Ultrasonic waves of frequency 32 kc/s are allowed to pass through a rising stream of hot turbulent air. Spaced microphones are used to investigate the diffraction pattern formed beyond the air stream, and spaced thermistors are used to record the temperature fluctuations in the air stream. The arrangement is used to study various aspects of random diffraction and to verify assumptions often made in the investigation of ionospheric irregularities. It is shown that the results are broadly in agreement with theory, in spite of various certainties about the applicability of the theories. In particular, reliable measurements of the velocity of the air stream and of the scale of the temperature fluctuations can be obtained from observations made at a few fixed points in the diffraction pattern.