DESCRIPTION OF THE OZARK REGION. 19 



day to day or from hour to hour, there are certain changes that move in cycles or 

 oscillations, having uncertain units of time. The cause of these cycles or oscillations 

 is not definitely known, but it has been pointed out by one or two authorities that 

 the wet and cool periods and the dry and warm periods on the earth appear to cor- 

 respond somewhat with the periods of maximum and minimum sun spots, which in 

 turn are thought to be due to changes in the sun's photosphere. A few students of 

 the subject claim that these weather changes go in cycles of 3 years, then a longer 

 one of about 9 or 11 years, and still a greater cycle of 30 to 35 years, the last being 

 more clearly pronounced; but no general agreement has yet been reached regarding 

 them. 



Studies of the mean daily, mean monthly, and mean annual temperatures, while 

 important in themselves, give us a nearly unchangeable factor, and one not par- 

 ticularly interesting or striking in its features. For instance, the variations in the 

 mean annual temperature of Missouri, covering a period of 25 years or more, rarely 

 exceed 3°, and are often less than 1°. It matters not what the extremes may have 

 been during the four seasons, or 12 months, at the close of each 365 days the final 

 value is practically the same year after year. Another example in which the mean 

 temperature gives but little satisfaction as to the variations in the weather that have 

 taken place during a certain period is that for April, 1910, which, for the State of 

 Missouri, differed from its 20-year normal by only 0.3°. Yet the frosts of that month 

 were the most destructive from the viewpoint of the agriculturist and horticulturist, 

 and the weather generally was the most disagreeable from the physiological side that 

 has occurred during, possibly, the past 30 years. 



The factor that is the most important, in my opinion, is the daily minimum tem- 

 perature, which is as a rule nothing more than the lowest night temperature. The 

 night temperatures explain the interesting cause why the crops do not grow and why 

 the season is delayed. The days may be warm and bright, but vegetation as a rule 

 will not flourish as long as the nights continue abnormally cold. 



To the above statements regarding the importance of the daily 

 minimum temperature in its relation to plant growth it might also 

 be added that the duration of the minimum temperature, if it is 

 sufficiently low to injure vegetation, is a most important factor. If a 

 critical temperature continues for only a very short time, as sometimes 

 happens, little or no damage may occur, while a longer period of 

 duration may produce disastrous results. 



Mr. Reeder further states : 



It appears from climatological data * * * that the springs of the past 10 years 

 experienced quite marked temperature departures from the normal. The most 

 interesting as well as the most important question to be answered is, How long will 

 the cold period last? Unfortunately our climatological data do not cover a sufficient 

 length of time to enable us to work out the number of years to each cycle. While the 

 records show periods of both mild and cold springs for the 30 years, the change during 

 the last 20 years from mild springs to unusually cold springs is not only of marked 

 interest to the climatologist but bears rather more serious import to the orchardists, 

 whose earnings have been affected, and who are of course interested in the question 

 of whether mild springs will ever come again. 



As we are unable to answer this question positively, and as it is rather more pleasant 

 to be optimistic than pessimistic, we will look to the future springs from the brighter 

 side. It is quite evident that freezing temperatures in May are abnormal for Mis- 

 souri, especially most of that part south of latitude 39° 40' N. The frost isochronals 

 of the last four years, while unusually late, rather indicate a return to more normal 

 275 



