Page:The American Cyclopædia (1879) Volume XI.djvu/453

 METEOEOLOGY 441 with considerable certainty that evaporation is two or three times greater in sunshine than in shade ; is four or five times greater in hot sum- mer days than in cold winter days ; is greater and on the other hand, by leaving the remain- ing air dryer, allows a more perfect radia- tion into space of the terrestrial heat, thereby increasing the diurnal variations of tempera- ture. The diurnal variation in the expansive pressure exerted by the vapor is illustrated by diagram IX., showing the results of observations made at Vienna. The annual varia- tion in vapor tension is given for the same station in the following diagram, No. X. By the relative humidity is meant the absolute hu- midity expressed as a fraction of the total quantity of moisture that the air would contain if perfectly satura- DIAGRAM ix.-Diumai Change of Vapor Pressure. ted. at %* observed temperature. As to its diurnal variation, the relative humidity is least in the hottest portion of the afternoon, and greatest shortly before sunrise, as shown in diagram XI. ; similarly, as to its annual variation, it is least in the summer months and greatest in the winter, as shown by comparing the curves for January and July in the same diagram, or more fully by the an- nual curve (E. H.) given in diagram X., where in windy weather in proportion to the strength of the wind ; is greater in dry than in moist weather; greater from marshes and forests than from fields of melting snow ; greater from the latter than from the surface of the ocean ; and in general greater in the equatorial than in the temperate regions. The aqueous vapor in the atmosphere may be considered from several points of view, namely : as to the pressure it exerts in maintaining baromet- ric equilibrium; or as to its absolute quantity ; or, again, as to its relative quantity as compared with the amount re- quired to saturate the atmos- DIAGRAM X. Annual Changes of Rela- tive Humidity (R. ff. of Vapor Pressure (F. P.), and of Tempera- ture (T.). DIAGRAM XI. Diurnal Change of Relative Humidity. phere. The condensation of a portion of the vapor into cloud or rain, on the one hand, diminishes the barometric pressure, thereby giving rise to local areas of low barometer, a comparison between the humidity and the temperature can be easily made. The irregu- larity of geographical distribution of moisture is by no means so great as might be expected.