Air density is computed using three inputs: atmospheric pressure, temperature, and the moisture content of the air. In order to calculate the elevation data necessary for accurate shooting, ballistic software must model the density of the air. at a barometric pressure of 29.92 inches of mercury, with zero percent relative humidity. The base condition of the ICAO Standard atmosphere is a temperature of 59 degrees F. What is referred to as “Army Standard Metro Conditions”, used in some ballistic programs, is a BP of 29.53 inches of mercury at a temperature of 59 degrees F. The highest pressure recorded in the world was 32.01inches at Agata, USSR, on 31 December, 1968. The lowest BP recorded in the western hemisphere was 26.05 inches of mercury on October 19, 2005, during Hurricane Wilma. In addition, some programs use either inches of mercury or millibars. If local weather data is reported in millibars, it can be converted to inches of mercury by dividing the number of millibars by 1000, then multiplying by 29.53. If the reader is more accustomed to seeing barometric pressure (BP) data expressed in millibars, note that inches of mercury can be converted to millibars by dividing the reading in inches of mercury by 29.53. For the purposes of this article, we will refer to that pressure in inches of mercury. Hurricanes and storms are usually low-pressure systems, while clear weather is usually accompanied by higher pressure. At altitudes where humans can live, barometric pressure decreases approximately at the rate of one inch of mercury per 1000 feet of elevation gain.Īir pressure at the bottom of the column, i.e., the surface of the earth, varies as well, with weather. The earth’s atmosphere is essentially a column of air.Īs you might expect, because of gravity there is more pressure at the bottom of that column than at the top – since the top is essentially a vacuum, i.e., outer space. Barometric Pressure and Ballistic Software Barometric Pressure and Ballistic Software
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