Brrr! It’s cold outside! There is a thick blanket of snow on your front yard. You put on your warmest sweaters and head outside to catch your school bus. You see your friends and you’ll start talking. You might notice that you can see a cloud of fog form every time you breathe out. It’s like you are making clouds. Why does it happen?
You’ve been doing it since you were a little kid. On a cold winter day, you take a deep breath, let it go and watch big, foggy billows come out of your mouth and float away. In fact, if you and your friends breathe out together at the same time, you’ll might even create a great big cloud of air in front of you. Why can you see your breath when it’s cold outside? Why doesn’t the same thing happen on a warm sunny day?
Believe it or not, there is nothing magical about this. It is only science at play.
Why do we breathe?
You very well know, that in order to survive, we need to breathe and in order to breathe we need oxygen. The air surrounding us has about 68% Nitrogen, 21% Oxygen and around 1% of Argon. Our lungs help us to exchange these gases. They are always moist and warm. But why are our lungs moist?
Well on an average, 70 % of our body is made up of water. Just like oxygen, water is mandatory for our survival. Most of our body processes like digestion, excretion and inhalation need water. Thus most of the organs in our body contain water and they are moist.
Why Can We See Our Breath In The Cold?
Now when we breathe, we inhale oxygen rich air and exhale carbon dioxide rich air. But carbon dioxide is not the only component of the air that we breathe out. We exhale a slight bit of carbon dioxide and some water vapour too. So where does this water vapour come from?
Since our moist lungs are responsible for performing most of our breathing exercises, they exhale out a certain amount of water vapour every time we breathe. You can check this by holding your palms in front of your mouth and breathing out. Do this a few times and then rub your palms together. You will see that there is some sort of moist lubrication. This means that every time you exhale, you give out water vapour.
In cold months, the moist air that you breathe out is at a higher temperature than the cold air in your surroundings.
When this warm and moist exhaled air comes in contact with the cool air, the moisture in the exhaled air condenses to form tiny water droplets. This process is called condensation. The moist air from your breath condenses along with the chilled air outside to form tiny water and ice droplets which collectively look like a cloud. This cloud of breath is not visible during the summers because the air outside your body is as warm as the air that you exhale.
Thus, we can see these tiny water droplets as a cloud or fog in our breath.
This is what happens when we speak in very cold weather. Well it’s because the moisture in your breath is experiencing a rapid change in dew point. Dew point is the temperature at which water vapor starts to condense out of the air and form a liquid just like the dew on your front lawn.
When it is warm, like in the summer or inside your body, the dew points are high because heat makes water molecules move around fast and turn into gas. So warmer air can hold more water that’s why it can get so muggy in the summer. But cold air slows these molecules way down and they begin to condense. As a result, lower temperatures often mean less moisture in the air.
Since the cold air is not capable of holding as much water and moisture as the warm air, it condenses the air that you breathe out and forms a nice foggy cloud.
Now our lungs, no matter what time of the year, are warm and muggy. When you exhale, the air leaving your body is actually completely saturated with moisture. That means the relative humidity of your breath is a hundred percent. And when the water vapor in this warm, sultry, saturated air strikes the cold air, those water molecules slow down and quickly condense into liquid. It’s the same mechanism that causes fog.
So when you see your breath on a cold day, it is formed from the same phenomenon as clouds and fog and dew. Although we cannot state the exact temperature at which your breath will start to turn into fog, it is estimated to be roughly around 45 degrees Fahrenheit.