Atmosphere & heat

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050 MeteorologyChapter 1

Schematic

ISA lapse rate 1.98C per 1000 ft in troposphere; tropopause near -56.5C.

Composition & structure

Key fact

Nitrogen ~78%, oxygen ~21%, argon ~0.93%, CO₂ trace. Water vapour 0–5% (decreases with altitude). Ozone in stratosphere filters UV. Layers: troposphere → tropopause → stratosphere → mesosphere → thermosphere (ionosphere).

Definition — Tropopause

Separates troposphere from stratosphere. ISA mid-latitudes: ~11 km / FL360, −56.5°C. Poles: ~8 km / 26 000 ft, ~−50°C. Equator: ~16–18 km / 54 000–60 000 ft, ~−75°C. Higher in summer, lower in winter. Jet stream cores at tropopause breaks.

Key fact

Stratosphere: isothermal (~−56.5°C) then temperature increases (ozone heating); absolutely stable, minimal weather. Stratopause ~50 km, ~0°C.

International Standard Atmosphere

Key fact

MSL: +15°C, 1013.25 hPa, 1.225 kg/m³. Temperature lapse: 1.98°C / 1000 ft (0.65°C/100 m) to tropopause. Pressure lapse: 27 ft/hPa (8 m/hPa) MSL–5.5 km; 50 ft/hPa above. Half pressure ~18 000 ft / 500 hPa. Half density ~20 000 ft.

Formula — Density altitude

Density alt = Pressure alt + (120 × ISA deviation)

Worst performance: hot, high, humid. 4% true altitude error per 10°C ISA deviation (warmer → under-read/safe on altimeter).

Exam trap

QFF uses actual temperature lapse to MSL (synoptic charts). QNH uses ISA lapse (27 ft/hPa) for altimeter. At MSL: QFE = QNH = QFF. Comparing QFF to QNH requires temperature context.

Heating the atmosphere

Key fact

Insolation (short-wave): ~45% reaches surface; heats ground → air by conduction, convection, turbulence. Terrestrial radiation (long-wave): net heat outflow at night; clouds act as blanket. Max diurnal temperature: ~noon + 2 h; min: ~sunrise + 30 min. Greatest DV: deserts, clear skies, calm winds, over land.

Key fact

Specific heat: water highest, rocks lowest — sea has small DV; land large DV. Convection and condensation (latent heat) contribute most to atmospheric warming.

Inversions

Type	Cause
Radiation/nocturnal	Clear, calm night; ground cools
Valley	Cold dense air drains into basin
Subsidence	Old high pressure; sinking air warms
Frontal	Warm air over cold
Tropopause	Isothermal layer aloft

Key fact

Inversions are absolutely stable. Low-level radiation inversions promote windshear (max at top of inversion).

Humidity

Key fact

Relative humidity = (actual mixing ratio / saturation mixing ratio) × 100%. RH increases as temperature decreases (inverse of diurnal temperature range). Dew point ≤ temperature; difference greatest in dry air. Cloud base ≈ (T − dewpoint) × 400 ft (DALR below cloud, SALR above).

Adiabatic lapse rates & stability

Formula — Lapse rates

DALR = 3°C / 1000 ft  (1°C / 100 m)   — dry/unsaturated
SALR = 1.8°C / 1000 ft (0.6°C / 100 m) — saturated
ELR  ≈ 2°C / 1000 ft average (measured)
DPLR ≈ 0.5°C / 1000 ft — dew point lapse (unsaturated)

Key fact

Absolute stability: ELR below SALR. Absolute instability: ELR above DALR. Conditional instability: SALR below ELR below DALR. Neutral: ELR = DALR (dry) or ELR = SALR (moist). Unstable → cumuliform, showers, good vis. Stable → stratiform, continuous rain, poor vis.