TAT AND OAT Q. What is the difference between OAT (outside air temperature) and TAT (total air temperature)?
A. Static air temperature (SAT) is what would be measured from a balloon, and is the temperature given in a forecast or report. It is also referred to as outside air temperature (OAT). Total air temperature (TAT) is obtained by a probe having velocity with respect to the air. Because of kinetic heating on the upstream side of the probe, TAT is warmer than SAT. SAT is computed from TAT and other flight conditions by an air data computer for dry air. There is less kinetic heating in saturated air than dry air. Indicated outside air temperature (OAT) is measured by a simple sensor in the airstream – essentially a thermometer. Typically, OAT values will be SAT or OAT plus approximately 80% of the difference from SAT to TAT. Surface temperature varies with pressure along the airfoil. At the leading edge where pressure is the highest, the surface temperature will also be higher than further aft. If the local surface temperature on the airfoil is warmer than freezing, no ice will form. Infra-
