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Background: Fire whirls are often reported to occur in wildland fires and can induce serious difficulties in firefighting by abruptly modifying fire behaviour, exposing firefighters and even causing casualties. Aims: The aim of this study was to examine the formation of fire whirls on a windward slope. Methods: Straight and V-shaped heated wires and burners were used to emulate fire spread fronts of various fire intensities. Different slope angles and ambient wind speeds were considered, to observe the formation and behaviour of fire whirls. Key results: A fire whirl appeared over a straight-line fire in the presence of wind, and a couple of counter-rotating fire whirls appeared over the two flanks of a V-shaped line fire. Conclusions: Two interaction regimes were experimentally observed and theoretically explained for the interaction of two fire whirls. We found that an effective wind speed instead of the local wind speed in sloped terrains can quantify the critical condition for the formation of fire whirls over line fires of different burning intensities. Implication: A critical effective wind speed correlation that couples slope angle, burning intensity and radiative fraction of a line fire can predict fire whirl risk in wildland fire fighting. A fire front spreading on a windward slope typically has a V-shape that can induce fire whirls, posing a risk to firefighters. We considered ambient wind, slope angle and line fire characteristics to predict the fire whirl risk. [ABSTRACT FROM AUTHOR]