162 STATE HORTICULTURAL SOCIETY. 



I have thus far directed your attention almost exclusively to mean tempera- 

 ture — to the averages of months and seasons. These suffice, indeed, to indicate 

 the length of the growing period and the average severity of the winter. But 

 there is another aspect of climate which possesses at least equal importance; 

 though in climatic discussions it has been largely overlooked. Published tables 

 give us means of the year, and of the several seasons; and their authors seem 

 to think that in this they have brought to view all the important elements of 

 climate which bear on health and production of crops. A little reflection, 

 however, shows that the extremes of climate are of equal importance with the 

 means. It signifies little that the growing season begins in March, if liability 

 to killing frosts continues to the middle of May, as in Tennessee. A mean 

 October temperature of 60° is comparatively valueless after a September 

 freeze. The mean temperature of a season may be mild, or even delightful 

 at the same time that one or two days have brought a destructive cold. One 

 killing frost is as bad as a dozen, for vegetation has but one life to destroy. 

 It is the liability to these exceptional temperatures which we must know before 

 forming final judgment on the adaptability of a district for a particular crop. 

 A winter which averages mild may be marked, like the climate of St. Louis, 

 by one, two or three mornings destructive to everything which would triumph- 

 antly survive all the rest of the season. Every fruit-raiser knows that it is 

 not the average weather of winter or spring which endangers his buds or his 

 trees. It is the one or two nights of the whole season which brings him ap- 

 prehension — especially if accompanied by high wind. It is of no consequence 

 that the winter mean of St. Louis is 33°, and that of Grand Haven 21°, or of 

 Traverse City 24°, if the thermometer falls sometimes to -22° at St. Louis, and 

 never sinks below -16° at Grand Haven or Traverse City. It is precisely 

 against these exceptional extremes that lake Michigan exerts its most striking 

 influence. 



There are two ways to consider extremes of climate. We may consider the 

 mean minimum of a locality, or its extreme minimum for a series of years. 

 There is a lowest point reached by the thermometer at each locality every win- 

 ter. Different winters may vary greatly in the severity of the coldest day, but 

 we may take the average of a series of winters. This is the mean minimum. It 

 indicates the lowest temperature which the locality is as likely to experience as 

 to escape. Now, from this point of view, the Michigan climate stands forth 

 singularly favored. I have with me a chart of mean minima, which members 

 of the society are at liberty to inspect. You will be surprised to notice how 

 the lines passing through points having the same mean minimum are bent 

 northward along the region of the lakes. They do not trend east and west, 

 as they must under the normal influence of latitude, but they run literally 

 north and south in the vicinity of lakes Michigan and Huron. The isothermal 

 of -15° strikes from Mackinac through Manitowoc, Milwaukee, and New Buf- 

 falo to Fort Riley, in Kansas, near the parallel of 39°. Here is a deflection 

 over nearly seven degrees of latitude, or about 480 miles in a straight line. 

 The meaning of this is that the most excessive cold at Mackinac, for a period of 

 twenty-eight years, is not, on the average, greater than at Fort Riley, 480 miles 

 further south. It is one degree less than at Chicago for a term of eleven 

 years. There is but one feature about our climate which is more striking, and 

 that is the isotherms for extreme minima. 



Suppose we note the lowest point reached by the thermometer in a series of 

 years, at each of fifty localities. These points are the extreme minima of the 



