Recent experiments have led to the discovery of the blue whirl, a small, stable

flame that evolves from a fire whirl, and burns typically sooty hydrocarbons without

producing soot. The distinct physical structure of the flame is investigated through

digital imaging techniques, which suggest that the transition and shape of the flame

may be influenced by vortex breakdown. The thermal structure of the blue whirl reveals

a peak temperature around 2000 K, and that most of the combustion occurs in a

relatively small, visibly bright vortex ring. The formation of the flame is shown to occur

over a variety of surfaces, including water and flat metal, all of which indicate that the

formation of the blue whirl is strongly influenced by the flow structure over the

incoming boundary layer. Finally, a schematic structure of the blue whirl is proposed,

based on the measurements presented here and previous literature on fire whirls and

vortex breakdown.