44 MICHIGAN SURVEY, 1908. 



Arthur, is much more generally distributed throughout the year. The 

 rainfall is also about 1/3 more in the south, the northern mean is 21.72 

 inches and the southern one 30.22 inches. In the north there is about 

 1/5 more snow than rainfall, 21.72 as contrasted with 25.41 inches j 

 while in the south about % more of the precipitation occurs as snow, 

 30.22 of rain as contrasted with 38.4 inches of snow. While in both 

 regions the greater precipitation is in the form of snow, the longer grow- 

 ing season of the plants in the south makes more of this moisture avail- 

 able ; but on the other hand, on account of the higher temperature, more 

 is needed. While about one-half of the rainfall in both regions occurs 

 during the growing season, yet the evaporation is much greater in the 

 south so that the relative humidity is less when compared with the 

 north. (Cf. Transeau, '05, a). It seems probable that the relative 

 humidity of Isle Eoj'ale is greater than on the adjacent mainland on ac- 

 count of its insular location and imperfect drainage. 



Mention should also be made of the long period of daylight in the 

 north because this is of great importance to a vegetation whose period 

 of growth is limited to such a short summer. 



To one accustomed to the hot summers farther south, the cool summer 

 of Isle Royale is very agreeable and invigorating. Moderately heavy 

 clothing is needed for comfort except during the middle of the day when 

 the heat at times is very oppressive. This was especially the case dur- 

 ing our examinations of the rock ridges. Thus on July 10 on the Jack 

 Pine Ridge (III, 5) the thermometer on a mat of Cladonia recorded 

 93° F. in the sun, wh.ile at the same time (2 P. M.) in the sun, but ex- 

 posed to a cool breeze, it recorded 76° F. Such temperatures would not 

 attract special attention were it not for the fact that usually the tem- 

 perature is so much lower. The nights are very cool, and at Washington 

 Harbor on Aug. 22 there was a frost in the valley along Washington 

 Creek (II, '04). During our camp at the Light-house, when shore winds: 

 accompanied a storm, the temperature became so low that a fire in the 

 evening was necessary for comfort. On July 15 there was a brisk east 

 wind, with a mean temperature of about 50° so that the vapor of ones 

 breath was visible all day. The lake breeze is at times very noticeable 

 as one passes from Rock Harbor into the channel at Middle Islands. It 

 is quite probable, as Jackson ('50, p. 420) suggests, that this cold lake 

 air is a factor in the production of the stunted tree growth. 



The low temperature of the wet, densely forested cedar swamps is 

 worthy of special mention. As Foster, remarks ( '50, p. 420) "Under the 

 shade of the crags, and among the thick evergreen swamps of white 

 cedar, it not unfrequently happens that perennial ice is found, covered 

 by a layer of turf. Mr. Blake discovered a considerable area of ice thus 

 preserved in midsummer, near Rock Harbor." Unfortunately our party 

 did not find such conditions although such "cold islands" were kept 

 in mind with the idea that under such conditions "glacial relicts" might 

 be expected if these areas were of sufficient extent. 



6. Seiches. The rapid and temporary changes of the water level in the 

 harbors has-been the b?isis of much comment. This was very marked at 

 Tobin Harbor and at Washington Harbor. Its influence upon Washing- 

 ton Creek was quite marked, ■ at times it would be ponded for some 

 distance up stream while on other days it would be a briskly flowing 



