Static Air Temperature (SAT) is a specialized measurement used primarily in fields involving high-speed movement through the atmosphere, most notably aviation. It represents the true thermodynamic condition of the air, independent of the motion of any object passing through it. Understanding this precise temperature is fundamental for accurately calculating performance metrics and ensuring the safety of aircraft operations. SAT helps determine the properties of the air mass, which affects how engines perform and how the aircraft interacts with the surrounding environment.
Defining Static Air Temperature
Static Air Temperature (SAT), often called Outside Air Temperature (OAT) or True Air Temperature, refers to the temperature of the air when it is completely undisturbed. In a theoretical sense, it is the temperature a thermometer would register if it were stationary relative to the surrounding air molecules. This measurement reflects the intrinsic thermal energy of the atmosphere at a specific location and altitude.
The temperature is a property of the air mass itself, representing its thermodynamic state before any external forces act upon it. SAT is the baseline temperature used in meteorological models and for certain flight calculations.
The Difference Between Static and Total Air Temperature
The distinction between Static Air Temperature (SAT) and Total Air Temperature (TAT) arises from the physics of an object moving rapidly through the air. As an aircraft flies, air molecules rush into the temperature sensor, forcing them to slow down against the probe’s surface. This rapid deceleration converts the air’s kinetic energy of motion into thermal energy, causing the temperature reading to increase artificially.
This phenomenon is known as kinetic heating or “ram rise.” The Total Air Temperature (TAT) is the actual temperature measured by the probe after the air has been brought to rest adiabatically, meaning without any heat exchange with the surroundings. TAT is defined by the relationship: SAT plus the Ram Rise equals TAT.
Ram rise becomes a significant factor at speeds above approximately 200 knots and increases proportionally to the square of the aircraft’s speed. For modern jet airliners cruising at high Mach numbers, the ram rise can easily be 30 degrees Celsius or more. This substantial difference means that Total Air Temperature must be calculated and then corrected to find the true Static Air Temperature.
Measuring Static Air Temperature
Directly measuring Static Air Temperature from a fast-moving aircraft is impossible due to the unavoidable kinetic heating effect. Instead, specialized probes are used to measure the Total Air Temperature (TAT), and this reading is then used to calculate the SAT. These probes are designed to efficiently capture and slow the airflow, maximizing the conversion of kinetic energy into heat.
The air is brought to rest within a specially designed chamber inside the probe, which measures the resulting Total Air Temperature. However, no probe is perfectly efficient at converting all kinetic energy into heat; some heat is inevitably lost. This inefficiency is quantified by a value called the recovery factor, which for modern probes is typically around 0.98.
The aircraft’s air data computer takes the measured TAT, the recovery factor of the probe, and the aircraft’s Mach number (speed relative to the speed of sound) to mathematically reverse the kinetic heating effect. This complex calculation allows the system to accurately determine and display the true Static Air Temperature. The final SAT value is a calculated result, not a direct measurement.
Real-World Applications and Significance
The accurate determination of Static Air Temperature is necessary across several specialized fields, particularly in aviation and meteorology. In flight, SAT is a primary input for the air data computer to determine True Airspeed (TAS), which is the aircraft’s actual speed relative to the air mass. TAS is used for accurate navigation, flight planning, and determining the aircraft’s performance limits.
SAT is also important for calculating engine performance, especially for jet engines, where the thrust output is highly dependent on the density and temperature of the intake air. Meteorologists rely on SAT data collected from aircraft to create precise weather models and upper-air soundings, providing an accurate representation of atmospheric conditions.
The use of the Kelvin temperature scale is necessary in these calculations. This is because the speed of sound, which is needed to determine the Mach number, varies with the square root of the absolute static temperature.