Frost formation on air-heat exchanger surfaces deteriorates the performance of heat pumps, which is a critical issue for the electrification of the civilian sector energy demand especially for cold regions. Therefore, studying the frost formation on cold plates under forced convection is important. In the present study, three-dimensional frost structure was measured using the replica method under different forced convection conditions. A plastic shell on the frost layer was fabricated using a remote control measurement system by supplying Formvar solution. The surface of the plastic shell was then filled with light-curing resin to create a frost replica. The three-dimensional structure of the replica was measured using X-ray micro CT. An in-house machine learning semantic segmentation framework was used to efficiently process many XCT images. The effects of environmental conditions on local frost crystal morphology were investigated. The relative humidity and temperature of the air flow and cooling plate surface temperature greatly influenced the frost shape. Sheath-like crystals were observed at a cooling plate temperature of −15 °C, and plate-like crystals were formed at −25 °C. These results are consistent with the snow crystal morphology. This paper shows that the three-dimensional structure of frost can be measured under various environmental conditions.