Why do You See Your Breath on Cold Days?

We all know that we breathe in oxygen and breathe out carbon dioxide.  But CO2 is not all that we breathe out.  The air inside of our lungs and mouth are very moist and some of this moisture travels outside our bodies in the form of water vapor when we breathe out.  Most of the time we don't notice this, but we can clearly see it when it's cold outside.  That's because when the outside air temperature is cold enough, the water vapor we breathe out is forced to change from a gas to a liquid through the process of condensation.  This is the same process that forms clouds in the skies above, so you're essentially making your own personal cloud just by breathing outside on a cold day!

So how cold does it have to be to make your own cloud?  Well it's not an easy answer because there are a couple of factors involved.  Not only is it dependent on the temperature outside, but also on the humidity.  When you breathe out, your warm, moist breath mixes with the colder, drier air outside.  The combination of your breath and the outside air must reach 100% relative humidity in order for you to see your breath.  100% relative humidity can be reached by lowering the temperature or raising the humidity.  You raise the humidity by breathing out, but if the outside air is warm or really dry (low relative humidity or dew point) it is less likely that you'll see your breath.  If the air outside is more humid, you can see your breath at higher temperatures than you would if the air was dry.  This is why you only sometimes see your breath when temperatures are in the 50s or upper 40s.  If the relative humidity is already above about 50% at these temperatures, the moisture in your breath should be sufficient to bring the mixed relative humidity to 100%.  If the outside relative humidity is under 50% though, even the addition of your moist breath may not be enough to reach 100% relative humidity.  You can even see your breath at 60 degrees, but this is uncommon, as the outside air would have to be at 80% relative humidity or higher to start with which is pretty muggy.

If the air temperature is cold enough though, you are guaranteed to see your breath.  This is right about 45 degrees.  Below this temperature, no matter the outside humidity, your breath will likely supply enough moisture to saturate the air around it and cause the formation of a cloud. 

Now you have a lot to think about the next time you're pretending to be a dragon!

Have a weather related question you'd like me to answer?  Comment below or hit me up on Twitter and I'll be happy to answer your question in a future blog post!