DYNAMIC METEOROLOGY. 407 



either by convection or radiation into the atmosphere from the earth's 

 surface will vary with the nature of that surface; the relative quan- 

 tities are roughly expressed in a numerical table (xiii). 



The relative amounts of heat that reach the earth's surface will de- 

 pend upon the percentage of cloudiness and the altitude of the sun some- 

 what as given in a numerical table (xiv). 



(III) Resulting density of the air. — By a combination of the numbers 

 given in the preceding tables and charts there may, in a rough way, be 

 indicated by relative numbers written upon the tri-daily maps what it 

 is expected will be the amount of heat and moisture added to any region 

 of the lower atmosphere during the interval between any two weather 

 charts. The effe:;t of this heat and moisture is to forcibly alter the den- 

 sity of the atmosphere, hot or moist air being lighter than cold or dry 

 air, assuming, of course, the barometric pressure to be the same in both 

 cases. These relative densities are shown in the Table xv, for per- 

 fectly dry air (relative humidity, 0) and for perfectly saturated air (rel- 

 ative humidity 100). 



By means of Table xvone is able finally to construct a map showing 

 relative densities of the lower or surface air over the United States 

 during the next eight or twenty-four hours. After much experience 

 one becomes able to make a rapid general mental summary of these 

 diverse influences without the labor of recording the items upon the 

 daily maps, and this is practically the method followed in daily routine 

 work. 



Now, the normal distribution of density is that on which the general 

 movements of the atmosphere depend, and the deviations shown by the 

 above map of densities will give a clue to the new disturbances that will 

 l)erhaps be initiated during the day. The normal distribution of tem- 

 l^erature, pressure, and winds is shown by the monthly and annual 

 maps. (Omitted.) Similar charts should be constructed for both hem- 

 ispheres when studying international meteorology. 



The Table xv, just explained, shows that the relative density of the 

 atmosphere is much more likelj" to be disturbed by changes of tempera- 

 ture than of moisture; thus at 70° of temperature an increase of tem- 

 perature by 10° diminishes the density 0.018, while at the same tempera- 

 ture an increase of moisture up to full saturation of the atmosphere 

 diminishes the density 0.008. An abnormal change of temperature is 

 therefore to be carefully looked for as a cause of further disturbance, 

 but after a disturbance is once set up the moisture becomes the most 

 important consideration, since its condensation alters the vertical dis- 

 tribution of temperature. 



(IV) Wind, Friction, and Pressure. — The general distribution of den- 

 sity over the earth's surface determines the flow of the denser air of the 

 polar regions toward and under the lighter air of the equator, and Pro- 

 fessor Ferrel has shown that from this (and the rotation of the earth on 

 its axis) results the general distribution of pressure prevailing through- 



