Downloaded from https://www.everyspec.com Figure 2-9. Snow Height and Mass Flux When blown by strong winds, snow crystals are broken and abraded into roughly equal size grains with rounded or subangular corners. More particles occur in the size range of 0.02 min to 0.4 mm, where the size is the effective diameter as— Figure 2-10. Blown Snow Crystal Diameter Formula —in the plane of measurement. Smaller sizes tend to occur at lower temperatures. Within the basic cold regions, the typical temperature range during periods of blowing snow is 14 ° F to −4 ° F (−10 ° C to −20 ° C).
Within the cold and severe cold regions, snowfall is common at temperatures between −10 ° F and −20 ° F (−23 ° C to −29 ° C). Blowing snow may occur at temperatures as low as −40 ° F (−40 ° C). (3) Snowload. A third important effect of snow is the structural load imposed by accumulated snow upon buildings, shelters, vehicles, or other relatively large military items. Snowload extremes are not applicable to operations; however, designers of the above equipment may wish to consider the following extremes, which are for snowloads on the ground. Snowloads on military equipment would usually be less than on the nearby ground.
(a) Portable equipment usually involves small items, such as tentage, which may be moved daily. This equipment generally will shed snow, but in instances where it does not, distortion will be noticeable and daily cleaning mandatory. The design criterion for this equipment is based on 24-hour snowfalls. The snowload value is 10 lbs/ft 2 (48.9 kg/m 2 ), which is equivalent to a depth of 20 inches (508 mm) of snow with a specific gravity of 0.1. (b) Temporary equipment usually involves large items on which snow can collect, rigid shelters, portable hangars, etc., which can be cleared of snow between storms.
This equipment will not sag much due to the snow loading but may collapse when its limits are exceeded. The design criterion for this equipment is based on snowfalls associated with storms lasting longer than one day. The snowload value is 20 lbs/ft 2 (97.7 kg/m 2 ), which is equivalent to a snow depth of 40 inches (1016 mm) with a specific gravity of 0.1. (c) Semipermanently installed equipment is usually demountable and not very mobile. Snow is not removed between snowfalls. The design criterion for this equipment is based on seasonal accumulation of snow.
The snowload value is 48 lbs/ft 2 (235 kg/m 2 ), which is equivalent to a snow depth of 96 inches (2438 mm) with a specific gravity of 0.1. c. Icing phenomena. Icing phenomena include glaze (freezing rain), hoarfrost, and rime, which cause problems of ice accretion on aircraft and other materiel, and ice fog, which interferes with visibility. Although reliable and systematic data on ice accumulation are scarce, fairly large areas of the United States and Europe can expect to endure seven or more ice storms per year. The effects of the storms may last from a few hours to several days. In the same areas, probably one storm per year on the average is severe enough to cause some damage.
In perhaps one year out of two or three, ice accumulation will probably be a half-inch or more. Therefore, if all-weather operation of materiel is desired within the areas where icing may occur the operational design value should be for onehalf inch of glaze with specific gravity of 0.9. This includes the colder sections within the basic design type, and all of the cold and severe cold areas. If equipment failure during the time of icing can be tolerated, the question of withstanding more severe 19 AR 70–38 • 15 September 1979