24 ECONOMIC BOTANY OF ALABAMA. 



Statistics from 23 different weather stations, all based 

 on at least 15 years' observations, have been compiled 

 from Bulletin "W" of the U. S. Weather Bureau, and 

 from the annual summary of the Alabama section of 

 that Bureau for 1911. The data here published include 

 only the mean annual temperature, the average length 

 of the growing season (period between the last killing 

 frost in spring and first killing frost in fall) , the aver- 

 age annual precipitation, and the percentage of the total 

 annual rainfall that comes in the four warmest months, 

 June to September inclusive, and in the six warmest 

 months. May to October, inclusive. 



It has already been pointed out (page 19) that some 

 parts of Alabama are characterized by wet winters and 

 others by wet summers ; and the percentages of rainfall 

 for the four warmest months seem to bring out the con- 

 trasts in this respect better than do those for any longer 

 or shorter period or for any other portion of the year. 

 The six months percentages are added to facilitate com- 

 parison of conditions in Alabama with those in the rest 

 of the United States, as mapped by Dr. Henry Gannett 

 on Plate 2 of U. S. Geological Survey Water Supply Pa- 

 per No. 234, published in 1909. That map represents 

 the percentage of rainfall for "the six warmer months, 

 April to September, inclusive" ; but in Alabama and most 

 other parts of the eastern United States October is 

 usually a little warmer than April, and furthermore it is 

 usually drier than April in the regions that have dry 

 summers, and wetter than April in the. regions that have 

 dry winters, so that the figures for May to October give 

 greater contrasts than those for April to September 

 would.* 



*It is interesting to note that in general where the summers are 

 wettest the soils are sandiest, and vice versa, in the southeastern 

 United States at least. Of course the correlation is not absolute, 

 and there are many areas of clay soil in regions with wet sum- 

 mers, and of sand in regions with dry summers, for the texture of 

 the soil depends on many other factors than seasonal distribution 

 of rainfall, which indeed has hitherto scarcely been recognized as 

 a factor in the problem at all. To attempt to explain this corre- 

 lation would be out of place in such a report as this, as it seems 

 to be a matter of soil chemistry primarily. It is possible, though, 

 that the relation may be partly reciprocal, or accidental. For ex- 

 ample, the black belt of Alabama and Mississippi is characterized 



