Abstract: An apparatus for use in an ultralight aircraft, glider, or sailplane to detect and navigate within thermal updrafts is disclosed. The apparatus includes a right temperature sensor and a left temperature sensor mounted on the wings of the aircraft. A difference circuit and difference indicator measure and indicate whether the right or left sensor is warmer and indicate a measured temperature difference. A derivative circuit and derivative indicator measure and indicate whether the air temperature is increasing or decreasing and indicate a rate of temperature change. The derivative circuit may be responsive to any of the right temperature sensor, the left temperature sensor, both the right and left temperature sensors, or to a third temperature sensor mounted on the aircraft. The apparatus detects thermal updrafts and indicates to the pilot when and how rapidly to turn to obtain the maximum lift from the thermal updraft.