An eddy covariance system including an open path infrared gas analyzer (IRGA) was used from 26 August 1996 to the present to monitor carbon and water vapor fluxes and the energy balance over a no-till field in east-central Illinois. The no-till field is in a corn (Zea mays L.) and soybean (Glycine max L. Merrill) rotation with corn grown during the summers of 1997, 1999, 2001, and soybean in 1998, 2000, and 2002. Eddy covariance measurements were collected continuously and averaged for 30 minute periods. These measurements were used to evaluate the effects of weather on corn and soybean growth and to evaluate the effects of short term weather extremes on growth and yield. Weekly plant samples were collected to provide crop growth data to compare with the eddy covariance estimates of plant growth. In this paper we will describe the effects of these weather extremes on light and water use efficiency, and how corn, a C4 crop, and soybean, a C3 crop, respond to weather conditions.