Why is a "haaa" breath warm but a "fwoo" breath cold?
― A pursed-lip breath pulls in more than twice its own volume of room air before it reaches your hand
A warm "haaa" for cold hands, a cool "fwoo" for hot soup. We've been switching between warming and cooling breath since childhood. But the breath coming from inside your body should only have one temperature. What actually changes isn't the breath itself — it's the "journey" it takes before it reaches your hand.
On a cold walk home, you bring your numb hands up to your mouth and breathe "haaa." A gentle warmth spreads across your palms.
Back home, you scoop up a spoonful of steaming soup. This time you purse your lips and blow "fwoo." The breath you send out feels cool and refreshing.
If a small child asked you, "Why does the same mouth make both a warm breath and a cold one?" — how would you answer?
There are only two reasons
A thin, fast breath drags in the surrounding air as it travels. By the time it reaches your hand, there's more room air in it than actual breath, and the temperature has dropped.
A fast wind strips away the thin, warm layer of air wrapped around your skin, and speeds up evaporation too. A slow, moist "haaa" does the opposite — it leaves heat behind.
The moment either breath leaves your mouth, both are said to be at almost the same temperature — about 34°C. The difference arises over just a few centimetres to a dozen or so centimetres between your mouth and your hand.
A thin stream of breath travels while "dragging in" the air around it
When you purse your lips, the same amount of breath passes through a smaller opening, so the flow speeds up. Shrink the opening's diameter to a quarter, and the area shrinks to a sixteenth. Push the same volume through, and the speed works out to nearly sixteen times faster.
Where a fast stream meets the still air of the room, countless small eddies form at the boundary. These eddies grab surrounding air and drag it into the stream. The stream widens as it travels, and its contents are steadily replaced by "room air."
Look at the right side of Figure 1. The breath from pursed lips spreads out like a cone as it travels. As the up-and-down arrows show, 20°C air is drawn in from the sides. By the time it reaches the hand, the temperature works out to roughly 24°C. That's well below skin temperature (about 32°C), so it feels cold.
Meanwhile, the "haaa" on the left side of Figure 1 is breathed out with the mouth wide open and the hand held close. The stream is thick and slow. The core of a thick stream is said to travel several times the width of the mouth opening without mixing with the surrounding air. So it reaches the hand at nearly the full 34°C.
Fast-moving air strips away your skin's "warm layer" and its moisture
Right next to your skin there's always a thin layer of air warmed by your body heat. In still surroundings, this layer wraps your skin like a small blanket. A fast wind strips this layer away again and again, constantly replacing it with fresh air. So even air of the same temperature carries away more heat the faster it moves.
Moisture behaves the opposite way. A "haaa" breath carries plenty of water vapour from inside your body. When it hits cold hands, some of that vapour condenses back into water — the same thing that happens when you breathe "haaa" onto a window or a pair of glasses and they fog up. When water vapour turns back into liquid, it hands over the heat it had been holding onto.
A "fwoo" breath, on the other hand, reaches you only after mixing with the room's dry air. Water keeps evaporating from moist skin or the surface of soup. Evaporation carries heat away, which makes things feel even colder. Blowing on soup cools it faster because you're blowing away the rising steam and continuously delivering dry air to the surface.
You'll often see the explanation that "air leaving a pursed mouth cools as it expands." It's true that expanding air does cool down. But the pressure inside your mouth during ordinary blowing is thought to be only about 1% above atmospheric pressure. The temperature drop from that difference is less than 1°C — nowhere near enough to explain the roughly 10°C difference you actually feel. The real driver is the "dragging in" of air.
Summary
The moment breath leaves your mouth, its temperature is the same whether you say "haaa" or "fwoo." A pursed breath becomes a thin, fast stream that mixes in more than twice its own volume of room air by the time it reaches your hand, and cools down as a result. Fast-moving air also strips away the warm layer and moisture on your skin. Breath from a wide-open mouth arrives still warm and moist, leaving heat behind.
What changes the breath's temperature isn't your body — it's the journey.
The thinner the breath, the more it gets diluted by room air before it arrives.
You can read about how moving air alone makes you feel cool in Why does an electric fan feel cool?, and about the water vapour in your breath in Why does your breath turn white in winter?.
- Purse your lips, hold your palm about 3cm from your mouth, and blow "fwoo." It shouldn't feel as cold as you'd expect.
- Keeping the same blowing style, move your hand gradually away to 20–30cm. The farther away, the cooler and crisper it feels.
- Now open your mouth wide, bring your palm to about 5cm, and breathe "haaa." Then move your hand 30cm away and breathe the same way, comparing how the warmth changes.
If you have a cooking thermometer, hold its tip in the breath at different distances and you can watch the numbers drop as the dragging-in effect takes hold. Be careful not to put the thermometer in your mouth.
Want to know more? ― terms, formulas, and textbook connectionsClearly labelled by level, from middle-school science to university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school physics
- HS+Advanced high-school content, or textbook sidebar material
- Univ.Not covered in high school — university specialist content (fluid dynamics, heat transfer engineering)
- ResearchNot even settled "textbook fact" at university level — something researchers are actively investigating
MSTerms: this phenomenon has names
- Jet: a stream that shoots vigorously out of a small opening and travels in roughly one direction. Breath from pursed lips is one example.
- Entrainment: the process by which a fast stream draws in the surrounding still fluid through eddies at its boundary, growing thicker as it goes.
- Heat of condensation: the heat released when water vapour turns back into water. It's the same amount of heat that was absorbed during evaporation.
MSHSChecking with a formula: what temperature does a pursed breath reach at the hand?
When breath and room air mix, no heat is created or destroyed. If you know the mixing ratio, you can work out the temperature at the hand. Let's calculate for pursed lips with the hand 10cm away.
| In symbols | T − Troom = ( Tbreath − Troom ) × m0 ／ m, where m ／ m0 ≒ 0.32 × x ／ d |
| In words | How much warmer than room temperature the breath is at the hand = how much warmer than room temperature it was on leaving the mouth × the amount of breath exhaled ÷ the amount of flow after entrainment. The flow's volume grows in proportion to how many times the opening's diameter the distance is |
| Where the formula comes from | The first half is conservation of heat (mixing doesn't change the total heat). The 0.32 in the second half is a value measured experimentally for how much surrounding air a round jet draws in (Ricou & Spalding, 1961) |
| Symbol | Meaning and unit |
|---|---|
| T | Temperature of the breath reaching the hand (°C) |
| Tbreath, Troom | Breath temperature on leaving the mouth, room air temperature (°C) |
| m0, m | Amount of breath leaving the mouth, amount of air flowing at the hand's position (mass per second) |
| x, d | Distance from mouth to hand, diameter of the pursed opening (cm) |
| Room air temperature | 20°C |
| Breath temperature on leaving the mouth | said to be about 34°C |
| Skin temperature of the palm | said to be about 32°C |
| Diameter of the pursed opening | about 1cm |
| Distance from mouth to hand | 10cm |
| How much warmer is the breath than the room? | 34 − 20 = 14 (°C) |
| How many opening-diameters is the distance? | 10 ÷ 1 = 10 (times) |
| How much bigger is the flow at the hand than the exhaled breath? | 0.32 × 10 = 3.2 (times) |
| How much warmer than the room at the hand? | 14 ÷ 3.2 ≒ 4.4 (°C) |
| Temperature of the breath at the hand | 20 + 4.4 = 24.4 (°C) |
| How much cooler than skin? | 32 − 24.4 = 7.6 (°C) |
More than two-thirds of the air reaching the hand is room air drawn in along the way. The breath arrives as a wind almost 8°C cooler than skin. For "haaa," with a mouth width of about 4cm and a hand 5cm away, the distance is only about 1.25 times the opening. The core of a jet is said to stay unmixed up to about 5 times the opening's diameter, so the breath reaches the hand at nearly 34°C.
HSHS+Conservation of heat and how much adiabatic expansion actually cools things
HSWhen you mix two objects at different temperatures, "heat lost by the hotter side = heat gained by the cooler side" — that's conservation of heat. The first half of the formula above applies this to mixing air with air. It has the same shape as the classic problem of finding the temperature when you mix water and hot water.
HS+A gas cools when it expands without exchanging heat (adiabatic expansion). The relationship between temperature and pressure is given by Poisson's equation; for air, temperature is proportional to pressure raised to about the 0.29 power. If the pressure inside the mouth is only 1% above atmospheric, the temperature drop from expansion is less than 1°C. The "pursing makes it expand and cool" explanation can't account for the difference you actually feel.
Univ.Self-similarity in turbulent jets, and the entrainment hypothesis
A round jet leaving the mouth becomes a turbulent jet after travelling a few times the opening's diameter. From there on, the distribution of speed and temperature spreads outward while keeping the same shape at every distance — a state called "self-similarity." The spreading angle stays roughly constant, the flow rate grows in proportion to distance, and the centreline speed and temperature difference shrink in inverse proportion to distance. The idea that the speed at which surrounding fluid is drawn in is proportional to the centreline speed is called the "entrainment hypothesis," popularised when Morton, Taylor and Turner applied it to rising chimney smoke in 1956. How fast a wind cools skin is expressed through the heat transfer coefficient for forced convection (the Nusselt number).
📖 For the derivation of the formulas and further reading: Jet (Japanese Wikipedia) ／ Adiabatic process (Japanese Wikipedia)
ResearchWhat's still not fully understood
- Lip shape and how breath mixes. The entrainment ratios measured experimentally are mostly for round nozzles. Exactly how much the mixing changes for a soft, irregular opening like human lips isn't precisely known.
- How far exhaled breath actually travels. How breath from talking, coughing, or breathing spreads indoors has been studied intensively in recent years from an infection-control perspective. The combined effect of warm, moist breath rising and being diluted by entrainment is still the subject of detailed calculation and experiment.
- Individual differences in feeling cold. How strongly skin perceives "cold" depends not just on temperature but on how fast heat is being carried away. Modelling how this differs by body part and by person is still a work in progress.
In other words, this article too reflects "the explanation as currently understood." The 24°C figure is a rough estimate based on an idealised round jet.
Connections to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science: "water vapour and dew point," "how heat travels" | Glasses fogging with "haaa," skin cooling from wind |
| HS | Physics: "conservation of heat" | Calculating the temperature when breath mixes with room air |
| HS+ | Physics: "adiabatic change, Poisson's equation" | Why the "expands and cools" explanation is only a minor effect |
| Univ. | Fluid dynamics: "turbulent jets," heat transfer engineering: "forced convection" | The entrainment ratio of 0.32, how fast wind carries away heat from skin |
| Research | Exhaled-breath dispersion, thermal-comfort modelling | The effect of lip shape, how far breath reaches |
| ― | Everyday connections | For numb hands, open your mouth wide and warm them up close; for hot food, purse your lips and blow from a little farther away — that's the efficient way to do it |
- Ricou, F. P. & Spalding, D. B. (1961) Measurements of entrainment by axisymmetrical turbulent jets. Journal of Fluid Mechanics 11, 21–32.
- Morton, B. R., Taylor, G. I. & Turner, J. S. (1956) Turbulent gravitational convection from maintained and instantaneous sources. Proceedings of the Royal Society of London A 234, 1–23.
- Pope, S. B. (2000) Turbulent Flows. Cambridge University Press (chapter on round jets).
- Wikipedia (Japanese): "Jet" (噴流)
- Wikipedia (Japanese): "Adiabatic process" (断熱過程)
※This article is a general-audience science explainer. The figures given are rough estimates meant to help you understand the underlying mechanism. Breath and skin temperature vary from person to person, and also depend on room temperature and how you blow.