CLIMATIC CONDITIONS OF THE UNITED STATES. 379 



ditions are not as easily controlled by artificial means as are moisture conditions; it is much 

 easier to make a desert moist than to make winter into summer. So it comes about that 

 large areas of the arid Southwest are annually producing nearly as much as their tempera- 

 ture conditions allow, while only exceedingly small areas in the Portland region are produc- 

 ing as much per year as their moisture conditions might allow. These latter areas are of 

 course under glass; greenhouses are the only localities where the natural winter is trans- 

 formed into an artificial summer." 



And even greenhouse conditions fail to give summer light during the 

 winter months. Quoting further from the author just mentioned, 



"It appears that the most efficient climate for plant growth, in the United States, is that 

 of peninsular Florida, as far as moisture and temperature conditions are concerned. But 

 this cUmate is not the most comfortable for human beings; its moisture ratio is too high in 

 the season of active plant growth. The reason why the climate of southern California 

 is so generally regarded as better than that of Florida [the temperature conditions being 

 similar for the two regions] is to be found in the facts, (1) that the moisture ratio here is 

 very low (making it a very poor cUmate for plant growth but a very pleasant one for human 

 beings, and (2) that the moisture ratio is here artificially raised for plants (by irrigation), 

 but not thus generally raised for human beings. The southern California cUmate for 

 cultivated plants is an artificial one, in as true a sense as is that of a greenhouse in winter 

 in Maine. In the latter case the value of the temperatiu-e index is artificially increased." 



It should be remarked, in conclusion, that the main essentials of 

 these moisture-temperature charts are also shown by the charts of 

 vapor-pressure of water in the air (plates 63 and 64), and that the same 

 is true in a more general way (but not so true in detail) of the four 

 precipitation charts, plates 46 (fig. 2), 47, 49, and 50. The chart for 

 normal annual precipitation (plate 52) also indicates some of these 

 features. None of the charts showing temperature conditions and none 

 of the other moisture charts (aside from those for vapor-pressure just 

 mentioned) exhibit fundamental similarity to the moisture-tempera- 

 ture charts here considered. 



V. CARTOGRAPHICAL COMBINATION OF TEMPERATURE AND 



MOISTURE INDICES. 



While the arithmetical combination of temperature and moisture 

 indices (giving such moisture-temperature indices as those just con- 

 sidered) does not give a chart that promises to be of great general 

 value in the study of the geographical distribution of species or vege- 

 tation types, a chart formed by separately plotting the isoclimatic 

 lines for moisture and for temperature values upon the same map 

 appears to be much more promising for the present purposes. Such a 

 combination chart was first presented, in a roughly generaUzed form, 

 by Merriman's system of hfe-zones, in which each temperature prov- 

 ince was divided into two portions according to moisture conditions. 

 Livingston^ published three charts representing combinations of mois- 

 ture and temperature data, and emphasized this general method of 

 studying chmatic data. 



1 Livingston, (1913, 1). 



