A 1000 km long cloud-line over the major axis of the Gulf Stream was detected in imagery from a number of satellites on April 24, 2001. Analysis of environmental conditions shows that such a cloud-line is formed when the synoptic low-level wind is parallel to the Gulf Stream axis and the sky is clear, conditions that rarely occur in the high-temperature and high-moisture Gulf Stream region. The PSU-NCAR fifth-generation Mesoscale Model (MM5) is used to study the cloud line. Results show that upward motion of the air in the middle of the Gulf Stream, caused by mesoscale solenoidal circulation induced by the large surface thermal gradient, is the source for the large cloud-line formation. This cloud-line formation mechanism is different from that of commonly observed ship cloud lines induced by ship-stack emissions, and its extent is much longer than that of cloud lines induced by lake effects.