Commercial passenger jets typically cruise around 35,000 feet, referred to by pilots as Flight Level 350 (FL350). At this height, the atmosphere is dramatically different from the air near the ground. The air is extremely thin and profoundly cold, presenting unique challenges for aircraft systems. This altitude is well above the clouds and weather systems, placing the aircraft in a high-altitude realm governed by distinct atmospheric physics.
The Specific Temperature Range at 35,000 Feet
The temperature at 35,000 feet is not a single fixed number but is generally frigid. According to the International Standard Atmosphere (ISA) model, a theoretical baseline used in aviation, the temperature at this altitude is approximately -54.34°C (-65.8°F). This calculation assumes a standard sea-level temperature of 15°C and a consistent rate of cooling with altitude.
In reality, the actual temperature typically falls within a range of -50°C to -65°C (-58°F to -85°F). This variation depends on geographic location, such as proximity to the equator or the poles, and the season. While the ISA provides a necessary reference for instrument calibration, pilots must always account for deviations from this standard.
The Significance of the Tropopause Boundary
An aircraft flying at 35,000 feet often finds itself near or within the tropopause, the atmospheric boundary separating the troposphere below from the stratosphere above. The troposphere is the lowest layer where nearly all weather occurs and where air temperature consistently decreases with altitude. The tropopause acts as a lid on this lower turbulent layer.
The height of this boundary is not uniform; it can range from 20,000 feet over the poles to over 60,000 feet near the equator. When an aircraft enters the tropopause, the temperature lapse rate—the rate of temperature decrease with height—abruptly slows down or stops. Commercial aircraft favor this altitude because this stability typically offers a smoother ride due to the lack of significant weather and moisture.
The Mechanism of Cooling: Atmospheric Lapse Rate
The air temperature plummets with altitude due to the atmospheric lapse rate, which describes how temperature changes as height increases. In the troposphere, air pressure decreases significantly as altitude rises because there is less air pressing down. This drop in pressure causes a pocket of air to expand.
When air expands without exchanging heat with its surroundings, a process called adiabatic cooling occurs, causing the air’s temperature to drop. For dry, unsaturated air, the temperature decreases at a steady rate of approximately 3°C per 1,000 feet, known as the dry adiabatic lapse rate.
If the rising air reaches its dew point and moisture begins to condense, the release of latent heat partially offsets the cooling. This results in the moist adiabatic lapse rate, a slower rate of cooling at about 1.5°C per 1,000 feet. The rapid temperature decrease up to the tropopause is a direct consequence of this pressure-driven expansion and cooling. Once the tropopause is reached, the lapse rate effect ceases, and the temperature stabilizes.
How Aircraft Measure Air Temperature
Aircraft employ specialized systems to measure the surrounding air temperature, but high speed complicates this measurement. The actual temperature of the undisturbed air is known as Static Air Temperature (SAT), also called Outside Air Temperature (OAT).
As the aircraft moves quickly, air molecules compress and frictionally heat up against the temperature probe. This effect, known as ram rise, causes the measured temperature to be higher than the actual SAT. The temperature displayed to the flight management system, which includes this heating effect, is called the Total Air Temperature (TAT).
TAT is a required input for calculating engine performance and determining the aircraft’s true airspeed. Aviation systems use the measured TAT and the aircraft’s speed to mathematically back-calculate the SAT.