576 



CORRELATION OF DISTRIBUTIONAL FEATURES. 



4. TEMPERATURE-MOISTURE PROVINCES BASED ON THE PRODUCT INDEX. 



It will be recalled that our three moisture-temperature charts (plates 

 69 to 72) show a form of climatic zonation that is very similar to that 

 shown by the chart based on the mean aqueous-vapor pressure for the 

 period of the average frostless season. For comparison with the vege- 

 tational areas, only the generalized chart of moisture-temperature 

 indices using the physiological temperature summation (plate 72, 

 figs. 18 and 25) has been employed. 



To avoid confusion, it is first necessary to point out that certain 

 climatic provinces are shown as practically the same by the moisture- 

 ratio chart (plate 58, figs. 16 and 23) and by the one here considered 

 (plate 72, figs. 18 and 25). The arid province corresponds very well 

 with the province of very low moisture-temperature values. The 

 semiarid province represents much the same area as the province of 

 low product values, but the north-south boundary of the latter province 

 lies much farther west at its southern end (Texas), and farther east at 

 its northern end (South Dakota). This line on the moisture-ratio 

 chart is practically the Hudson Bay-Gulf of Mexico line, as has been 

 noted, while the north-south line just mentioned has a very different 

 position. Also, the eastern projection of the province of low product 

 values is not represented on the chart of moisture-ratios; it roughly 



