standard atmosphere

Examples of standard atmosphere in the following topics:

• Gauge Pressure and Atmospheric Pressure

  • In most measurements and calculations, the atmospheric pressure is considered to be constant at 1 atm or 101,325 Pa, which is the atmospheric pressure under standard conditions at sea level.
  • Atmospheric pressure is due to the force of the molecules in the atmosphere and is a case of hydrostatic pressure.
  • Depending on the altitude relative to sea level, the actual atmospheric pressure will be less at higher altitudes and more at lower altitudes as the weight of air molecules in the immediate atmosphere changes, thus changing the effective atmospheric pressure.
  • Atmospheric pressure is a measure of absolute pressure and can be affected by the temperature and air composition of the atmosphere but can generally be accurately approximated to be around standard atmospheric pressure of 101,325 Pa.
  • In this equation p0 is the pressure at sea level (101,325 Pa), g is the acceleration due to gravity, M is the mass of a single molecule of air, R is the universal gas constant, T0 is the standard temperature at sea level, and h is the height relative to sea level.

• Celsius Scale

  • From 1743 until 1954, 0°C was defined as the freezing point of water, and 100°C was defined as the boiling point of water, both at a pressure of one standard atmosphere, with mercury as the working material.
  • Although these defining correlations are commonly taught in schools today, by international agreement the unit "degree Celsius" and the Celsius scale are currently defined by two different temperatures: absolute zero and the triple point of Vienna Standard Mean Ocean Water (VSMOW; specially purified water).
  • "The output of the heat exchanger is hotter by 40 degrees Celsius" and "Our standard uncertainty is ±3°C").

• Absolute Temperature

  • Absolute temperature is the most commoly used thermodyanmic temperature unit and is the standard unit of temperature.
  • The kelvin (or "absolute temperature") is the standard thermodyanmic temperature unit.
  • By international agreement, the unit kelvin and its scale are defined by two points: absolute zero and the triple point of Vienna Standard Mean Ocean Water (water with a specified blend of hydrogen and oxygen isotopes).
  • Here, the size of helium atoms relative to their spacing is shown to scale under 1950 atmospheres of pressure.

• Measurements: Gauge Pressure and the Barometer

  • Gauge pressure is the pressure of a system above atmospheric pressure.
  • Barometers are devices used to measure pressure and were initially used to measure atmospheric pressure.
  • Early barometers were used to measure atmospheric pressure through the use of hydrostatic fluids.
  • As the atmospheric pressure changes, the pressure exerted by the atmosphere on the fluid reservoir exposed to the atmosphere at the base changes, increasing as the atmospheric pressure increases and decreasing as the atmospheric pressure decreases.
  • The height of the liquid within the glass column then gives a measure of the atmospheric pressure.

• Convection

  • More extreme measures are sometimes taken in very cold (or hot) climates to achieve a tight standard of more than 6 hours for one air turnover.
  • In the case of Earth, the atmospheric circulation is caused by the flow of hot air from the tropics to the poles, and the flow of cold air from the poles toward the tropics.
  • For example, the pot of water on the stove in is kept warm in this manner; ocean currents and large-scale atmospheric circulation transfer energy from one part of the globe to another.
  • If the water vapor condenses in liquid droplets as clouds form, heat is released in the atmosphere (this heat release is latent heat)  .
  • Thus, an overall transfer of heat from the ocean to the atmosphere occurs.

• The Evaporating Atmosphere

  • At 100 °C and atmospheric pressure, equilibrium is not reached until the air is 100% water.
  • The Earth's atmosphere is not unchanging.
  • Collisions between water molecules in the atmosphere allows some to condense and some to remain in vapor.

• Greenhouse Gases and Global Warming

  • The gases of the atmosphere are "selective absorbers"; energy in the visible part of the electromagnetic spectrum passes through the atmosphere directly to the Earth's surface (with some reflection occurring as well).
  • The gases in the atmosphere, primarily CO2 and water vapor are highly absorbent in the infrared part of the spectrum.  
  • The atmosphere absorbs the infrared radiation from the Earth, preventing it from escaping to space.  
  • Thus the greenhouse effect is a continuous cycle of absorption and emission of energy between the Earth and atmosphere.
  • This causes the Earth's atmosphere and surface to be warmer than otherwise expected.

• Scattering of Light by the Atmosphere

  • Rayleigh scattering describes the air's gas molecules scattering light as it enters the atmosphere; it also describes why the sky is blue.
  • This law applies to all electromagnetic radiation, but in this atom we are going to focus specifically on why the atmosphere scatters the visible spectrum of electromagnetic waves, also known as visible light.
  • In this case, the light is scattered by the gas molecules of the atmosphere, and the refractive index of air is 1.

• Visible Light

  • Visible wavelengths pass through the "optical window", the region of the electromagnetic spectrum which allows wavelengths to pass largely unattenuated through the Earth's atmosphere (see opacity plot in.
  • A consequence of the existence of the optical window in Earth's atmosphere is the relatively balmy temperature conditions on Earth's surface.
  • The surface of the planet then emits energy primarily in infrared wavelengths, which has much greater difficulty escaping (and thus causing the planet to cool) due to the opacity of the atmosphere in the infrared.
  • This is a plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation.
  • Most UV wavelengths are absorbed by oxygen and ozone in Earth's atmosphere.

• Ultraviolet Light

  • After atmospheric filtering, only about 3% of the total energy of sunlight at the zenith is ultraviolet, and this fraction decreases at other sun angles.
  • Most UV-B and all UV-C is absorbed by ozone (O3) molecules in the upper atmosphere.
  • This is a plot of Earth's atmospheric opacity (opposite of transmittance) to various wavelengths of electromagnetic radiation, including visible light.
  • Visible light passes relatively unimpeded through the atmosphere in the "optical window."
  • Most UV wavelengths are absorbed by oxygen and ozone in Earth's atmosphere.