An apparatus that produces flow and heat transfer in horizontal narrow channels was used to explore the physics of the onset of subcooled nucleate boiling for R-11 and Novec 649. The apparatus uses a downward-facing electrically-heated thin foil as a heater, with the outside of the heater coated with a liquid crystal to display full-field temperature distributions for the heater surface. Video recordings were made of the temperature fields for a given set of operating conditions, accompanied by video recordings of the boiling behavior of the fluids in the channel itself. These two recordings were superimposed spatially so that the influence of various bubble dynamics could be directly correlated to the temperature response of the heater. Data have been obtained in a series of experiments performed in a horizontal channel, approximately 1.2 mm high x 20 mm wide x 357 mm long at mass fluxes indicative of laminar single phase flow in the channel. These data show that the enhancement of heat transfer behind the "boiling front", i.e., the point in the channel where bubbles are observed to first nucleate, is caused primarily by sliding bubbles rather than the activation of additional nucleation sites. These results tend to confirm the speculation made with an apparatus that did not allow visual observation of the boiling phenomena that surface temperatures near the boiling front can be depressed even though active bubble nucleation is not occurring there. This leads to boiling curves that involve a "turning angle" type of behavior that connects single-phase convection heat transfer to nucleate boiling heat transfer. The results reported in this paper provide a much clearer physical description of the controlling features of the transition from convection to boiling than has been available in the past. (C) 2011 Elsevier Ltd. All rights reserved.