BELL SYSTEM SLEET STORM MAP 117 



A particularly important feature of this map in so far as its applica- 

 tion to the design of telephone plant is concerned, is that it repre- 

 sents actual destructive effects upon existing telephone plant and is 

 not directly governed by the extent of precipitation measured by the 

 Weather Bureau or otherwise, which in certain areas may have been 

 very heavy but from which no destructive effects were experienced. 



For convenience in preparing and interpreting the sleet storm map, 

 all storms have been classified as either heavy or medium and the 

 frequency of their occurrence has been considered in four sub-divi- 

 sions. The heavy storm was defined as one in which the diameter 

 of the ice covering the wires was 3/4" or greater and is represented 

 on the map by the dark pins. The medium storm was defined as 

 one in which the diameter of the ice covering the wires was less than 

 3/4" yet sufficient to cause appreciable damage to the aerial plant, 

 and is represented on the map by the light pins. In certain cases 

 where the thickness of ice was not recorded, sleet storms have been 

 classified by the amount of damage caused, commensurate with storms 

 in which the ice deposit was known. 



The four sub-divisions of storm frequency considered are repre- 

 sented by different markings and groupings of the pins corresponding 

 to the class of storm experienced, light marking being used on the dark 

 pins and dark marking on the light pins. At least one storm every 

 three years is represented by unmarked, closely spaced pins; at least 

 one storm every six years is represented by a single mark across the 

 face of the pins the latter being more widely spaced and staggered; 

 and storms occurring less frequently than every six years are repre- 

 sented by a cross on the face of the pins which are more closely spaced 

 vertically than horizontally on the map. The fourth subdivision is 

 in the nature of an exception to the third in that it covers those cases 

 in which only one storm has been recorded. These single storms are 

 represented by a single dot on the face of the pins, the latter being 

 widely but evenly spaced. These various arrangements may occur 

 singly or in combination resulting in the differently shaded areas 

 presented by the map shown in Fig. 2. No shading or markings appear 

 in those areas in which no sleet storms have been reported either 

 because no aerial plant is maintained or because that area does not 

 experience sleet. 



The effect of the wind has been taken into consideration in the 

 preparation of the sleet storm map only when such consideration 

 would change the classification of the area involved. In the design 

 of the aerial plant, however, the horizontal force exerted by the wind 

 on the wires and therefore on the poles, is an important factor. For 



