This week’s weather question is: How cold does it have to be to see your breath?

There is no exact temperature, but a good bet is 45°F or below. (This number can change based off other atmospheric conditions like relative humidity).

We breathe in oxygen and exhale carbon dioxide, but there are also tiny water molecules in our breath.

On warm days, we don’t see anything because the water molecules can move around freely and remain water vapor, but when temperatures are cool enough – water molecules pack closer together to conserve energy.

This causes condensation or a little cloud to form when you breathe out.

For clouds or fog to form in general you need three ingredients:

  1. Warm, moist air
  2. Cooling
  3. Cloud Condensation Nuclei (basically, something for the water vapor to condense on and stick to).

A great way to show how this works is by making a cloud in a jar.

If we pour boiling water into a jar – it forms a warm, moist environment.

Putting ice on top of the lid creates cooling.

And spraying a little hair spray in there will act as cloud condensation nuclei.

This is the exact process that happens when you breathe out on a cold day.

Just instead of hairspray, it is the water vapor in your breath that condenses once it hits the cold air.