Air temperature

050-01-02Pablo Asensio Martínez2026-04-013 min

International Standard Atmosphere (ISA)

ICAO defines a standard atmosphere for calibration and comparison:

  • Mean Sea Level (MSL): Temperature 15°C, Pressure 1013.25 hPa.
  • Lapse Rate: Temperature decreases 2°C per 1000 ft (0.65°C/100m) in the troposphere.
  • ISA Tropopause: At 36,090 ft (11 km).
  • Lower Stratosphere: Constant temperature of -56.5°C from the tropopause up to approx. 20 km (65,000 ft).

Temperature Calculations

To calculate the ISA temperature at a given altitude (in thousands of feet): $T_{ISA} = 15 - (2 \times Altitude)$ Example at FL240 (24,000 ft): $15 - (2 \times 24) = 15 - 48 = -33^\circ C$.

ISA Deviation: Difference between the actual temperature (OAT) and the ISA temperature. $Deviation = T_{Actual} - T_{ISA}$ Example: If at FL380 the actual T is -48°C. ISA T is -56.5°C. Deviation = -48 - (-56.5) = ISA +8.5°C.

Heat Transfer

Greenhouse Effect

  1. Radiation:
    • Solar (Insolation): The sun emits shortwave radiation (UV, visible). The atmosphere is almost transparent to it.
    • Terrestrial: The Earth absorbs solar radiation and re-emits it as longwave (Infrared) radiation. This is the main heat source for the troposphere (it is heated from below).
    • Greenhouse Effect: Gases like water vapor and CO2 absorb terrestrial longwave radiation, retaining heat.
  2. Conduction: Transfer by direct contact. Heats or cools only the layer of air in immediate contact with the surface (a few centimeters). Important for the formation of nocturnal inversions.
  3. Convection: Vertical transfer by movement of air masses. Warm air (less dense) rises (thermals).
  4. Advection: Horizontal transfer of heat by the wind.

Diurnal Variation

Daily Radiation and Temperature Cycle

  • Day: The surface heats up by insolation. Maximum temperature is usually in the afternoon (approx. 15:00), when heat received exceeds heat emitted.
  • Night: The surface cools by terrestrial radiation. Minimum temperature occurs just after sunrise.
  • Effect of Clouds:
    • Day: Reflect solar radiation -> Lower temperatures.
    • Night: Absorb and re-emit terrestrial radiation (blanket effect) -> Higher temperatures (less cooling).
  • Effect of Wind: Mixes the air, reducing extreme surface cooling at night (hinders inversions).

Temperature Inversions

An inversion occurs when the temperature increases with altitude in the troposphere. They are very stable layers.

Thermal Inversion

  1. Radiation Inversion (Nocturnal):
    • Occurs on clear and calm nights.
    • The ground cools rapidly by radiation, cooling the air in contact by conduction.
    • Common in winter over land. Can cause radiation fog.
  2. Subsidence Inversion:
    • Associated with High Pressure (Anticyclone) systems.
    • Air descends (subsidence) and warms by adiabatic compression.
    • Creates a warm layer over a colder, stagnant layer near the ground.
    • Common in winter over continents and in subtropical oceans.
  3. Valley Inversion:
    • Cold air (denser) drains down slopes to the valley floor at night (katabatic wind), displacing warm air upwards.
  4. Frontal Inversion:
    • Warm air rising over a cold air mass (fronts).

Albedo

It is the reflectivity of a surface (0 to 1).

  • High Albedo: Fresh snow (0.80), Thick clouds. Reflect a lot of heat, warm up little.
  • Low Albedo: Asphalt, Forests, Dark ocean. Absorb a lot of heat.