B737 Max 8 cockpit || Southwest Airlines Registration: N8904L

On the 737 the TAT probes are electrically heated, so to prevent the heat being applied to the probe from causing erroneous readings, they use air from the packs to draw outside air over elements using a Venturi. This allows for accurate measurement of temperature when on the ground and at low airspeeds. However, if probe heat is on but the bleeds are off (i.e. just after startup or shutdown), the probe heating will heat up the air within them, giving false readings. Once either the packs are turned back on or the probe heating is turned off the readings will return to normal, but it can take a few minutes for the heat to dissipate.