Short notes 02: Pressure

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Diurnal & Semi-Diurnal Pressure Variation

• Atmospheric pressure shows a semi-diurnal (12-hour) oscillation.
• Most pronounced in tropical and sub-tropical latitudes.
• Tropics/Sub-tropics: ~3 hPa variation.
• Temperate latitudes: ~1 hPa variation.
• Pressure maxima: ~1000 & 2200 hrs (local).
• Pressure minima: ~0400 & 1600 hrs (local).
• Caused mainly by solar heating of the atmosphere.

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Pressure Systems: Low, High, Trough & Ridge

• Low / Cyclone / Depression
  • Surface convergence.
  • Rising air (ascent) → cooling, cloud, precipitation.
  • NH: Counter-clockwise circulation.
  • SH: Clockwise circulation.
  • Associated with bad weather.
• High / Anticyclone
  • Surface divergence.
  • Subsidence (sinking air) → adiabatic warming.
  • Produces stable air, light winds, fair weather.
  • Often associated with haze, fog, poor visibility.
• Trough
  • Elongated area of low pressure.
  • Winds back in NH (anti-clockwise change).
  • Surface convergence and ascent.
  • Pressure rises away from trough axis.
• Ridge
  • Elongated area of high pressure.
  • Subsidence and divergence.
  • Pressure falls away from ridge axis.
  • Associated with good weather.
• Col
  • Neutral area between two highs and two lows.
  • Very weak pressure gradient.
  • Light winds, stagnation → fog/haze common.

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Isobars, Contours & Charts

• Isobar: Line joining points of equal pressure (surface charts use QFF).
• Isotherm: Line of equal temperature.
• Isallobar: Line of equal pressure tendency.
• Contour / Isohypse: Line of equal height on constant-pressure charts.
• Close isobars: Strong pressure gradient → strong winds.
• Wide isobars: Weak pressure gradient → light winds.

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Pressure vs Height

• Atmospheric pressure = weight of air column above a surface.
• Pressure always decreases with height.
• Decrease is rapid near surface, slower aloft.
• Cold air (dense) → pressure decreases faster with height.
• Warm air (less dense) → pressure decreases slower with height.

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Standard Pressure Levels (ISA – Approximate)

• 850 hPa: ~5,000 ft (FL050)
• 700 hPa: ~10,000 ft (FL100)
• 500 hPa: ~18,000 ft (FL180)
• 400 hPa: ~24,000 ft (FL240)
• 300 hPa: ~30,000 ft (FL300)
• 200 hPa: ~38,000 ft (FL380)

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Altimeter Fundamentals

• Altimeter is an aneroid barometer.
• Measures pressure, displays height/altitude.
• Reading depends on sub-scale setting.

Altimeter Settings

• QFE: Pressure at aerodrome datum.
  • On ground → reads zero.
  • In air → height above aerodrome (AGL).
• QNH: QFE reduced to MSL using ISA.
  • On ground → reads aerodrome elevation.
  • In air → altitude above MSL.
• QFF: QFE reduced to MSL using actual temperature.
  • Used for meteorological charts only.
  • Never used for altimetry.
• 1013.25 hPa (SPS): Indicates Pressure Altitude / Flight Level.

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QNH, QFE, QFF Relationships

• Aerodrome at MSL → QNH = QFF = QFE.
• Aerodrome AMSL & colder than ISA → QFF > QNH.
• Aerodrome BMSL & colder than ISA → QNH > QFF.
• Rule of thumb near surface:
  • 1 hPa ≈ 8 m ≈ 27 ft

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Pressure Altitude & True Altitude

• Aircraft flying at constant IA follows a constant pressure surface.
• Warm air: Pressure surfaces higher → TA > IA.
• Cold air: Pressure surfaces lower → TA < IA.
• [ High to low – you are below ] or [ Warm to cold – don’t be bold ]

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Subsidence & Inversions

• Subsidence: Large-scale sinking air.
• Occurs in high pressure systems.
• Causes adiabatic warming, stability.
• Leads to subsidence inversion.
• Traps pollution, haze, fog.

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Upper-Air Charts

• Used because aircraft fly at Flight Levels.
• Show wind, temperature, and height of pressure surfaces.
• Closer contour lines → stronger winds.
• Heights often shown in decametres (gpm).

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Key One-Line Exam Facts

• 850 hPa ≈ 5,000 ft.
• 500 hPa ≈ 18,000 ft (half atmospheric mass below).
• 200 hPa ≈ 40,000 ft.
• Pressure falls ~34 hPa per 1,000 ft near MSL.
• Low pressure → convergence & ascent.
• High pressure → divergence & subsidence.

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