208 THE MECHANICS OF THE EARTH'S ATMOSPHERE. 



centimetre, and relative humidity 50 per cent,, it is desired to find what 

 conditions this mass of air will pass through wheu it is carried without 

 change of heat into the higher strata of the atmosphere, and therefore 

 into a lower pressure, and at what approximate altitudes above the sea- 

 level the different conditions will be attained. 



We first seek from the diagram the point that corresponds to the ini- 

 tial stage. We find it as the intersecting point of the horizontal iso- 

 therm 27 and the vertical isobar 750. We remark that it lies almost ex- 

 actly on the dotted line 22. This indicates that our mass of air must 

 contain 22.0 grams of aqueous vapor in each kilogram of its owu weight 

 in order to be saturated. Since however it has only a relative humid- 

 ity of 50 per cent., therefore it contains 11.0 grams of water per kilo- 

 gram. We note this for future use. Furthermore, we go along down 

 the isobar 750 to the scale of altitude that is found at the lowest edge 

 of the diagram, and here we read off 100 metres. The point of the 

 scale of altitude therefore lies about 100 metres below the sea-level 

 adopted by us as a base, and therefore we have to subtract 100 metres 

 always from all the direct readings on the altitude scale, in order to 

 obtain the altitude above sea-level. If now we raise our atmospheric 

 mass upward, then the series of conditions which it runs through will 

 be directly given by that line of the Alpha system that passes through 

 the initial condition.* An engraved line not being given for this case 

 we therefore interpolate such an one (i. e., the — . . — . . line of the 

 diagram). If the number of intersecting lines appears to be bewil ler- 

 ing, then we take a strip of paper and lay it parallel to the system 

 under consideration, when all confusion disappears. In order now to 

 recognize the condition in the neighborhood of the altitude 700 metres 

 we seek for the point 700 -f 100 = 800 on the scale of altitudes, and go 

 perpendicularly up until we intersect our Alpha line. The intersection 

 gives this point at pressure 687 milimetres, and temperature 19.3° C. 

 But we ought to use the Alpha line only to that point in which it itself 

 intersects the dotted line 11 (or the line of absolute weight of con- 

 tained water). The attainment of this line indicates that we have ar- 

 rived at a condition in which the air is only just able to contain 11 

 grams of water per kilogram in the form of aqueous vapor. Since now 

 we have 11 grams per kilogram, therefore with any further cooling con- 

 densation begins. The pressure for the point at which precipitation 

 commences is 640 milimetres ; the temperature is 13.3° C. This is not 

 the temperature of the original dew-point, but it is lower. The dotted 

 line, eleven, intersects the isobar 750 at 15.8° C, and this is the initial 

 dew-point. But since besides cooling our air has also experienced an 

 increase in its volume, therefore the vapor has remained volatile to a 



*The letters a, /J, y, that designate the systems are to be found in the small circles 

 at the edge of the diagram. For each of these there corresponds one line of the sys- 

 tem that it designates. A line of special dots and dashes in the diagram indicates 

 the change of condition of the air in our illustrative example. 



