Earth's Atmosphere, Weather, and Climate
Domain IV covers atmospheric composition and structure, the energy and moisture that drive weather, the systems that produce fronts and storms, and the difference between short-term weather and long-term climate. Items ask you to read a station model or sounding, explain a cloud or storm, and evaluate climate evidence.
Structure and composition
Dry air is about 78% N₂, 21% O₂, 0.93% Ar, and 0.04% CO₂, with variable water vapor. Layers by temperature trend: troposphere (weather; average lapse rate about 6.5 °C/km), stratosphere (ozone absorbs UV, so temperature rises with height), mesosphere (coldest), thermosphere, exosphere. Pressure falls roughly exponentially with altitude.
Moisture and stability
Relative humidity compares vapor content with capacity; dew point is the temperature at which saturation occurs. Rising air cools at about 10 °C/km dry and 6 °C/km moist; the lifting condensation level sets cloud base. Cumulonimbus signals deep instability.
Pressure, fronts, and storms
The pressure gradient force starts air moving; the Coriolis effect deflects it (right in the Northern Hemisphere); friction slows surface wind and turns it across the isobars. Cold, warm, occluded, and stationary fronts mark air-mass boundaries, and surface flow converges into midlatitude wave cyclones. Tropical cyclones need sea-surface temperatures above about 26.5 °C, low wind shear, and Coriolis effect - hence none form at the equator.
Climate
Weather is the current state of the atmosphere; climate is its statistics. Controls include latitude, altitude, continentality, ocean currents, and topography. Greenhouse gases absorb outgoing infrared radiation, warming the surface; albedo and water-vapor feedbacks amplify or damp change; Milankovitch cycles pace glacial-interglacial shifts; ENSO redistributes Pacific rainfall.
Worked example
Surface air at 25 °C with a dew point of 15 °C rises at 10 °C/km and reaches 15 °C at 1 km - the lifting condensation level, so cloud base is 1 km. Above that, cooling slows to about 6 °C/km because latent heat is released, so at 3 km the parcel is near 3 °C.
Traps
Confusing the ozone hole with greenhouse warming; saying carbon dioxide traps incoming sunlight; blaming seasons on Earth-Sun distance; and thinking the Coriolis effect drives wind speed when it only deflects it.