When it’s very cold outside, if we exhale with your mouth we see a sort of appear speech bubble which vanishes within a few moments. It hardly happens if we exhale with the noseunless the temperatures are very cold. And of course our breath is never seen when it’s not very cold outside. The “cloud” into which our breath transforms in these cases is composed of condensing water vapor in tiny droplets of liquid water in contact with the cold outside air.
The air we inhale already contains a certain amount of water vapor upstream, which increases significantly in our lungs due to the natural humidity inside them. Since the environment in our lungs is warm, the air we exhale has a high temperature, approximately 31-35°C. And since the hotter the air, the better it is at taking in water vapour, our breath has a very high humidity saturation point. This limit can also almost be reached: the air we exhale can in fact even reach the 90% relative humidity. In short, there is a lot of water in the form of vapor in our breath.
When this warm, humid air leaves our body, it meets the colder, drier air of winter days and at this point the water vapor condenses, i.e. passes from the vapor phase to the liquid phase: that is, it transforms into droplets of water which – as in “normal” clouds – become visible even if for a short time. To be clear, condensation is the same process by which we can fog up a mirror if we breathe on it.
This process is driven not only by the rapid cooling of water vapor exiting our lungs, but also by the fact that the air on our breath cools significantly, making it less capable of holding water in vapor form. For example, an aria a 34°C with the 90% of relative humidity becomes saturated (i.e. not capable of holding any more water vapor) when it drops to approx 32°Ctherefore with a modest temperature change that can occur rapidly – that is, before the water vapor from our breath disperses – when the outside air temperature is sufficiently low.
Of course, the details of this process depend on many other factors, for example the humidity of the outside air. Also exhaling through your nose or mouth makes a difference: the air that comes out of the mouth is in fact more humid because it also collects the humidity of the oral cavity, therefore it is more inclined to form the “cloud” when we breathe on a cold day.
