Satellite IR imagery and in situ hydrographic data are used to understand better the mesoscale variability of the California Current system and to explore the relation of its surface and subsurface thermal structures. In August 1982, one of the cool filaments commonly seen in satellite IR images during summer off Northern California was sampled hydrographically during an OPTOMA (Ocean Prediction Through Observation, Modeling, and Analysis) cruise. Based on the cool anomaly seen in a series of IR images and the in situ hydrographic data, and based on geostrophic calculations, a jet entrained upwelled coastal water and advected it rapidly through the warmer waters offshore, providing an important cross-shore transfer process. The anomaly was about 50 km wide and extended vertically to about 40 m. The temperature patterns at the surface were well correlated with those to about 30 m; below 50 m, the surface temperature was better correlated with horizontal temperature gradients, i.e., the ''thermal wind'', consistent with strong horizontal advection by the geostrophic jet. The cool anomaly was also relatively dense. Based on investigation of the T-S properties, the coastal water had been diluted through mixing with Columbia River plume water on the northern edge of the anomaly. Near the temperature front, in the shoreward sampled portion of the anomaly, and just south of the anomaly, interleaving occurred. There was also a surface saline anomaly embedded in the cool anomaly near the temperature front. The location of the temperature front in the in situ data and in the IR imagery corresponded well. The temperature front was tracked in a series of IR images over 2 weeks. The reorientation of the seaward extremity of the anomaly, from westward to northwestward, and the southwestward displacement of its southern boundary at speeds of 2 to 5 km/d were consistent with the propagation of mesoscale features evident in successive quasi-synoptic surface dynamic topography maps. The southwestward displacement was probably also related to surface Ekman transport.