188 NATUEAL HISTORY BULLETIN 



EXPOSURE OP PRAIRIE TO EVAPORATION 



One other characteristic is also possessed by all types of 

 prairies, — namely, all are exposed at least at times to excessive 

 evaporation. The factors chiefly concerned in evaporation are 

 temperature, relative humidity of the air controlled in large 

 part by temperature, and the wind. 



The maximum heat of the day is reached at about two o'clock 

 in the afternoon. The "two-o'clock sun" beats down upon the 

 southerly and southwesterly slopes of rough areas, and equally 

 affects uniformly flat areas; the prevailing winds of summer to 

 which the prairie flora is exposed are from the south or south- 

 west, and the sun-scorched southwesterly slopes are also most ex- 

 posed to these winds, the flat prairie again suffering almost equal- 

 ly ; hence southwesterly slopes and large flat areas suffer from the 

 combined influence of the afternoon sun and the prevailing sum- 

 mer winds. 



Countless illustrations of the effect of this exposure may be 

 observed throughout Iowa and the neighboring states. They are 

 most strikingly shown in the rougher parts of the state, and no 

 where more strikingly than in the loess bluffs which border the 

 Missouri valley in Iowa. From Sioux City to Hamburg these 

 bluffs present bare surfaces to the west, their somewhat north- 

 westerly direction along the east side of a broad valley exposing 

 them to both sun and summer winds. These surfaces are wholly 

 treeless excepting where by a bending of the line of bluffs a por- 

 tion of the latter is protected, and they are covered with a typical 

 dry-prairie flora. It is only on the east side of ridges which form 

 the bluffs, and in pockets and valleys which are protected on the 

 south and west by ridges, that groves appear, and in these groves 

 the prairie flora is in part or wholly displaced by a mesophytic 

 flora, the extent of this displacement being determined by the de- 

 gree of exposure and the consequent density of the grove. 



Very striking illustrations of the effect produced by a bend 

 in the line of bluffs may be found in a number of localities on the 

 Iowa side of the IMissouri. Thus in Lyons township, Mills county, 

 in Lewis and Crescent townships in Pottawattamie county, in St. 

 Johns and Little Sioux townships, Harrison county and Belvidere 



