Why Does Your Nose Run When You Step Outside on a Cold Day?
― Your Nose Is a Humidifier That Warms and Moistens the Air You Breathe
You don't have a cold, yet on a winter morning you step outside and a clear, watery drip runs from your nose. This is actually a sign that your nose is doing its job. Before cold, dry air reaches your lungs, your nose warms it to near body temperature and moistens it. A runny nose in the cold is that moisture overflowing.
On a winter morning you leave the house and start walking to the station. The air is cold, and your breath shows white.
After about five minutes, your nose tingles and a trickle of watery fluid runs out. You have no fever and your throat doesn't hurt.
Then you step into a warm building, and before you know it the dripping has stopped. Where did all that fluid come from?
There are two main reasons
The breath that leaves your body is warm and full of water vapour. When it passes through a nose entrance chilled by cold air, some of the vapour turns back into water. It is the same thing that fogs a window in winter.
The inside of your nose senses cold, dry air and sends a signal along nerves to the small glands that make mucus. More water is made to moisten the air, and whatever isn't used overflows.
Both reasons lead back to the same place. Your nose is an "air-conditioning room" that stops outside air from going straight into your lungs. First, let's look at how much work it does.
With every breath, your nose adds water to the air
The deep parts of your lungs are delicate, and cold, dry air arriving straight from outside is thought to damage them. So over the short distance to the back of your throat, your nose warms the air you breathe in and brings its humidity to almost 100%.
The problem is that cold air can hold hardly any water to begin with. A cubic metre of air can carry only so much water vapour, and the limit drops as the temperature falls. It is about 4.8 grams at 0°C, and about 37.6 grams at 34°C, close to body temperature.
Look at Figure 1. The left bar is the water in the outside air. The right bar is the water in the air when it reaches the back of your throat. The difference (the dotted box above the left bar) is made up entirely by your nose, using its own water.
For 0°C air, even a normal walk means your nose hands about 17 grams of water to the air every hour. That is roughly one tablespoon. The inside of your nose keeps making this water without a break.
Your breath "condenses" at the cold nose tip
As the cold air you breathe in takes heat from the nose, the part near the entrance cools down. Then warm, moist breath comes back from deep inside the body.
Look at Figure 2. The breath flows from right to left. As it passes the chilled entrance (on the left), the amount of water the air can hold drops. The vapour it can no longer hold turns back into droplets on the inside of the nose. It is the same as the inside of a window fogging up on a winter morning.
This condensation is not a loss for the nose. It recovers water and heat from the breath, which can be used to warm and moisten the next breath in. In effect, the nose recycles some of its heat and water. But the colder it is outside, the more water comes back, and what the nose cannot hold drips out.
The other reason: nerves open the nose's "tap"
A runny nose in the cold can be more than condensation alone can explain. That is where a nerve reflex comes in.
Inside the nose are nerve endings that sense cold and dryness. When cold, dry air touches them, the news reaches a relay point just before the brain, and a return signal goes to the glands in the nose. The glands then make a rush of watery fluid.
This reflex is thought to exist to add water for moistening the air. Colder, drier air needs more water, so the design makes sense. But the extra has nowhere to go and drips from the nose tip.
This cold-weather runny nose has even been called "skier's nose" in medical papers, because so many people at ski resorts suffer from it. A nasal spray that blocks the nerve signal telling the nose to make fluid has been reported to reduce the dripping. That is one sign that the nerve reflex really is at work.
If you cover your nose with a scarf or mask, some of the heat and water in your breath stays in the cloth. The next breath in passes through it and arrives a little warmer and more humid. Many people feel their nose runs less, because the nose has less work to do.
Summary
A runny nose in the cold is not a sign of illness. It is the "working water" of a nose that warms and moistens the air. Breath condenses at the cold nose tip, and nerves that sense cold air tell the nose to make more water. Together, these two effects produce the clear drip. However, if you have thick yellow or green mucus that lasts, a fever, or facial pain, it may be a cold or sinus inflammation, so see a doctor.
A winter runny nose is water overflowing from the humidifier that is your nose.
Each hour, your nose uses about a tablespoon of water to fit cold air to your body.
Why your breath looks white is explained in "Why Does Your Breath Look White in Winter?", and how the airflow in your nose alternates between the two sides is explained in "Why Does Only One Side of Your Nose Get Blocked?"
- On a cold day, hold a chilled hand mirror under your nose and breathe out slowly through it. The mirror fogs at once because your breath is full of water vapour.
- After walking outside for five minutes, notice whether your nose runs. The next day, walk the same route with a scarf over your nose and compare.
- After going into a warm room, time how long it takes for the dripping to stop. As the nose entrance warms, condensation decreases.
People differ in how they feel this. Many people are said to get a runny nose in the cold more easily as they age.
For those who want more ― terms, formulas and links to textbooksEach part is labelled by level, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school biology or physics
- High school+Advanced high school material, or a textbook sidebar
- UniversityUniversity specialist subjects (physiology, thermodynamics) not taught in high school
- ResearchNot taught even at university as settled fact; researchers are still studying it
Junior highTerms: this phenomenon has names
- Saturation vapour content: the largest amount of water vapour that 1 cubic metre of air can hold. It gets smaller as the temperature falls.
- Dew point and condensation: the dew point is the temperature at which cooling air starts to turn its vapour back into droplets. Droplets forming on a cold surface is condensation.
- Cold-induced rhinorrhoea: the watery runny nose that comes from breathing cold air. It is separate from colds and allergies.
Junior highHigh schoolChecking with a formula: how much water does the nose add and recover in an hour?
We estimate the water the nose adds to the air, and the water it recovers from the breath by condensation. Take a person walking normally on a morning at 0°C and 50% humidity. Values such as the temperature at the nose entrance are assumed, to help the explanation.
| In symbols | m ＝ V ×（ a_out − a_in ） |
| In words | Water added = volume of air passed × (water per m³ of outgoing air − water per m³ of incoming air) |
| Where it comes from | Conservation of water (vapour coming in + water added by the nose = vapour going out). For air at almost 100% humidity, the water a is simply the saturation vapour content, set by temperature alone. The condensed amount uses the same formula, with the direction reversed. |
| Saturation vapour content at 0°C | about 4.8 g (per cubic metre) |
| Saturation vapour content at 34°C (air at the back of the throat) | about 37.6 g |
| Saturation vapour content at 30°C (assumed temperature of the chilled nose entrance) | about 30.4 g |
| Air per breath (while walking) | about 0.5 litres, about 16 breaths a minute |
| 8 litres converted to cubic metres | 0.008 cubic metres |
Symbols: m is the amount of water (grams), V is the volume of air passed (cubic metres), and a_out and a_in are the water per cubic metre in the outgoing and incoming air (grams).
| Water in outside air (50% humidity) | 4.8 × 0.5 ＝ 2.4 g |
| Water the nose adds per cubic metre | 37.6 − 2.4 ＝ 35.2 g |
| Air breathed in per minute (litres) | 0.5 × 16 ＝ 8 |
| Water the nose adds per minute | 35.2 × 0.008 ≒ 0.28 g |
| Water the nose adds per hour | 0.28 × 60 ≒ 17 g |
| Water recovered by condensation from 1 cubic metre of breath | 37.6 − 30.4 ＝ 7.2 g |
| Water recovered by condensation per minute | 7.2 × 0.008 ≒ 0.058 g |
| Water recovered by condensation per hour | 0.058 × 60 ≒ 3.5 g |
| Share of the added water that is recovered | 3.5 ÷ 17 ≒ 0.21 |
By this calculation, the nose gives the air about a tablespoon (about 17 grams) of water an hour, and recovers about 20% of it by condensation of the breath. If the recovered water cannot stay in the nose, it drips out as a runny nose.
High schoolHigh school+How heat and water "go and return"
High schoolThe mucous membrane inside the nose is rich in blood vessels, and the heat carried by the blood warms the air. Water evaporates into the air from the mucus covering the surface. Evaporation also takes heat away, so the membrane cools further.
High school+The nose entrance, cooled on each breath in and warmed on each breath out, works as a "heat exchanger" that temporarily stores heat and water. Because the in-breath and out-breath use the same passage in turn, heat and water taken from one flow can be handed to the other.
UniversityTemperature dependence of saturation vapour content, and the nasal reflex
The steep rise of saturation vapour content with temperature is explained by the Clausius–Clapeyron equation. The slider in Figure 1 uses an approximation of it, the Tetens equation. The cold-induced rhinorrhoea reflex is thought to start in sensory nerves of the nasal lining and run through the parasympathetic nerves (cholinergic) to drive the glands. A nasal spray of an anticholinergic drug, which blocks this signal, has been reported to reduce the dripping.
📖 Derivations and further reading: Clausius–Clapeyron equation (Wikipedia, Japanese) ／ Saturation vapour content (Wikipedia, Japanese)
ResearchWhat is still not clear
- Is the trigger "cold" or "dryness"? Cold air is also dry air, so studies disagree about which is the main trigger of the reflex.
- The role of cold-sensing molecules. Sensor molecules that detect cold in the skin are known to be present in nasal nerves too, but how much they contribute to the runny-nose reflex is still being studied.
- Why some people run more easily. It is said to be more common in older people and in people with nasal allergies, but the reasons are not fully understood.
In other words, this article is also "an explanation of what is known so far." The amount of condensation can be estimated by physics, but some details of the nerve reflex remain unknown.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Junior high | Science, year 2: "Changes in the weather (saturation vapour content, dew point)" | Figures 1 and 2, checking with a formula |
| High school | Biology: "autonomic nerves"; Physics: "heat and temperature" | The nerve reflex, how the membrane warms the air |
| High school+ | Advanced: heat exchange | How heat and water "go and return" |
| University | Physiology, thermodynamics | Clausius–Clapeyron equation, parasympathetic reflex |
| Research | Rhinology, molecular biology of the senses | The reflex trigger, individual differences |
| ― | Links to daily life | Covering the nose with a scarf or mask, when to see a doctor |
- Silvers WS. The skier's nose: a model of cold-induced rhinorrhea. Annals of Allergy, 1991.
- Cole P. The Respiratory Role of the Upper Airways: A Selective Clinical and Pathophysiological Review. Mosby, 1993.
- National Astronomical Observatory of Japan (国立天文台), ed. Chronological Scientific Tables (理科年表), table of saturation vapour pressure. Maruzen Publishing (丸善出版)
- Wikipedia (Japanese), "Saturation vapour content" (飽和水蒸気量)
※This article is a general-audience science explainer. The figures given are rough guides to help you understand the mechanism. If you notice symptoms of concern, such as the colour or smell of nasal mucus, a fever, or facial pain, please consult a doctor.