6.1 Introduction to the atmosphere

The atmosphere is a gaseous boundary coupled to Earth’s surface, not a solid lid; it supplies gases, pressure, energy regulation and radiation filtering.

Focus on exchanges with biosphere, hydrosphere and lithosphere: gases, particles, water and energy cross the boundary. Most weather and water vapour are in lower layers.

Evaporation moves water from ocean to atmosphere; precipitation returns it to the hydrosphere.

The atmosphere is coupled through ongoing flows and has no sharp solid edge.

Do not list functions without naming the exchange that supports life.

Unequal solar heating creates pressure and temperature contrasts; rising, sinking and rotating air redistribute energy through idealized Hadley, Ferrel and Polar cells.

Warm air expands and rises, cool air sinks, and pressure gradients move air. The cells help explain wind and rainfall belts, but they do not erase temperature differences.

More direct equatorial sunlight warms air that rises and transports energy poleward aloft before descending elsewhere.

To explain broad circulation and energy transport, not predict every local wind or storm.

The three-cell model is idealized; mountains, seasons and oceans modify real circulation.

Greenhouse gases absorb outgoing infrared radiation; aerosols are suspended particles that scatter or absorb radiation.

Water vapour, CO2, methane and nitrous oxide absorb and re-emit outgoing infrared radiation. CO2 and water vapour are abundant greenhouse gases, while methane also has a strong warming effect. Many aerosols scatter sunlight; black carbon absorbs radiation and can darken snow or ice.

CO₂ is a greenhouse gas; soot on snow is black carbon aerosol that lowers albedo. Both affect radiation, but through different classes.

Classify by physical form and radiation pathway. Water vapour is a greenhouse gas but its atmospheric abundance responds dynamically to temperature, so it is usually treated as a feedback rather than a direct mitigation target.

‘Warms’ does not identify the category; classify by physical form and mechanism.

The natural greenhouse effect makes Earth habitable; human increases in long-lived greenhouse gases enhance it and warm the climate.

Short-wave sunlight enters; the surface emits long-wave infrared; gases absorb and re-emit some energy. Extra GHGs require a warmer surface–troposphere system to restore balance. Global warming is temperature trend; climate change includes wider responses.

Adding CO₂ does not add sunlight; it slows outgoing infrared, so equilibrium is restored at a higher mean temperature.

Specify natural versus enhanced effect and the short-wave/long-wave energy pathway.

The atmosphere does not warm by trapping all heat forever; energy still leaves, but the balance shifts.