Why does a candle flame always point upward?
― There's no such thing as a downward-burning flame, because a flame is a "light gas"
Tilt a candle a little, or point a match downward — the tip of the flame always turns toward the sky. A flame isn't a burning "thing." It's the path that glowing, superheated gas takes as it floats up through the air. That's why, in weightless space, a flame becomes a round ball with no up or down.
A birthday cake has many candles stuck in it. Even one poked in at an angle still has a flame pointing straight up.
A child asks, "Why does fire always go up? It'd be fine if it burned downward too."
Adults tend to answer, "Because that's just what fire does." But there's actually a proper reason. And once you know that reason, you can even predict what happens in space.
There are only two reasons a flame points up
A flame is really hot gas, glowing as it burns. Gas expands as it heats up, and at the same size it becomes lighter. The gas in a candle flame weighs only about a fifth as much as the surrounding air.
The light gas is pushed up from below by the heavier air around it. Once it rises, fresh air flows in from the sides and below to take its place. This flow stretches the flame upward into a slender shape.
In other words, the flame isn't climbing under its own power. The surrounding air is heavier, so it dives underneath and lifts the flame up. Let's look at this step by step.
Why is hot gas light?
Near a candle's wick, melted wax turns to vapor and combines with oxygen in the air to burn. The heat this releases brings the surrounding gas to temperatures over 1000°C. The hottest spot is said to reach around 1400°C.
Gas particles fly around more violently as they heat up, pushing each other further apart. The same amount of gas expands to take up more space. So if you compare equal one-litre containers, the hot gas is much lighter.
Look at Figure 1. The left box is ordinary room air. The middle box is hot gas of the same size. As you turn the dial up to raise the temperature, the contents of the middle box keep getting lighter, and the upward arrow on the right grows longer.
At 1400°C, the hot gas weighs only 0.18 times as much as air — like a cork floating in water. Just as a cork is pushed up by water, the light gas is pushed up by the heavier air around it.
This upward push is about 80% as strong as gravity. You could almost say the flame's gas is "falling" upward with nearly the same force that makes things fall down.
The surrounding air shapes the flame
When hot gas escapes upward, it doesn't leave empty space behind. Air immediately flows in from the sides and below. That incoming air carries fresh oxygen with it.
This fresh air mixes with the wax vapor and burns. The gas, now hot, escapes upward again. This cycle — in from below, out from the top — runs without pause. The glowing gas is stretched along by this flow, so the flame bulges at the bottom and tapers to a point at the top, forming a "teardrop" shape.
Look at the left side of Figure 2. As the arrows show, air flows in from the sides near the base of the flame and escapes as hot gas upward from the tip. The shape of the flame is simply a picture of this airflow.
So what happens somewhere with no weight at all? Inside a space station, both the hot gas and the cool air are floating. There's no "heavy thing" below to push the light gas upward.
So no upward flow forms. The flame stays wrapped around the wick, taking on a round shape. Fresh oxygen only seeps in slowly from all around. Because the burning is gentler, the flame is reported to become small, blue, and dim. The flame "pointing up" is a shape that only exists because Earth has weight.
Point a match downward, and since the flame still heads upward, it wraps around the unburned part of the stick from below. The stick heats up sooner, and the fire moves toward your hand faster. Point it straight up instead, and the flame moves away from the stick, making it more likely to go out. Holding a match tilted slightly downward aims for a middle ground between the two.
As hot gas escapes upward, it rubs against the surrounding air and creates small swirls. These swirls keep peeling away one after another, so the tip of the flame flickers in fine detail. A small flame is known to flicker at a rhythm of around 10 times per second. This is why a flame never stays completely still, even in a windless room.
Summary
A flame is glowing, superheated gas. Hot gas expands and becomes much lighter than the surrounding air. Because the heavier air dives in underneath and pushes the lighter gas up, the flame stretches upward. The flow of air — in from below, out from the top — is what gives the flame its teardrop shape. In weightless space, this flow doesn't form, and the flame becomes round.
A flame doesn't point up because fire wants to go up.
It's because the surrounding air is heavier, and dives in underneath.
For what actually burns in a candle flame, see "Is it the wick of a candle that burns?." For why your face gets hot standing beside a campfire — the heat of light — see "Why does standing by a campfire only heat your face?." And for how weightlessness works on a space station, see "Do things float on the space station because there's no gravity?."
- Light an incense stick and stand it up in a windless room. Watch the smoke rise straight up, then start to flicker after a while. The smoke makes visible the path hot gas takes as it escapes upward.
- Light a candle, and compare holding your palm 20cm to the side of the flame versus 40cm directly above it. The spot above should feel warmer, even though it's farther away — because hot gas is being carried upward. Don't bring your face or hair close to the flame.
- Slowly tilt a plate with a candle standing on it. Check that even as the candle leans, the tip of the flame stays pointing upward.
When using fire, keep flammable objects away and have a cup of water ready before you start. Always make sure the flame is out when you're done.
Want to know more? ― Terms, formulas, and textbook connectionsWe've labeled which level each part belongs to, from middle-school science to university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school physics or chemistry
- HS+Advanced high-school material, or textbook sidebar content
- UnivNot covered in high school — university-level specialist subjects (fluid dynamics, combustion engineering)
- ResearchNot yet settled even at university level — something researchers are actively investigating
MSTerminology: this phenomenon has names
- Convection: the flow in which gas or liquid, heated and made lighter, rises, while cooler, heavier material sinks in to replace it. The movement of air around a flame is this convection.
- Buoyancy: the upward force a surrounding gas or liquid exerts on an object. The lighter an object is compared with its surroundings, the more strongly it's pushed up.
- Combustion: the change in which a substance combines with oxygen to release heat and light. In a candle, it's vaporized wax that's burning.
MSHSCheck with a formula: how fast does the flame's gas rise?
We'll find the upward acceleration on the hot gas, then estimate its speed after traveling the length of the flame. This uses the same formula as the dial in Figure 1.
| As symbols | a = g × (1 − T0 ÷ T), v = √( 2 × a × L ) |
| In words | Upward acceleration = gravitational acceleration × (1 − surrounding temperature ÷ gas temperature). Rising speed = the square root of 2 × upward acceleration × flame length |
| Where it comes from | A gas's weight (density) is inversely proportional to its absolute temperature (Charles's law). The difference between the buoyant force from the surrounding air and the gas's own weight becomes the upward force. The speed reached by accelerating under that force takes the same form as the speed of a falling object |
| Symbol | Meaning and unit |
|---|---|
| a | Upward acceleration on the hot gas (m/s²) |
| g | Gravitational acceleration (9.8 m/s²) |
| T0 | Absolute temperature of the surrounding air (K, °C plus 273) |
| T | Absolute temperature of the gas inside the flame (K) |
| L | Length of the flame (m) |
| v | Rising speed of the gas at the flame's tip (m/s) |
| Absolute temperature of surrounding air (20°C) | 293 K |
| Hottest point of a candle flame (about 1400°C) | said to be 1673 K |
| Gravitational acceleration | 9.8 m/s² |
| Length of a candle flame | about 3 cm (0.03 m) |
| Weight of 1 litre of 20°C air | about 1.2 g |
| How many times lighter than air is the hot gas | 293 ÷ 1673 ≒ 0.175 |
| Weight of 1 litre of hot gas | 1.2 × 0.175 = 0.21 g |
| Buoyant fraction | 1 − 0.175 = 0.825 |
| Upward acceleration | 9.8 × 0.825 ≒ 8.09 m/s² |
| 2 × acceleration | 2 × 8.09 = 16.18 |
| Multiplied by flame length | 16.18 × 0.03 ≒ 0.485 |
| Square root (rising speed) | the square root of 0.485 is about 0.70 m/s |
| Converted to km/h | 0.70 × 3.6 = 2.52 km/h |
This gives an estimate of roughly 70cm per second at the flame's tip — about walking pace — as gas escapes upward. In reality, drag from the surrounding air likely slows this down a bit. Even so, it's a considerable pace relative to the size of the flame. This flow is what stretches the flame upward.
HSHS+The ideal gas law, and gas that "falls" upward
HSAt constant pressure, a gas's volume is proportional to its absolute temperature (Charles's law). From the ideal gas law, this can be restated as: density is inversely proportional to absolute temperature. Carbon dioxide and water vapor produced by combustion differ slightly in weight from air, but this article's estimate ignores that.
HS+The size of the buoyant force equals the weight of the air displaced (Archimedes' principle). The difference between this and the hot gas's own weight becomes the upward force, and dividing by the gas's weight gives the acceleration. The formula above expresses this as the density difference with the surrounding air, divided by the air's density — the same idea as the "Boussinesq approximation" commonly used when that difference is small.
UnivNatural convection and diffusion flames
A flame like a candle's, where fuel and oxygen mix only at the point of burning, is called a "diffusion flame." The shape of a diffusion flame is determined by how fast fuel and oxygen are transported. On the ground, natural convection driven by buoyancy carries oxygen in, so the flame stretches upward. How strong buoyancy is relative to viscosity is expressed by a dimensionless quantity called the "Grashof number," used to characterize flame length and flicker behavior. In a weightless environment, convection disappears, and oxygen is transported only by molecular diffusion. As a result, the flame becomes spherical, its temperature drops, soot becomes harder to form, and it turns blue and dim.
📖 For the derivation of the formulas and further reading: Grashof number (Wikipedia, Japanese) ／ Buoyancy (Wikipedia, Japanese)
ResearchWhat isn't fully understood yet
- How flames extinguish in space. Experiments on the International Space Station have reported "cool flames" — fuel droplets that keep burning at low temperature even after the visible flame goes out. How this works is still under investigation.
- The rhythm of flame flicker. Researchers are studying how the roughly 10-times-per-second flicker rhythm changes with flame size and gravity strength. When multiple flames are placed side by side, their flickering has been reported to sometimes synchronize and sometimes alternate.
- How fire spreads in weightlessness. How things burn and spread inside a spacecraft can't be fully verified with ground-based experiments alone. Experiments burning large pieces of fabric inside uncrewed cargo spacecraft have also been carried out.
In other words, the contents of this article are also "an explanation based on what's currently known." The reason a flame points upward is well established, but questions remain about the finer details of how flames behave.
Textbook connections (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science: combustion, how air warms (convection) | What a flame is, hot gas rising |
| HS | Physics: gas laws, buoyancy | Figure 1, checking with a formula |
| HS+ | Physics: Archimedes' principle, Boussinesq approximation | How a density difference gives acceleration |
| Univ | Fluid dynamics / combustion engineering: natural convection, diffusion flames, Grashof number | Figure 2, how flame shape relates to gravity |
| Research | Microgravity combustion | Flame experiments on the space station |
| ― | Everyday connections | How to hold a match, not placing hands or objects directly above a candle |
- Faraday, M. "The Chemical History of a Candle" (Japanese translation by Takeuchi Keito, Iwanami Bunko / 岩波文庫)
- Law, C. K. "Combustion Physics", Cambridge University Press, 2006.
- Ross, H. D. (ed.) "Microgravity Combustion: Fire in Free Fall", Academic Press, 2001.
- Charles's law (Wikipedia, Japanese)
- Grashof number (Wikipedia, Japanese)
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding of the underlying mechanism. A flame's temperature and length vary with the type of candle and where it's placed. Always observe fire under adult supervision.