Earth's Atmosphere and Hydrosphere
This domain covers atmospheric composition and structure, energy balance, winds and weather, the water cycle, and ocean circulation. Exam items emphasize cause and effect: why the troposphere cools with height, how humidity depends on temperature, and how pressure gradients and the Coriolis effect shape global winds.
Core rules and concepts
- Structure. The troposphere holds most weather and cools at ~6.5°C/km (environmental lapse rate); the stratosphere warms with height because ozone absorbs UV; the mesosphere cools; the thermosphere is hot but thin. Pressure decreases exponentially with altitude.
- Composition and energy. Dry air is ~78% N₂, 21% O₂, 0.9% Ar, and ~0.04% CO₂. Insolation is absorbed, reflected (albedo), or re-emitted; greenhouse gases absorb and re-radiate infrared, warming the surface. The greenhouse effect is distinct from ozone depletion.
- Winds. Air flows from high to low pressure; the Coriolis effect deflects it right in the Northern Hemisphere and left in the Southern. Hadley, Ferrel, and Polar cells, jet streams, and prevailing westerlies/easterlies follow from differential heating and rotation.
- Moisture and weather. Relative humidity = actual vapor pressure ÷ saturation vapor pressure × 100%; dew point is the temperature at which saturation occurs. Rising air cools adiabatically; condensation forms clouds. Fronts and air masses produce weather changes.
- Oceans. Salinity and temperature drive thermohaline circulation; wind-driven gyres and Ekman transport cause upwelling. Groundwater moves through aquifers according to porosity and permeability.
Worked example
At 25°C, saturated air holds about 20 g/m³ of water vapor. If the actual content is 10 g/m³, relative humidity is 10/20 × 100 = 50%. If the air cools to 15°C, where saturation capacity is about 13 g/m³, RH rises to 10/13 × 100 ≈ 77%. Reasoning: the numerator stays fixed unless vapor is added or removed; cooling lowers the denominator, so RH rises toward saturation and clouds may form.
Traps that cost marks
- Explaining global warming by saying the ozone hole lets in heat; ozone depletion and greenhouse warming are different mechanisms.
- Claiming the Coriolis force creates wind; it only deflects moving air.
- Forgetting that warm air holds more vapor, so RH can fall as temperature rises even with constant vapor.
- Reversing Ekman transport or upwelling: winds push surface water, and deeper water rises to replace it.