Flames flicker because hot gases rising from the fire interact with cooler, denser air surrounding it, creating small instabilities that push the flame around. This happens naturally on Earth due to gravity and is so predictable that a typical candle flame flickers at a frequency between 10 and 12 Hz, roughly 10 to 12 oscillations per second.
How Gravity Drives the Flicker
When a flame burns, it heats the air directly around it. That hot air becomes less dense and rises, pulling in cooler, heavier air from the sides and below to replace it. This cycle of rising hot air and incoming cool air is called buoyant convection, and it’s the primary engine behind flickering.
The problem is that this exchange doesn’t happen smoothly. When a lighter fluid (hot combustion gases) sits beneath a heavier fluid (the surrounding cool air), the boundary between them becomes unstable. Small disturbances at that boundary grow into rolling, mushroom-shaped waves that tug the flame sideways and upward in irregular pulses. This is the same type of instability you see when a dense liquid sits on top of a lighter one and the two begin to mix. In a flame, these instabilities develop continuously, producing the constant dance you see.
The classic teardrop shape of a candle flame is itself a product of gravity. Hot gases accelerate upward, stretching the flame into a point at the top. Remove gravity entirely, and you get something radically different. NASA experiments in microgravity have shown that without buoyancy-driven convection, flames burn as small, nearly perfect spheres. There is no rising column of hot air, no cool air rushing in from below, and critically, no flickering. The flame just sits there, round and still.
What You’re Actually Seeing
The visible light in a flame comes from tiny carbon-rich soot particles that form during combustion. As fuel burns incompletely, it produces these particles, which get heated to extreme temperatures and glow with continuous, blackbody-type radiation, the same kind of light emission that makes a heated piece of metal glow red or white. Even very young soot particles, barely formed in the flame, are capable of producing visible thermal emissions.
When the flame flickers, what you’re really watching is these glowing particles being carried along with the shifting gas flow. As pockets of hot gas roll and fold into the surrounding air, the concentration and temperature of soot particles change from moment to moment, making the flame appear to brighten, dim, and sway. The color shifts you sometimes notice, from bright yellow to darker orange near the edges, reflect local differences in temperature and soot density as the turbulent mixing unfolds.
Why Some Flames Flicker More Than Others
The intensity and character of flickering depends on how smoothly or chaotically the gases are flowing. Fluid flow exists on a spectrum from laminar (smooth, orderly layers) to turbulent (chaotic, swirling). A small, well-shielded candle flame can burn in a mostly laminar state, with gentle, rhythmic oscillations. A campfire or a gas burner with a strong fuel flow produces much more turbulent combustion, with wild, unpredictable flickering.
The transition from steady to turbulent depends on the speed of the gas flow relative to how easily the gas resists deformation. Engineers quantify this with the Reynolds number. When the flow speed is low and the gas is viscous, the flame stays orderly. As speed increases or conditions change, the flow crosses a critical threshold and turbulence kicks in. For jet diffusion flames, researchers have identified at least four distinct regimes the flame passes through on its way from laminar to fully turbulent, each triggered by different instability mechanisms in and around the flow.
Even a draft across a candle is enough to disrupt the delicate balance of rising hot gas and incoming cool air. That’s why candles flicker more near open windows or in rooms with air circulation. You’re adding an external disturbance on top of the flame’s natural buoyancy-driven oscillation.
How Oxygen Changes the Behavior
Oxygen concentration has a surprising and somewhat counterintuitive effect on flickering. At normal atmospheric oxygen levels (about 21%), a single candle flame shows no strong periodic oscillation. But when researchers raised the oxygen concentration above 70%, that same single candle began flickering with a clear, measurable rhythm at about 11.5 Hz.
The relationship gets more complex with multiple flames. A bundle of 14 candles that flickered strongly at normal oxygen levels actually stopped oscillating when oxygen concentration was pushed above 90%. As oxygen increased, the flickering frequency decreased, but the rate at which the candles consumed fuel increased. More oxygen means more intense combustion, which changes the temperature gradients and gas flow patterns around the flame. At very high oxygen levels, the combustion becomes so efficient and the heat release so uniform that the instabilities driving the flicker can actually be suppressed.
This is one reason why different fuels and burner designs produce different flickering patterns. Anything that changes how quickly fuel meets oxygen, how hot the combustion zone gets, or how the surrounding air circulates will alter the balance of forces that create or suppress flicker.
The Rhythm Behind the Randomness
Flickering may look random, but it has a characteristic frequency that’s remarkably consistent. For candle-sized diffusion flames, that frequency sits in the 10 to 12 Hz range, determined mainly by the diameter of the fuel source and the strength of the buoyant flow. Larger flames with wider fuel jets tend to oscillate at slightly lower frequencies, while smaller ones oscillate faster, but the range stays narrow.
When multiple candles are placed together, their flames can synchronize. Groups of three or more candles in close proximity begin oscillating in phase, sometimes merging into a single larger flame that pulses as a unit. The collective behavior follows its own rules: the flickering frequency decreases as more candles are added to the group, because the effective size of the combined flame increases.
This rhythmic quality is part of why people find candlelight so visually appealing. The oscillation at 10 to 12 Hz falls in a range that’s fast enough to create a sense of movement but slow enough for the eye to track, producing that characteristic warm, living quality that no steady light source can replicate.