What gives flames their tongue-like shapes?

Flames get their “tongue-like” shapes from the way burning gases move and interact under earthly gravity.

When you strike a match, you get a flame of burning gases. Vaporized fuel from the match mixes with air and produces heat by a chemical reaction. Flames get their “tongue-like” shapes from the way these gases move and interact under earthly gravity.

Picture it this way. Near the match head, a column of hot gas is created. This column is fairly wide – wider than the match itself. Hot gases have more energy than colder gases and can more easily shake off the tug of gravity. So the column of hot gas starts to rise – and it rises faster and faster. Since the amount of gas is constant, as it rises faster, the column is stretched thinner.

A similar thing happens when you put your thumb over the end of a garden hose. You make the water stream narrower, so the water travels faster. The result for a flame is a wide column near the match, and a thin column higher up – in other words, a teardrop shape. But flames flicker, and their shapes are constantly changing. The motion of gases near a flame is chaotic. These gases expand, rise, and swirl through each other. If you look at a flame’s shadow, you can actually see these swirling hot gases rising even outside the region of the flame.