be realised from what has already been said that these three 
great surfaces of discontinuity are not mathematical abstractions. They 
have a very real physical existence characterised by great stability,which 
amounts, in the case of the stratosphere at least, almost to rigidity. Across 
them, when undisturbed, air does not pass, and when temporarily destroyed 
they reform as soon as the disturbing conditions have passed. 
But these are not the only surfaces of discontinuity which play a very 
real part in the physics of the atmosphere. While the three surfaces just 
described are of a more or less permanent nature, we now recoguise a 
constant succession of temporary surfaces of discontinuity which form and 
pass away in our own latitudes. Their presence is revealed in many ways. 
On the synoptic charts lines can be drawn which divide regions in which 
the conditions at the surface as regards temperature, humidity, and wind 
velocities are entirely different (Fig. 3). These lines are simply the 
Intersection at the earth’s surface of the surface of discontinuity between 
two bodies of air. 
