58 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 63 



Bureau has taken pains to determine the relative humidity during 

 fogs observed during ten years at Chicago on Lake Michigan. Ob- 

 servations were made on ii8 foggy days by Dr. Frankenfield, whose 

 results are given as follows : 



Relative humidity 90 per cent (or more) in 75 per cent of days. 

 Relative humidity 80 to 90 per cent in 13 per cent of days. 

 Relative humidity below 80 per cent in 12 per cent of days. 



The observer noted dense fog on one occasion when the relative 

 humidity was as low as 52 per cent ; on another, when it was 58 per 

 cent. 



The Pacific coast, as a whole, is much foggier than the Atlantic 

 coast, because the winds on the Atlantic are mostly off-shore and 

 consequently carry less moisture than the westerly on-shore winds of 

 the Pacific. 



In the interior of the United States, especially the western half, 

 the average number of foggy days per year is less than ten each 

 year; in the Lake region the number rises to fifteen or twenty per 

 annum. In isolated localities, local conditions increase this number 

 greatly. 



At Colorado Springs genuine fogs occur, sometimes very dense 

 and lasting all day, but they are uncommon and scarcely worth 

 mentioning were not their existence so often denied. (Ely.) 



In the Adirondack Mountains fogs and mists are not uncommon 

 along the rivers and on the lake shores in the early morning in the 

 summer and autumn. They are examples of the radiation fogs 

 already referred to and, like dew and frost, they are associated with 

 clear weather. The presence of a light fog over an Adirondack lake 

 in the early morning foretells a bright, sunny, warm day. 



Fogs are not at all unusual in the Alleghany and Blue Ridge 

 Mountains. They follow river courses and settle in low valleys. 

 The humidity attendant on the melting of snow or during the rains 

 of early summer or autumn is not so readily exchanged for dryer 

 air in the long narrow valleys as at the seaboard. In many localities 

 the high ridges on either side shut out the direct rays of sunlight 

 for several hours ; while at the seaboard there are no such natural 

 barriers. 



At some of the higher elevations in the Blue Ridge Mountains 

 of Pennsylvania, fog is noted during the summer and autumn. One 

 observer, himself a tuberculous patient, recorded at Mount Pocono, 

 in Monroe County, Pa., elevation 2,000 feet, fifteen days with fog 

 part of the day, usually early morning, and seven with fog all day, 



