28 DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



centigrade thermometer. * The tabulated numbers are, as seen, all very small, and 

 r per cent of any of them is easily found with sufficient accuracy. 



When the pressure is given in millimeters ol mercury, table 1 1 a of the Appendix 

 gives the virtual-temperature correction for saturated air, expressed as above in 

 degrees of the centigrade thermometer. 



When the observations are made in inches of mercury and degrees of the Fahr- 

 enheit thermometer, it will be usually found most convenient, also, to calculate the 

 virtual-temperature correction in Fahrenheit degrees and to perform at a later 

 stage the transition to the other system of units. The virtual-temperature correc- 

 tion for saturated air, expressed in Fahrenheit degrees, and with the pressure in 

 inches of mercury as argument, is given in table 12 a of the Appendix. 



23. Virtual-Temperature Diagrams. The calculation of the virtual-tempera- 

 ture correction for every special observation of a long series will be found to be a great 

 waste of time. In such cases it is easy to find a curve, the virtual-temperature 

 diagram, giving the relation between virtual temperature and pressure. This 

 diagram is obtained by the following procedure: On coordinate paper the pres- 

 sures are measured out along the axis of the ordinates and the temperatures along 

 the axis of the abscissa;. In this plane the observed values of temperature and 

 pressure give a number of points by use of which a curve representing the relation 

 of pressure and temperature is drawn. This curve immediately gives the temper- 

 atures corresponding to the pressures 1 100, 1050, 1000, 950, m-bars, serving 



as argument in table 7 m. The corresponding virtual-temperature corrections for 

 saturated air are taken from this table with great ease, no interpolation respecting 

 the pressure now being required. By means of these corrections a second curve is 

 drawn, the curve of virtual temperature for saturated air. The curve for the virtual 

 temperature corresponding to the observed relative humidities will run between 

 these curves at a horizontal distance from the curve of real temperature, which is r 

 per cent of the horizontal distance between the two curves. This final curve is then 

 easily drawn by estimation, in accordance with the observed relative humidities. 



The method is easily understood by inspection of the diagrams accompanying 

 the examples worked out in Chapter VI. In each of them the curve to the left 

 is that of real temperatures, as immediately given by the observed temperatures 

 and pressures. The curve to the right is that of virtual temperature for saturated 

 air, as obtained by the use of table 7 m as described above. The line between the 

 two others is the required curve of virtual temperatures, as drawn by means of the 

 observed relative humidities. 



The observations being made in millimeters of mercury and centigrade degrees, 

 or in inches of mercury and Fahrenheit degrees, the virtual-temperature curve will 

 be obtained in exactly the same way, table 1 1 a or 12 a of the Appendix being used 

 to obtain the curve, as shown in the examples of Chapter VI. 



*The values of the vapor-tension f used for calculating table 7 m have been taken from Broch's well-known 

 table (Travaux et M^moires du Bureau International des Poids et des Mesures, T. I, Paris, 1881) for temperatures 

 above zero, and from Juhlin's table (Bihang till k. svenska Vetenskapsakademiens Handlingar, T. 17, Afdelning I, 

 Stockholm, 1891), for temperatures below zero. 



