DATA OF SUNSHINE AND CLOUDINESS. 



327 



From those data we may gather the following general conclusions: 

 Phe climate of the Dismal Swamp region is characterized l>y a mild 

 winter, with normal positive temperatures of 4° to G° C, and by a long 

 and hot, but usually not extremely hot, summer. The normal daily 

 variation in temperature is comparatively small. Even the normal 

 annual range is only between 8° and 9° C, and the departure of the 

 normal variation in any month of the year from that of any other 

 month does not exceed about 2° C The normal number of days in 

 the year which have a temperature above 6° C. (43° F.), which is gen- 

 rally regarded as the minimum temperature for vegetative activity 

 n most plants of the Temperate Zone, is, at Norfolk, about five-sixths 

 of the whole. The sum total of temperatures above (5° V. during that 

 period is for the latitude a considerable one, enough to permit the 

 Occurrence in the region of a number of tropical and subtropical 

 forms and to place it in the warm temperate belt. l Likewise important 

 as regulating the northward extension into this region of numerous 

 warm temperate and tropical forms is the distribution of killing 

 frosts, from which about eight, months of the year are normally free. 



SUNSHINE AND CLOUDINESS. 



Normal percentages of possible sunshine. 



Station. 



Norfolk 



Cape Henry 



Hatteras 



Wilmington 



Jan. 



Feb. 



Mar. 



Apr. 



May. 



June. 



July. 



46 



45 



I'.i 



52 



52 



52 



52 



42 



51 



50 



50 



56 



55 



Si 



46 



47 



52 



55 



59 



55 



55 



47 



46 



52 



56 



54 



50 



48 



Aug. Sept. 



49 53 



49 56 



53 56 



47 I 52 



Oct. 

 58 



Nov. 



1 >■>■•. 



53 



53 



58 



51 



47 



58 



54 



53 



60 



55 



52 



An- 

 nual. 



Normal hours of sunshine. 1 



Station. 



Norfolk 



Cape Henry 



Hatteras 



Wilmington 



188.1 

 12!>.8 

 144.7 

 U9.6 



Feb. 



136.5 



154.7 

 143. 7 

 141.3 



-Mar. 



181. 5 

 185. 8 

 19.3. 4 

 L93.4 



Apr. 



305.3 

 19' 

 216.0 

 219.3 



May. 



June. July. Aug. 



338.31 238.9 232. «' 305.8 



245.8 242.1 2=57.1 305.8 



256.2 2:58.9 343.1, 216.9 



233.6 315. S 311.0 L95. i 



Sept. 



19 

 308.6 

 208.3 

 193, 3 



Oct. Nov. 



301.8 162.0 

 301.7, 155.9 

 203.0 167.4 

 210.5 173.7 



Dee. 



155 

 140.7 

 160.6 

 160.0 



An- 

 nual. 



2,324,0 

 3.305.4 

 3, 392. 2 

 3,296.9 



Normal cloudiness. 



Station. 



Jan. Felt. Mar. 



Norfolk ' 5.4i 5.5 5.1 



Cape Henry . I 5.8 | 4.9 5.0 



Hatteras .\ 5.4 1 5.3 j 4.8 



Wilmington ! 5.3 5.4! 4.8 



Apr. I May, 



June. July. 



4.8 ! 4.8 4.8 4.8 



5.0 4.4 4.5 4.7 



4.5 4.3 4.5 4.5 



4.4 4.6 I 5.0 5.3 



Aug. 



Sept. 

 4.7 



Oct. 



Nov. 



Dec. 



5.1 



4.2 



4.7 



4.8 



5.1 



4.4 



4.2 



4.9 



5.3 



4.8 



4.4 



4.2 



4.6 



4.7 



5.3 



4.8 



4.0 



4.5 



4.8 



An- 

 nual. 



4.9 

 1.8 



1.6 

 l.N 



1 Schimper (Pflanzengeographie, p. 445) regards the lino which divides the cold temperate 

 from the warm temperate bolt as approximately coinciding with the isotherm of 4-6° C. (43° F.) 

 for the coldest month. In the Dismal Swamp region the normal temperature falls slightly 

 below this point in January. 



a "Data as to sunshine are derived from the statistics of normal cloudiness and must be con- 

 sidered merely as a first approximation to the actual values of normal sunshine for these sta- 

 tions. For comparative purposes they may boused without serious error."— Mr. A. J. Henry, 

 Division Climate and Crops, U. S. Weather Bureau, in litt. 



3 Based upon the same data as the above table but expressed in hours. 



4 From the Report of the Weather Bureau for 1896-1)7, pp. 386 to 288. The data are "computed 

 from monthly means based on tridaily observations, Novomber, 1870, to June 30, 1888; thereafter, 

 frequent personal observations. Scale to 10.'' 



23592— No. 6—01- 



-2 



