The Atmosphere, Weather and Climate
Structure and composition
The atmosphere is ~78% nitrogen, 21% oxygen, with trace greenhouse gases (CO₂, water vapour, methane). Temperature falls through the troposphere (where weather occurs), rises in the stratosphere (ozone absorbs UV), and varies above. Pressure decreases with altitude.
Weather essentials
- Insolation varies with latitude, season and albedo; the tropics receive the most energy.
- Coriolis effect deflects winds (right in the northern hemisphere); it does not cause winds, only turns them.
- Pressure gradients drive wind; highs diverge, lows converge.
- Clouds and rain follow adiabatic cooling of rising air; orographic and frontal lifting are common triggers.
Climate
Climate is long-term average plus variability (typically 30 years). Controls: latitude, altitude, distance from sea, ocean currents, prevailing winds, and greenhouse gas concentrations. Feedback loops matter - water vapour and ice albedo amplify, while increased cloud cover may damp.
Traps
- Saying the Coriolis effect causes winds - pressure gradients do.
- Confusing weather (short-term) with climate (long-term statistics).
- Assuming ozone depletion causes warming; they are separate issues (ozone absorbs UV; greenhouse gases trap infrared).
- Ignoring that land heats and cools faster than water.
Worked example
Air at 20 °C rises 1000 m. Using a dry adiabatic lapse rate of 10 °C per 1000 m, it cools to 10 °C; if it reaches saturation, the saturated rate (about 6 °C per 1000 m) applies instead. Rule: rising air cools, sinking air warms; saturation changes the rate.
Rule of thumb
When explaining a climate pattern, name the latitude, the ocean current, the prevailing wind and the altitude - each can dominate locally.