IB Environmental systems and societies 6.1D Atmosphere System Notes

The Atmosphere as a Dynamic System

1. The atmosphere is a dynamic, constantly changing system shaped by physical and chemical processes from the surface to the upper layers.
2. These processes shape its composition, temperature, circulation, and the formation or destruction of gases such as ozone.
3. Because it receives constant inputs of solar radiation, aerosols, gases, and energy, it continually adjusts through feedbacks and exchanges with other Earth systems.

Physical Processes: Air Movement and Pressure

1. Warm air rises because it becomes less dense.
2. Cold air sinks because it is more dense.
3. This creates convection currents, the basis of global wind systems.
4. These convection currents drive global winds and weather systems, organised by the tricellular model (covered in 6.1B).
5. Air moves from high-pressure regions to low-pressure regions.
6. These pressure differences come from temperature contrasts, such as:
   1. warm equatorial air rising and creating low pressure,
   2. and cold polar air sinking and creating high pressure.

Chemical Processes: The Ozone–Oxygen Cycle

Ozone Production

1. In the stratosphere, molecular oxygen (O₂) absorbs high-energy ultraviolet (UV) radiation from the Sun.
2. This splits O₂ into two free oxygen atoms (O).
3. A free oxygen atom joins another O₂ to form ozone (O₃).

The Ozone–Oxygen Cycle (Dynamic Equilibrium)

1. UV radiation splits O₂ into O + O.
2. A free oxygen atom collides with O₂, forming O₃.
3. Another UV photon breaks O₃ back into O₂ + O.
4. The freed oxygen atom can rejoin O₂ again.
5. This creates a dynamic equilibrium that keeps stratospheric ozone levels stable.

The Atmosphere as an Open System

1. Inputs include solar radiation, volcanic gases, aerosols, and water vapour.
2. Outputs include infrared radiation, precipitation, and gas exchange with the biosphere and hydrosphere.
3. Flows include wind, convection, and chemical transformations.
4. Storages include atmospheric gases, clouds, and aerosol particles.

Why Air Thins and Cools with Altitude

1. Gravity strongly influences how atmospheric gases are distributed.
2. Molecules are pulled toward the surface, creating higher density and pressure at low altitudes.
3. Because gravity weakens with distance, the atmosphere becomes thinner at higher altitudes.
4. Temperature also changes with height according to the standard lapse rate.

Atmospheric Density and Pressure

1. Near the surface, air pressure is high because the mass of air above compresses the lower layers.
2. At higher altitudes there is less air above, so pressure and density fall.
3. As altitude increases:
   1. oxygen availability decreases,
   2. air becomes thinner,
   3. and temperature generally declines.

Why the Atmosphere Thins with Altitude

1. Gravity's Influence
   1. Gravity pulls gas molecules downward.
   2. Gravitational force weakens with height.
   3. So density is greatest near the surface and falls with altitude.
2. Pressure from Layers Above
   1. Lower layers support more weight from the air above.
   2. Higher up, less weight presses down, so molecules spread out.

Temperature Change with Altitude

1. The standard (environmental) lapse rate is about 6.5°C per kilometre, roughly 0.65°C for every 100 metres.
2. This happens because rising air expands under lower pressure and cools.
3. Actual lapse rates vary with humidity and stability:
   1. dry air cools faster,
   2. and moist air cools more slowly.

Consequences of Atmospheric Thinning

1. Low temperatures at altitude cause snow and ice to build up on peaks.
2. Biodiversity decreases with altitude, as fewer organisms tolerate cold and low oxygen.
3. Weather processes like cloud formation depend on how temperature and pressure change with height.