102 N CLIMATOLOGY 



The annual variation of N s is indicated on figure 4.10 by contours of 

 the difference between the maximum and minimum monthly means ob- 

 served throughout the year. It is quite remarkable how clearly climatic 

 differences are evidenced by the yearly range of N s. The prevailing 

 transport of moist maritime air inland over the west coasts of North 

 America and Europe is indicated by relatively small annual ranges (20 to 

 30 A'^ units), while, for example, the east coast of the United States with a 

 range of 40 to 50 A^ units or more reflects the invasion of that area from 

 time to time by such diverse air masses as arctic continental and tropical 

 maritime. The largest annual ranges of N ^ (90 N units) are observed in 

 the Sudan of Africa and in connection with the Indian monsoon. 



An additional A'^o map (fig. 4.11) was prepared from the minimum 

 monthly mean value of A^^ observed throughout the year to supplement 

 the range map in order that an estimation might be made of both the 

 minimum and maximum monthly mean N s expected durmg the year. 



A measure of the variability of the February and August mean values 

 of A'"^ is given by monthly range maps (fig. 4.12 and 4.13) determined 

 from monthly averages from 5 years of data. Ranges are given by the 

 maximum difference of the five individual monthly mean values. In con- 

 touring the two variability maps only those terrestrial regions having 

 reasonable data coverage are included. Dashed contours are shown for 

 areas of sparse or unreliable data. The general picture of the worldwide 

 distribution of A''^ variability is that of a number of continentally located 

 cells of moderate range accompanied by somewhat random small-scale 

 variation over ocean areas. Regions of large range, from 40 to as much 

 as 70 A^ units, are present, however, in Australia and on islands of the 

 adjoining oceans, on the African equatorial plateau near the Cameroons, 

 and in the Great Basin of the southwestern United States. Common to 

 all these areas of large year-to-year variability, at least during the summer 

 season, are high mean temperatures ranging from about 25 to 30 °C, the 

 variability being due to relatively small variations of humidity. It is felt 

 that when a more dense network of stations is available for a longer period 

 of record, say 10 years, areas of high monthly variability are likely to be 

 more extensive in tropical and desert areas than indicated on our present 

 maps. 



4.2.4. Climatic Classification by N^ 



The annual cycle of A^« at each station was examined for the purpose 

 of deriving similarities of climatic pattern. As one form of climatic 

 classification, the annual mean value of A'^s at each station was plotted 

 versus the annual range at the station. When this was done, several 

 distinct groupings of data seemed evident. These groupings, described 



