Why do you see your breath on a cold winter’s day (sometimes)?

This is another one of those weather topics that likely is not given too much notice in our daily life. When it’s cold out we tend to see our breath upon exhaling, typically anyways. Perhaps you have been more curious about when you don’t see your breath? Or, how on occasion you can make a rather substantial cloud that even gets carried away with the wind. What’s cool about this explainer is that it brings into focus the origins of clouds and fog when considering the microclimate your body provides. Afterall, your "visible breath" is a tiny cloud! So, what is happening in the atmosphere to turn your hot, unsaturated air leaving your lungs into visible condensation after encountering the outside cold air that is also unsaturated?

The “cloud” you see erupting with every breath you take is from the same physical processes that generate “mixing fog”. For saturation to occur in the air to allow for clouds or fog, the air has to either cool to its dewpoint temperature or sufficient water vapor needs to be added to exceed the air’s water vapor capacity limit, which can be defined by its dewpoint temperature. There is a third mechanism leading to saturation (on large or small scales). You can also attempt to mix two air masses of different temperature and moisture characteristics.

Consider that an air mass’s upper limit on its holding capacity for water vapor is temperature dependent. But, there’s an important relationship to note. Water vapor capacity is not linear as temperature increases according to the Clausius-Clapeyron equation! In a nutshell, hotter temperatures have progressively even greater holding capacities. That is what makes it possible for mixing of two unsaturated air parcels to reach a new equilibrium temperature that now suddenly has a water vapor capacity limit lower than the combined water vapor contributions provided by those two air parcels! Any water vapor amount in the mixture beyond the newly established saturation limit must condense into visible cloud material. Sometimes, the combined water vapor is still not enough for saturation to occur. This is why not every breath you take (or when opening your freezer door) results in a tiny cloud!

Photo Credit: Jonny William Malloy