Atmospheric pressure

050-01-03Pablo Asensio Martínez2026-04-012 min

Pressure Systems

Pressure Systems: High, Low, Ridge, Trough and Col

  • Isobars: Lines joining points of equal atmospheric pressure. On surface charts, they represent pressure reduced to sea level considering the actual temperature (QFF).
  • High Pressure (Anticyclone): Region of relatively high pressure. Air descends (subsidence), which inhibits cloud formation. Associated with good weather, clear skies, and dry conditions.
  • Low Pressure (Depression): Region of relatively low pressure. Air rises, favoring cloud formation and precipitation.
  • Ridge: An elongated extension of a high-pressure area. Associated with good weather and warm/dry air.
  • Trough: An elongated extension of a low-pressure area. Associated with cold air, clouds, and precipitation.
  • Col: Area of neutral pressure between two highs and two lows.

Pressure Variation with Altitude

Pressure decreases with altitude in a non-linear (exponential) manner. The rate of decrease is greater near the ground and smaller at high altitude.

  • Sea Level: 1 hPa ≈ 27-30 ft.
  • 18,000 ft (5,500 m): Pressure is approx. 500 hPa (half of sea level pressure). Here, 1 hPa ≈ 50 ft.
  • 39,000 ft: 1 hPa ≈ 105 ft.

Standard Pressure Levels (ISA)

Approximate relationship between Flight Levels (FL) and Pressure:

  • FL50 ≈ 850 hPa
  • FL100 ≈ 700 hPa
  • FL140 ≈ 600 hPa
  • FL180 ≈ 500 hPa
  • FL240 ≈ 400 hPa
  • FL300 ≈ 300 hPa
  • FL340 ≈ 250 hPa
  • FL390 ≈ 200 hPa

QFF vs QNH

Both are pressure settings reduced to Mean Sea Level (MSL), but they differ in how they correct for the fictitious column of air between the station and sea level.

Altimeter Settings: QNH, QFE, QNE

  • QNH: Reduces pressure using the Standard Atmosphere (ISA) temperature. It is the setting set on the altimeter to read the aerodrome elevation on the ground.
  • QFF: Reduces pressure using the actual air temperature. It is the value used to draw isobars on weather charts.

Relationship according to Temperature (Station above MSL)

Temperature's influence on aircraft altimeters

  • Warmer than ISA: The actual air column is less dense (more expanded) than the standard one. To reach the same station pressure, the "fictitious" sea level pressure (QFF) would have to be lower than that calculated with ISA (QNH).
    • QFF < QNH
  • Colder than ISA: The actual air column is denser (more compressed).
    • QFF > QNH
  • Equal to ISA: QFF = QNH.

(Note: If the station is below sea level, the relationship is reversed).