Why Does Your Breath Turn White in Winter?
― The Moment Invisible Water Vapor Meets Cold Air and Becomes Droplets
Breathe out on a hot summer day and you see nothing. Do the same in midwinter and a white puff appears. Your breath should be the same in both seasons, so why can you only see it in winter?
On a winter morning, you bury your face in a scarf and breathe out. A white, cloud-like puff spreads out in front of you. Many people have laughed with a friend and said, "Look, you can see your breath."
But on a hot summer day, the same breath shows nothing. The breath leaving your lungs shouldn't change much from season to season. So what is it that changes?
You see white breath only when two things come together
Inside your lungs it is about 37°C, close to body temperature, and the humidity is thought to be almost 100%. Your breath is warm, moist air packed with invisible water vapor.
The colder the air, the less water vapor it can hold. When warm breath is suddenly cooled by cold outside air, the surplus vapor appears as tiny water droplets.
Only when both are true do you see "white breath." Let's take them one at a time.
Reason 1: Breath is full of invisible water vapor
Inside your lungs it is about 37°C, close to body temperature, and always moist. The breath that comes out is thought to hold far more water vapor than the air around you. But water vapor is a colorless, transparent gas, so at this point you still can't see anything.
Reason 2: Cold air can't hold much water vapor
Air can hold only so much water vapor as gas. This limit is called the saturation vapor content. The limit is larger when the air is warm and smaller when it is cold.
When warm, moist breath meets cold outside air and cools quickly, the vapor above the limit for that temperature can no longer stay a gas. The overflow appears as tiny, visible beads of water (droplets). This works exactly like condensation, the water that forms on the outside of a cold drink glass.
Condensation happens when air touching a cold glass is cooled quickly, and vapor beyond the saturation vapor content turns into droplets. White breath is just the same thing happening in open air. The "dry ice smoke" used on stage and in films works the same way: it chills the surrounding air quickly and makes a mist of water droplets.
What you can check yourself
- Pour a well-chilled drink from the fridge into a glass and leave it out
- After a minute or two, check that the outside of the glass is covered in droplets
The inside of the glass isn't wet, yet the outside is. The air around the glass is cooled by touching it, and it gives up the water vapor it can no longer hold. What happens in the air with white breath happens here on the glass surface.
Summary
Your breath turns white in winter because (1) your breath holds plenty of water vapor, and (2) cold outside air can't hold much water vapor. You need both. In summer you can't see your breath simply because the outside air can still take in all that vapor.
White breath isn't a special kind of breath.
It's your ordinary breath, meeting cold air and showing itself for the first time.
The same "vapor cools and shows itself" effect happens on a lawn at night. Why the morning grass is wet even without rain is explained in this article.
For those who want more ― terms, formulas and links to textbooksFrom middle school science to active research, each topic is labeled by level
- Middle schoolCovered in middle school science
- High schoolCovered in high school Basic Earth Science
- High school+High school chemistry, or textbook advanced or sidebar content
- UniversityUniversity-level specialist material (atmospheric science) not taught in high school
- ResearchTopics researchers are still investigating, not yet taught as settled fact even at university
Middle schoolTerms: words around white breath
- Water vapor: Water in gas form. Colorless, transparent and invisible.
- Saturation vapor content: The most water vapor that air at a given temperature can hold as gas.
- Condensation: Gaseous water vapor turning into liquid water (droplets).
- Dew formation (condensation on surfaces): When air is cooled and the vapor it can't hold turns into droplets on surfaces.
Middle schoolHigh schoolChecking with a formula: how much water becomes droplets in one breath?
Hearing that "the excess becomes droplets" doesn't tell you how much that is. A calculation gives a clear number.
Water turned to droplets = vapor in breath − vapor the outside air can hold
| Vapor in breath | About 39.6 [g/m³] at 37°C and 100% humidity |
| Vapor outside air (0°C) can hold | About 4.8 [g/m³] at 0°C and 100% humidity |
| Volume of one breath | About 0.0005 [m³] (500 mL) for a resting adult |
These values come from the relationship between air temperature and the most vapor it can hold (saturation vapor content). The colder the air, the smaller this limit.
| Vapor in one breath | 39.6 × 0.0005 = 0.0198 [g] |
| Vapor 0°C air can hold | 4.8 × 0.0005 = 0.0024 [g] |
| Turned to droplets (difference) | 0.0198 − 0.0024 = 0.0174 [g] |
By this calculation, about 0.0174 g (17.4 mg) of water turns from gas to droplets with each breath.
17.4 mg is about the weight of a single grain of rice. Think of it as one grain's worth of water turning into visible mist with every breath, and white breath may feel much closer to home.
High school+UniversityWhy can warmer air hold more water vapor?
Saturation vapor content changes with temperature because the pressure of vapor that can coexist in air (saturated vapor pressure) rises exponentially with temperature. The basic idea is that at higher temperatures water molecules escape more readily, so more can stay as gas. This relationship is also described by the Clausius–Clapeyron equation, covered in the article on air pressure and boiling point.
The same mechanism behind white breath, where warm, moist air meets cold air and forms droplets, also applies to clouds in the sky and the contrails that airplanes leave high above. Only the scale differs; the physics is thought to be almost the same.
ResearchWhat we still don't understand
- Precisely predicting the size and number distribution of droplets in white breath, condition by condition, is still thought to be difficult. Temperature, humidity and the force of the breath all interact.
- How the fine droplets in exhaled breath (droplets and aerosols) spread, and how long they stay in the air, is a field still being studied, partly for infection control.
- Numerical models that predict exactly when and how much glasses or masks will fog up from the conditions are also thought to be still developing.
Even the white breath you see every winter is still a subject of research once you look at how each droplet behaves. Being familiar and being understood are two different things.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: water vapor and condensation / humidity | Basics of water vapor, condensation and dew; the glass observation |
| High school | Basic Earth Science: water vapor in the atmosphere | Saturation vapor content calculation, estimating droplet mass |
| High school+ | Chemistry: liquid–gas equilibrium (advanced) | Saturated vapor pressure and temperature |
| University | Atmospheric science, thermodynamics | Clausius–Clapeyron equation, links to clouds and contrails |
| Research | Aerosol science, infectious disease epidemiology (unsolved) | Predicting droplet size distribution, spread of exhaled droplets, numerical models of fogging |
| ― | Everyday observation | Condensation, link to dry ice smoke |
- Explanatory materials on the relationship between temperature and saturation vapor content, from sources such as the Japan Meteorological Agency (気象庁).
- General descriptions of humidity, saturated vapor pressure and the Clausius–Clapeyron equation in earth science and chemistry textbooks.
- Guideline values for resting tidal volume (about 500 mL for adults) in the respiratory physiology literature.
- Research reviews in aerosol science (general findings on the generation and spread of exhaled droplets).
* Values such as saturation vapor content and tidal volume vary with body size and conditions. This article uses commonly cited guideline values.
* This article is a general-audience science explainer. Values for saturation vapor content and breath volume vary by source and by person, and are given as guides for understanding the mechanism.