Aerodynamic characteristics of vegetated surfaces have been assumed to be relative stable over the growing season and primarily a function of canopy height. Roughness lengths and displacement heights derived from neutral conditions have shown that there is more variation in these parameters than predicted by the simple height relationships. In 1994 and 1995, 10 meter towers were placed in corn and soybean fields to collect windspeed data at a minimum of five heights above the surface. These anemometers were raised throughout the season to maintain a minimum height of 25 cm above the top of the canopy. We developed a more physical approach to screening the data and evaluating the aerodynamic characteristics than predicted by the neutral condition approach with logwind profiles. The screening method and subsequent analytical method allowed for use of 95% of the available data. The results show that roughness lengths vary across the season but within the day depending upon the windspeed and the canopy structure. This approach allows for a more rigorous examination of canopy aerodynamic properties.