Editing Atmospheric pressure (section)

== Altitude variation ==
{{See also|Blaise Pascal#First atmospheric pressure vs. altitude experiment}}{{further|Barometric formula|Vertical pressure variation}}
[[File:Storm over Snæfellsjökull.jpg|thumb|left|Cloud formation above [[Snæfellsjökull]] ([[Iceland]]), formed above the mountain by [[orographic lift]]]]
[[File:Atmospheric Pressure vs. Altitude.png|thumb|300px|Variation in atmospheric pressure with altitude, computed for 15&nbsp;°C and 0% relative humidity.]]
[[File:Plastic bottle at 14000 feet, 9000 feet and 1000 feet, sealed at 14000 feet.png|thumb|This plastic bottle was sealed at approximately {{convert|14000|ft|m|disp=flip}} altitude, and was crushed by the increase in atmospheric pressure, recorded at {{convert|9000|ft|m|disp=flip}} and {{convert|1000|ft|m|disp=flip}}, as it was brought down towards sea level.]]

Pressure on Earth varies with the altitude of the surface, so air pressure on mountains is usually lower than air pressure at sea level. Pressure varies smoothly from the Earth's surface to the top of the [[mesosphere]]. Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round. As altitude increases, atmospheric pressure decreases. One can calculate the atmospheric pressure at a given altitude.<ref>[http://archive.psas.pdx.edu/RocketScience/PressureAltitude_Derived.pdf A quick derivation relating altitude to air pressure] {{webarchive|url=https://web.archive.org/web/20110928003908/http://psas.pdx.edu/RocketScience/PressureAltitude_Derived.pdf |archive-url=https://web.archive.org/web/20100614221022/http://psas.pdx.edu/RocketScience/PressureAltitude_Derived.pdf |archive-date=2010-06-14 |url-status=live |date=2011-09-28 }} by Portland State Aerospace Society, 2004, accessed 05032011</ref> Temperature and humidity also affect the atmospheric pressure. Pressure is proportional to temperature and inversely related to humidity, and both of these are necessary to compute an accurate figure. The graph {{if mobile|above|on the right}} was developed for a temperature of 15&nbsp;°C and a relative humidity of 0%.

At low altitudes above sea level, the pressure decreases by about {{convert|1.2|kPa|hPa|abbr=on}} for every 100 metres. For higher altitudes within the [[troposphere]], the following equation (the [[barometric formula]]) relates atmospheric pressure ''p'' to altitude ''h'':


<big><math>\begin{align}
  p &=       p_0 \cdot \left(1 - \frac{L \cdot h}{T_0} \right)^\frac{g \cdot M}{R_0 \cdot L} \\
    &= p_0 \cdot \left(1 - \frac{g \cdot h}{c_\text{p} \cdot T_0} \right)^{\frac{c_\text{p} \cdot M}{R_0}}
       \approx p_0 \cdot \exp \left(-\frac{g \cdot h \cdot M}{T_0 \cdot R_0} \right)
\end{align}</math></big>

The values in these equations are:
{|class="wikitable sortable"
|-
! Parameter !! Description !! Value
|-
| ''h'' || style="text-align:left;"| Height above mean sea level || style="text-align:right;" | {{nbsp}}m
|-
| ''p''<sub>0</sub> || style="text-align:left;"| Sea level standard atmospheric pressure || style="text-align:right;"| 101,325{{nbsp}}Pa
|-
| ''L'' || style="text-align:left;"| Temperature lapse rate, = ''g''/''c''<sub>p</sub> for dry air || style="text-align:right;"| ~ 0.00976{{nbsp}}K/m
|-
| ''c''<sub>p</sub> || style="text-align:left;"| Constant-pressure specific heat || style="text-align:right;"| 1,004.68506{{nbsp}}J/(kg·K)
|-
| ''T''<sub>0</sub>|| style="text-align:left;"| Sea level standard temperature || style="text-align:right;"| 288.15{{nbsp}}K
|-
| ''g'' || style="text-align:left;"| Earth-surface gravitational acceleration || style="text-align:right;"| 9.80665{{nbsp}}m/s<sup>2</sup>
|-
| ''M'' || style="text-align:left;"| Molar mass of dry air || style="text-align:right;"| 0.02896968{{nbsp}}kg/mol
|-
| ''R''<sub>0</sub> || style="text-align:left;"| [[Universal gas constant]] || style="text-align:right;"| 8.314462618{{nbsp}}J/(mol·K)
|}