Measurements made in contrails formed in the wake of a Lear 35 have been analyzed with respect to their temperature, water vapor, and vertical velocity structure. The National Center for Atmospheric Research (NCAR) Sabreliner traversed the Lear's wake at distances varying from 150 m to 5 km and penetrated the contrails 243 times over the course of seven flights. The width of the plume is documented as a function of the distance between the leading and chase aircraft and the temperature, humidity and vertical motion components inside the plume have been analyzed and compared with a three dimensional wake vortex model. The analysis shows that the width increases very little within the first 5 km, in agreement with the vortex model. The increases in temperature and water vapor in the contrail 500 m from the exhaust were ≈0.6¿ and 8 ppmv, respectively. The vertical velocity change across the contrail was ≈4 m s-1. The agreement between observations and model predictions is quite good in comparisons of temperature. The model predicts larger water vapor mixing ratios, but the differences can be explained within the expected measurement uncertainties and lack of particle condensation in the vortex model. The model predicts larger excursions in vertical velocity but the trends are in excellent agreement between model and observations. ¿ 1998 American Geophysical Union