Fire whirls are reported to occur frequently in the wilderness and in urban areas due to the influence of ambient winds. Fire whirls that occur on sloped fuel surfaces are common in the wilderness and have received less attention despite their potential to significantly alter fire behavior. Particularly in terms of frequency and height, previous studies have been performed on flatlands but less so on slopes. This paper presents an experimental study of fire whirls in sidewind line fires, focusing on the frequency of occurrence and the height of fire whirls. Regarding the effect of a side wind, it is shown that a side wind increases the frequency of occurrence, while the velocity component parallel or perpendicular to the line fire has a competing effect. In contrast, an increase in the slope reduces the height of the fire whirl; this phenomenon has been justified on the basis of experimental data from our work and the literature and explained in terms of the mechanism of vortex generation and movement.