Downloaded from https://www.everyspec.com storms without permanent damage becomes important. For withstanding, the values as given in MIL-STD-210B are as follows: 3 inches (76 mm) glaze, specific gravity 0.9. 6 inches (152 mm) glaze and rime mixed, specific gravity 0.5. 6 inches (152 mm) rime near the surface increasing linearly to 20 inches (508mm) at 400 feet (122 m), specific gravity 0.2. (1) Deposits of hoarfrost, the only type of ice accretion that occurs when air temperatures are well below 32 ° F (O ° C), may be several inches thick but will have a specific gravity of less than 0.2. (2) Ice fog consists of suspended ice crystals averaging 5 to 20 micrometers in diameter.
In areas where sufficient water vapor is present, ice fog occurs mainly at temperatures below −20 ° F (−37 ° C), ice fog may be very dense, limiting visibility ot a few feet. Ice fog is often locally induced by the operation of motor vehicles, power plants, weapon systems. It is usually high in concentration of contaminants from the burning of hydrocarbon fuels and explosive fuels. It affects the performance of electro-optical systems because of its attenuation and degradation of electromagnetic radiation in the atmosphere. d. Hail. Hail occurs too infrequently to warrant specification of an operational extreme.
When hail-caused equip- ment failure would endanger life or limb, designers should consider the possibility of encountering hailstones up to 2 inches (51 mm) in diameter. The largest hailstone ever recorded measures 5.6 inches (142 mm) in diameter. e. Wind. Wind is probably the most complex of all climatic elements affecting materiel. Wind effects are difficult to analyze because wind is a vector quantity subject to rapid temporal and areal changes in speed and direction. In addition to parameters of average speed and direction, a complete description of wind includes the random motions of widely different scales and periods called atmospheric turbulence or eddies.
The wind forces on a structure result from differential pressures, positive and negative, caused by an obstruction to the free flow of the wind. Thus, these forces are functions of the velocity and turbulence of the wind and of the oreintation, area, and shape of the elements of the structure. (1) For operations, the following extremes, as given in MIL-STD-210B, are: a steady windspeed of 73 fps (22 mps) and a gust of 95 fps (29 mps). (2) The above operational windspeeds are for a height of 10 feet (3 m). Multiplication factors for obtaining speeds at the height of equipment are— Figure 2-11. Height—Windspeed Calculations 20 AR 70–38 • 15 September 1979