Bell System Sleet Storm Map 



By J. N. KIRK 



MANY, no doubt, have seen copies of the Bell System Sleet 

 Storm Map which has been prepared to show the relative 

 intensities and frequencies of sleet storms throughout the United 

 States. In the midst of the current sleet storm season it may be of 

 interest to discuss some of the factors which have led to the prepara- 

 tion of this map, and to outline in a general way, the means by which 

 its indications are utilized in the design, construction and upkeep of 

 the pole and aerial wire plant. 



A sleet storm to be destructive to telephone plant must be ac- 

 companied by such atmospheric conditions as will cause either a 

 relatively heavy deposit of ice with no wind or a deposit of ice with a 

 considerable amount of wind. It is neither difficult nor expensive 

 to construct the aerial plant so that it will withstand winds of rela- 

 tively high velocities provided the wires are free of sleet. A slight 

 deposit of sleet, however, rapidly increases the "sail area" against 

 which the force of the wind is directed and the resulting load con- 

 stitutes one of the most formidable and most difficult to anticipate 

 of any of the destructive agencies with which wire using companies 

 have to contend. Maintenance difficulties unfortunately, are not 

 necessarily at an end with a lessening or cessation of the wind, for this 

 change may often increase the precipitation of sleet, which undis- 

 turbed by the wind builds up around the wires, frequently stressing 

 them beyond the breaking point. It is not uncommon for a wire of 

 approximately 1 /10 of an inch in diameter to accumulate sleet or 

 ice under favorable atmospheric conditions to the extent of a cylin- 

 drical coating from one to two inches or more in diameter. Some 

 idea of the destructive effects of such ice loads upon both poles and 

 wire may be gathered from Fig. 1 in which ice coatings as much as 

 2^2 inches thick without wind were responsible for practically a com- 

 plete collapse of the pole and open wire plant. 



Any medium through which information on the past performance 

 of pole and wire plant when subjected to storm conditions, may be 

 collected, analyzed and arranged in convenient form for reference 

 is a very valuable aid in the design and construction of new plant 

 which may be subjected to similar conditions. It would be both 

 impracticable and uneconomical to attempt to design open wire pole 

 lines to withstand such loads as are imposed by the occasional and 

 unusual storm, such for example, as was responsible for the damage 



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