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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Altitude vs. Barometric pressure
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<blockquote data-quote="Hugnot" data-source="post: 2099192" data-attributes="member: 115658"><p>When I ascend from 3000 feet to 7500 feet I would expect the air density to decrease and my GPS would provide an elevation but as the temperature changed or if a storm rolled in or out with moisture and barometric pressure changes the air density would change. Various bullet stability calculations, the most common being the Miller program use barometric pressure and temperature to estimate Sg values.</p><p></p><p><em>ρ = mass of air / volume</em></p><p></p><table style='width: 100%'><tr><th>Altitude<br /> [ft (m)]</th><th>Temperature<br /> [°F (°C)]</th><th>Pressure<br /> [psi (hPa)]</th><th>Air density<br /> [lb / cu ft (kg / m³)]</th></tr><tr><td>sea level</td><td>59 (15)</td><td>14.7 (1013.25)</td><td>0.077 (1.23)</td></tr><tr><td>2 000 (610)</td><td>51.9 (11.1)</td><td>13.7 (941.7)</td><td>0.072 (1.16)</td></tr><tr><td>4 000 (1219)</td><td>44.7 (7.1)</td><td>12.7 (873.3)</td><td>0.068 (1.09)</td></tr><tr><td>6 000 (1829)</td><td>37.6 (3.1)</td><td>11.7 (808.2)</td><td>0.064 (1.02)</td></tr><tr><td>8 000 (2438)</td><td>30.5 (-0.8)</td><td>10.8 (746.2)</td><td>0.06 (0.95)</td></tr><tr><td>10 000 (3048)</td><td>23.3 (-4.8)</td><td>10 (687.3)</td><td>0.056 (0.9)</td></tr><tr><td>12 000 (3658)</td><td>16.2 (-8.8)</td><td>9.2 (631.6)</td><td>0.052 (0.84)</td></tr><tr><td>14 000 (4267)</td><td>9.1 (-12.8)</td><td>8.4 (579)</td><td>0.048 (0.77)</td></tr><tr><td>16 000 (4877)</td><td>1.9 (-16.7)</td><td>7.7 (530.9)</td><td>0.045 (0.72)</td></tr></table><p></p><p></p><p>The standard measurement type for air density is kilograms per cubic meter (Kg/m3). Many people also use the following:</p><p></p><p><em><strong>g/cm³ = gram per cubic centimeter</strong></em></p><p><em>1 g/cm³ = 0.001 kg / m³</em></p><p><em></em></p><p><em><strong>kg/L = kilogram per liter</strong></em></p><p><em>1kg/L = 1,000 kg / m³</em></p><p><em></em></p><p><em><strong>g/mL = gram per milliliter</strong></em></p><p><em>1g/mL = 1,000 kg / m³</em></p><p></p><p>Air density decrease with altitude increase.</p><p></p><p><em>The first reason is gravity. Earth's gravity pulls <strong>air</strong> as close to the surface as possible. The second reason is <strong>density</strong>. As <strong>altitude</strong> increases, the amount of gas molecules in the <strong>air decreases</strong>—the <strong>air</strong> becomes less dense than <strong>air</strong> nearer to sea level.</em></p><p></p><p></p><p>The amount of <a href="http://www.theweatherprediction.com/habyhints/40/" target="_blank">water vapor in the air</a> also affects the density. Water vapor is a relatively light gas when compared to diatomic Oxygen and diatomic Nitrogen. Thus, when water vapor increases, the amount of Oxygen and Nitrogen decrease per unit volume and thus density decreases because mass is decreasing.</p></blockquote><p></p>
[QUOTE="Hugnot, post: 2099192, member: 115658"] When I ascend from 3000 feet to 7500 feet I would expect the air density to decrease and my GPS would provide an elevation but as the temperature changed or if a storm rolled in or out with moisture and barometric pressure changes the air density would change. Various bullet stability calculations, the most common being the Miller program use barometric pressure and temperature to estimate Sg values. [I]ρ = mass of air / volume[/I] [TABLE] [TR] [TH]Altitude [ft (m)][/TH] [TH]Temperature [°F (°C)][/TH] [TH]Pressure [psi (hPa)][/TH] [TH]Air density [lb / cu ft (kg / m³)][/TH] [/TR] [TR] [TD]sea level[/TD] [TD]59 (15)[/TD] [TD]14.7 (1013.25)[/TD] [TD]0.077 (1.23)[/TD] [/TR] [TR] [TD]2 000 (610)[/TD] [TD]51.9 (11.1)[/TD] [TD]13.7 (941.7)[/TD] [TD]0.072 (1.16)[/TD] [/TR] [TR] [TD]4 000 (1219)[/TD] [TD]44.7 (7.1)[/TD] [TD]12.7 (873.3)[/TD] [TD]0.068 (1.09)[/TD] [/TR] [TR] [TD]6 000 (1829)[/TD] [TD]37.6 (3.1)[/TD] [TD]11.7 (808.2)[/TD] [TD]0.064 (1.02)[/TD] [/TR] [TR] [TD]8 000 (2438)[/TD] [TD]30.5 (-0.8)[/TD] [TD]10.8 (746.2)[/TD] [TD]0.06 (0.95)[/TD] [/TR] [TR] [TD]10 000 (3048)[/TD] [TD]23.3 (-4.8)[/TD] [TD]10 (687.3)[/TD] [TD]0.056 (0.9)[/TD] [/TR] [TR] [TD]12 000 (3658)[/TD] [TD]16.2 (-8.8)[/TD] [TD]9.2 (631.6)[/TD] [TD]0.052 (0.84)[/TD] [/TR] [TR] [TD]14 000 (4267)[/TD] [TD]9.1 (-12.8)[/TD] [TD]8.4 (579)[/TD] [TD]0.048 (0.77)[/TD] [/TR] [TR] [TD]16 000 (4877)[/TD] [TD]1.9 (-16.7)[/TD] [TD]7.7 (530.9)[/TD] [TD]0.045 (0.72)[/TD] [/TR] [/TABLE] The standard measurement type for air density is kilograms per cubic meter (Kg/m3). Many people also use the following: [I][B]g/cm³ = gram per cubic centimeter[/B] 1 g/cm³ = 0.001 kg / m³ [B]kg/L = kilogram per liter[/B] 1kg/L = 1,000 kg / m³ [B]g/mL = gram per milliliter[/B] 1g/mL = 1,000 kg / m³[/I] Air density decrease with altitude increase. [I]The first reason is gravity. Earth's gravity pulls [B]air[/B] as close to the surface as possible. The second reason is [B]density[/B]. As [B]altitude[/B] increases, the amount of gas molecules in the [B]air decreases[/B]—the [B]air[/B] becomes less dense than [B]air[/B] nearer to sea level.[/I] The amount of [URL='http://www.theweatherprediction.com/habyhints/40/']water vapor in the air[/URL] also affects the density. Water vapor is a relatively light gas when compared to diatomic Oxygen and diatomic Nitrogen. Thus, when water vapor increases, the amount of Oxygen and Nitrogen decrease per unit volume and thus density decreases because mass is decreasing. [/QUOTE]
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