Why does a candle flame take a teardrop shape?

The flame in a candle is caused by the burning of the wax, a process that liberates a large amount of energy in the form of heat. This heat, in turn, excites the molecules and atoms in the air and the carbonaceous combustion products of the burnt wax. These excited atoms and molecules get de-excited and emit the light we see. The flame is, thus, a collection of highly heated gas atoms and molecules, which having lower density than the surrounding air, lifts itself up. During this, it goes farther from the source of heat and gets cooled by transferring heat to the surrounding air, which rushes from the neighbourhood. As the height increases, the cool air exerts a transverse pressure on the flame from all sides making it teardrop-shaped. 

The progressively increased effective cooling of the flame at higher levels from the tip of the candle can be demonstrated by a simple experiment. Take a flat-bottomed plate with some ordinary water and hold it for a short while at about half the length of the original flame. Take care not to hold the plate for too long. You observe that the central portion has no mark but the outer regions have the black mark of the carbon soot indicating the cooler gas being at the periphery of the flame compared with the inner region. The hotter gas in the inner region converts all the carbon into CO{-2} but not at the periphery. Hence black soot is formed. Next, hold the plate above the flame and you will collect a smaller soot mark without the clear middle region. This is because the hot gas has got cooled by the time it has reached the top and the carbon in the gas escapes conversion into CO{-2}. The teardrop shape of the flame is mainly because of the transverse air movement around the flame.On Earth, gravity-driven buoyant convection causes the teardrop shape. In microgravity, where convective flows are absent, the flame is spherical.

Source: The Hindu