A case study of the formation of an instability line traversed by the tornado research airplane before, during and after development is presented. Five traverses at the 800-mb level and two traverses at the 700-mb level were flown normal to the line. Gradients of moisture and temperature and their changes in time and space as shown from the aircraft data are presented. The dew-point front at the 800-mb level was traversed in a distance of one mile and was observed on all 800-mb traverses. The instability line developed immediately ahead of this front.