Horizontal convective rolls (HCRs) and cloudstreets that occurred in northeastern Colorado are studied using Doppler radar data, cloud photography, and vertical soundings. Examination of the environment in which the HCRs form indicates a boundary layer wind speed that is much lower than those measured in previous studies. Evidence shows that the HCRs form in a well-defined boundary layer, yet the cloudstreets that develop seem to exist within a stable layer above. A relationship between the radar reflectivity echoes and the cloudstreets is firmly established. The echoes are aligned directly under the cloudstreets. The echoes at lower-levels are shown to be caused by convergence of particulate matter into updraft portions of the HCR circulation, yet evidence indicates the cloud-level echoes are due to backscatter from refractive index inhomogeneities at the surfaces of rising thermals. Examination of the echoes show that the strongest and tallest echoes are associated with cloud formation, and that echo maxima, spaced 3.0 to 8.0 km apart, exist along the HCR.