Pressure & wind

050 MeteorologyChapter 2

Pressure & density

Key fact

Pressure decrease with height lessens with altitude. Cold air: pressure falls faster with height (compact isobars) → APLR below 27 ft/hPa. Warm air: slower decrease. High to low, look out below — flying into lower pressure without resetting altimeter shows higher indication than true altitude.

Formula — Ambient pressure lapse rate (APLR)

ΔH = 96 × T(K) / P(hPa)   [ft per hPa]

Key fact

Density ∝ pressure, ∝ 1/temperature, ∝ 1/humidity. Surface density highest at poles; by ~26 000 ft (8 km) density equal latitudes; above tropopause at poles density eventually lower than tropics.

Q-codes & altimetry

Code	Definition
QFE	Pressure at aerodrome elevation (height datum)
QFF	QFE reduced to MSL using actual conditions
QNH	QFE to MSL using ISA lapse rate (27 ft/hPa)
SPS	1013.25 hPa — flight levels

Key fact

Transition altitude: climb — set 1013 passing TA. Transition level: descend — set QNH passing TL. Transition layer ≥ 500 ft. Regional QNH (RQNH): lowest forecast QNH in area for 1 hour.

Exam trap

Airfield below MSL: QFE greater than QNH. Difference QNH−QFE = elevation in hPa (approx). Warmer than ISA: true altitude greater than indicated (safe). Colder than ISA: true altitude less than indicated (danger).

Surface charts

Key fact

Isobars join equal QFF (MSL pressure). High/Low relative to surroundings. Features: L depression, H anticyclone, T trough, R ridge, C col, F flat. Col: winter fog; summer thunderstorms over land.

Wind fundamentals

Key fact

Wind from direction stated (met = true; ATC/ATIS = magnetic). Measured at 10 m / 30 ft AGL. Veers = clockwise; backs = anticlockwise (both hemispheres). Gust: +10 kt from mean; squall: +16 kt to ≥22 kt for ≥1 min.

Geostrophic & gradient wind

Key fact

PGF: high to low, ⊥ isobars. Coriolis (GF): deflects right NH, left SH; ∝ wind speed, ∝ sin latitude; negligible 0–15° N/S. Geostrophic wind: PGF = GF; parallel straight isobars; above ~2000 ft; GW ∝ 1/sin(latitude).

Key fact

Gradient wind (curved isobars): adds centripetal force. Around LOW (NH): CF opposes PGF → wind slower than geostrophic (sub-geostrophic). Around HIGH: CF with PGF → super-geostrophic, faster.

Formula — Buys Ballot's law (NH)

Back to the wind, low pressure on your left

Friction layer

Key fact

~1500–3000 ft AGL (avg 2000 ft). Wind backs and slows over land (30°, 50% of 2000 ft wind); over sea 10°, 70% speed retained. Thinner at night (mechanical only) → more effective backing/slowing. Strongest surface winds: mid-afternoon.

Local winds

Key fact

Sea breeze (day): sea→land, 10–15 kt, 10–15 NM inland, slack pressure gradient required. Land breeze (night): weaker, ~5 kt, ~5 NM. Anabatic: upslope by day. Katabatic: downslope at night (Bora, Mistral — violent gusts possible).

Key fact

Foehn/Chinook: moist air rises windward (SALR), descends leeward (DALR) → ~1.2°C/1000 ft net warming; warm, dry, gusty lee side. Harmattan: dry dusty NE wind from Sahara. Sirocco: dusty southerly from Africa to Mediterranean.