Data taken during the Air Mass Transformation Experiment (AMTEX) by the NCAR Electra aircraft have proven useful for investigating the structure of thermals penetrating into the turbulent inversion layer which caps the convective mixed layer. On 16 February 1975, two flight legs, one upwind and one crosswind, and each about 12 min long (∼80 km in length), were flown at a nominally constant altitude at about the level of the turbulent inversion layer. Because of the variations in height of this relatively thin layer, the airplane spent about equal amounts of time above and below the inversion layer. These two legs are further split into six sections for statistical analyses, each with somewhat different characteristics. Variances, co-variances, spectra and cospectra of potential temperature, the three air velocity components, and humidity are computed to illustrate the dynamic processes occurring in this region. Two spectral maxima occur in vertical velocity and temperature: one at a wavelength of about 1.5 times the mixed layer depth and the other at about 200–300 m, which seems to be related to the characteristic size of a penetrating thermal.