This work aims to provide effective strategies and practical tools to control the diameter of fibers, a long-lasting challenge in the application of free surface melt electrospinning, mainly by highlighting the importance of the solidification point. A systematic approach to mapping the solidification point and temperature profile in an electrohydrodynamic jet in the melt electrospinning process was developed experimentally through the backlit imaging technique and numerically through computational fluid dynamics. The effect of the different spin-line temperature profiles on the robustness of the process as well as the fiber morphology was investigated. Scanning electron microscopy analysis demonstrated that at high spin-line temperature profiles, the fiber diameter dropped by four times compared to the room temperature spin-line environment. Both in situ backlit images from the jets in the spin line and the numerical phase fraction analysis revealed an immediate solidification of the jet, which is elongated twice in the case of the high spin-line temperature profiles. The elongated freezing length for the high spin-line temperature profiles as a result of the delayed solidification was identified as one of the main factors contributing to the jet thinning and subsequent fiber diameter reduction. Based on the simulation, the temperature profile of the jet demonstrated an approximately 20 °C drop along the jet length in the nonsolidified portion (freezing length), proposing the viscosity drop as a second factor in the fiber diameter reduction mechanism. Ultimately, the molten film thickness on the plate was identified as a semiphysical confinement parameter, controlling the size of the formed cones and subsequently the fiber diameter, despite the free surface nature of the unconfined melt electrospinning.