Downloaded from https://www.everyspec.com b. Temperature, humidity, solar radiation. (1) Operational conditions. The design condition is a minimum temperature of −60 ° F (−51 ° C). (For testing pur- poses, this is a cold soak temperature.) Solar radiation (horizontal surface) is negligible and relative humidity toward saturation (because of low temperature, not high absolute humidity or vapor pressure). Windspeeds are less than 16 fps (5 mps). In rare cases where materiel is designed to operate solely in areas where the cold climatic design type applies, the reverse season, or expected maximum, temperature is 95 ° F (35 ° C). (2) Storage and transit conditions. Same as (1) above.
(3) Daily Cycle. No cycle is given because temperature, humidity, and solar radiation remain nearly constant throughout the 24-hour period. 2–8. Additional environmental elements. Several additional climatic or other environmental elements are known to have effects on some kinds of military materiel. The elements are discussed in the following paragraphs and, where possible, operational extremes are given. a. Rain. The world’s highest rainfall intensities are in areas that experience the constant high humidity conditions of the basic climates, particularly Southeast Asia. The operational value is an instantaneous (1 minute) rate of 0.03 inches per minute (0.80 mm/min).
Based on data from Southeast Asia, this is the value exceeded only 0.5 percent of the hours in the rainiest month. For certain classes of materiel (e.g., missiles, aircraft) that might be subject to erosion from the more extreme rainfall intensities, a design value of 0.07 inches per minute (1.80 mm/min), derived from the same area should be considered. This is the intensity that is exceeded only 0.1 percent of the hours in the most extreme month. Much higher rainfall intensities can occur, but they are normally of short duration and, usually are restricted to small areas. The highest rainfall intensity ever officially recorded is 1.23 inches per minute (31 mm/min).
(1) A nominal drop-size spectrum for the 0.5 percent extreme is— Figure 2-8. Drop Diameter Range (mm) (2) The above rainfall intensities may be accompanied by intermittent winds up to 60 fps (18 mps). Higher windspeeds occur in hurricanes and typhoons (up to 148 fps or 45 mps), along with intense rain that falls almost horizontally penetrating cracks around doors, hatches, and other vertical openings. Rain affects the performance of electro-optical systems because of its attenuation of electromagnetic radiation in the atmosphere. b. Snow. Three aspects of snow are discussed in relation to equipment design.
Falling snow also affects the performance of electro-optical systems because of the attenuation and degradation of electromagnetic radiation in the atmosphere. (1) Snowfall rate. No operational extreme is given for rate of snowfall accumulation because windblown. (See (2) below.) The greatest snowfall accumulation during a 24-hour period ever recorded in the United States, the snowier sections of which receive as much snow as any part of the world, was 76 inches (1930 mm), a rate of about 3 inches per hour (76 mm/hr). Crystal sizes of snow particles range from 0.05 to 20 mm diameter with a median range of 0.1 to 1.0 mm.
Larger sizes are associated with temperatures near freezing and light winds. (2) Blowing snow. Operational extremes for blowing snow are given in terms of horizontal mass flux of snow particles; that is, the mass of snow moving horizontally across a unit area per unit time. Mass flux decreases significantly with increasing height; highest fluxes are found below 2 inches (0.05 m). Therefore, extremes of blowing snow are given for height intervals up to 33 feet (10 m). Design values should be based on the height of the equipment.
The horizontal mass windspeed of 44 fps (13 mps) at a height above fluxes for operational extremes, with a ground or snow surface of 10 feet (3 m), are— 18 AR 70–38 • 15 September 1979