238 THE MECHANICS OF THE EARTH's ATMOSPHERE. 



then by the compression the cliauge of couditiou from c onward goes 

 further along the adiabatic cd of the dry stage. In this case a glance 

 at the diagram shows Immediately that for this change of condition the 

 initial temperature will be attained even at a ])ressure that lies far be- 

 low the initial i)ressure, and that in the farther progress towards ])res- 

 sures that are near the initial pressure, that is to say, in the des(!ent to 

 the old original level, much higher temperatures will be attained. At 

 the same time the quantity of moisture is much less since the dew-point' 

 curve aS'c (which however is not drawn in order not to confuse the dia- 

 gram), lies nearer the coordinate ])lane than the curve Sa-, and since the 

 curve of condition cd remains with S,. in the same plane which is itar-^ 

 allel to the plane PV. The quantity of moisture which in the initial 

 condition was Xa is now at the end Xd=-x^<iXa^ while for the temperature 

 the equation Ta^Ta holds good. Therefore after the passage over the 

 mountain one has warm dry air, whereas at first it was cool and damp. 



At the same time we see directly from the diagram {hat the charac- 

 teristic peculiarities of the foehu must stand out so much the plainer in 

 jjroportion as tlie point a is nearer to the curve of saturation, that is 

 to say, the warmer and moister the air is before its ascent and again,, 

 the longer the portion h e is, that is to say, the inore extensive is the 

 expansion in the rain stage, or in other words, the higher the summit 

 is that has to be surmounted. 



Therefore we understand also at once why it is that in the Alps, in- 

 dependent of the prevailing conditions of atmospheric pressure, north- 

 erly foehns are so much rarer than the southerly foehns, as also why 

 descending winds that have surmounted no summit, but have only 

 passed along over a plateau, as for example the bora, have not the char- 

 acteristic warmth of the foehn. 



(2) The interchange of air between cyclone and anticyclone in summer. 



Between an anti-cyclone and the cyclones that feed it, similar rela- 

 tions exist as between the masses of air on the two sides of a mountain 

 range to be surmounted by them. In cyclones one has to do with an 

 ascending current of air that afterwards descends in the anticyclone. 

 Hence arises the precipitation in the region of the cyclone, the dryness 

 and the clear sky in the region of anti-cyclone. But, whereas in the 

 foehn the ascent and descent occur at points in the neighborhood of each 

 other, so that in the short path there scarcely remains time for gain or 

 loss of heat, but the whole jirocess may in fact be considered as adiabatic ; 

 on the other hand very different relations obtain for the ascent and de- 

 scent in cyclone and anti-cyclone. These two opposite processes in gen- 

 eral occur at i)laces so distant from each other that in the transit from 

 one to the other extended opportunity is offered to take up or give out 

 heat. In this i)rocess during the summer season the increase of heat 

 prevails, but during the winter time the loss of heat ; the day-time also 



