Why is the breath visible on cold days?


Sometimes in autumn we step out of the house and find that every time we exhale, a veil of mist forms in front of our face. What is happening there and why do we only experience this in the cold season?

It has to be cold and damp outside: Then small clouds of mist appear in front of our mouths. Why does hoo produce cold air

Behind this is a natural law that also works on a much larger scale and shapes our weather events. It’s about the humidity in the air and the formation of clouds.

When taking a breath, two things happen: on the one hand, the air that is sucked in warms up, on the other hand, it becomes enriched with water in the lungs. This happens because the alveoli have extremely fine membranes to absorb fresh oxygen from the air and to release the waste products of metabolism, water and carbon dioxide, to them. The water molecules in the warmed air of the lungs do not exist there in liquid form, but rather buzz around as a gas.

If the air is then breathed into a frosty environment, it cools down together with the gaseous water it contains. But air has a fundamental physical property: it can only contain a certain amount of water in gaseous form.

How large the amount is depends – and this is crucial – on the temperature. The warmer the air, the more water it can contain: at zero degrees Celsius it is around five grams per cubic meter, at 30 degrees around 30 grams of water vapor.

That means: At some point the limit of gaseous water that the air can absorb will be reached: it is “saturated”. If it cools down further, that part of the water that it can no longer hold is separated: it changes from a gaseous to a liquid state, i.e. it condenses into tiny droplets that float in the air – the same mist-like breath we see before us in the cool season.

From a physical point of view, the process can be explained by the movements of the molecules. Put simply, a substance is gaseous if its particles – in this case the water molecules – have a lot of energy and therefore fly around quickly and over a wide area without influencing each other.

It only becomes liquid when the molecules lose so much energy that they can no longer move freely, but slow down and collide. The colder the ambient temperature, the slower they become and the more of them accumulate together. And they eventually form droplets that reflect the light. Then we see them as fog.

Rain clouds are created according to the same principle

However, whether our breath condenses does not depend solely on temperature. But also about how many water molecules are already in the ambient air. The more humid it is, the greater the chance that its saturation limit will be exceeded and fog droplets will form. Therefore you can see his breath on a cool, damp November morning, but not on a cold, sunny January day when the air is dry.

And only when it freezes a lot – around minus ten degrees – a person almost guaranteed to produce little clouds, because then many of the water vapor molecules in his breath condense so quickly that they immediately form fog.

The principle that makes the air we breathe visible also governs our weather. For example, warm air can accumulate moisture over the ocean in a similar way to what happens in the lungs. If it then rises, it cools down increasingly because the temperatures are lower at altitude.

The water vapor that has risen condenses into tiny droplets: clouds are formed. The water molecules clump together to form larger and larger spheres. After all, they can get so heavy that they fall to the ground – and then it rains.