Plume cross sections of buoyant combustion products from jet diffusion flames in a crossflow were investigated experimentally. The objectives were to determine the forces leading to dynamic similarity in the plume behavior near the flame and to measure the cross-sectional shape in terms of these dynamic variables. Stack diameters from 16.7 to 102.3 mm and jet-to-crossflow velocity ratios from 0.0625 to 1.33 were investigated, which span a range of Reynolds number from 760 to 18600. The shape and size of 222 plumes were found to be dynamically similar to heated buoyant jets in a crossflow. This demonstrates that no fluid dynamic transition in plume behavior occurs over these conditions and that the dominant forces are the horizontal momentum flux of the crossflow and the buoyancy of the combustion products. A practical implication of this work is used to establish the validity of small-scale modeling of thermal plumes from flares in crossflows and provide a method to interpret the results of these models to the full scale.