100% humidty when it's below zero?

How is this possible?

Right now, in the fine state of Minnesota, we have 100% humidity with a temp below 32 degrees.


Humidity is a measure of water vapor in the air. Even when it is below freezing, water vapor can still be in the air.

Water has the capability of transitioning from solid to gas phase directly, without going through liquid phase. This process, analogous to evaporation is called sublimation. What this means is that even ice has a vapor pressure, and you can get ice ‘evaporating’ (sublimating) or ‘condensing’ (don’t know if there is a word for the opposite of sublimation.) just like you can have water doing these things.

If the partial pressure of water vapor in the air is less than the equilibrium pressure, ice will sublimate. You may have noticed ice cubes shrinking in your fridge - this is because the environment is very dry to prevent frost build-up. These cubes are sublimating because of low humidity, losing mass and size.

If the partial pressure of water vapor is equal to the equilibrium pressure of ice at ambient temperature, it is 100% humidity. This means that while water vapor is sublimating from the ice, water vapor is also re-freezing on the ice, for a net of no change. Just like at 100% humidity a glass of water will be exchanging water vapor with the air, but will not have net evaporation.

If an object is colder in a below-freezing ambient environment, at high humidities you will get frost on that object. This is the analog of condensation/dew, except that you do not get liquid water but instead get ice crystals. So, even when it is below freezing you will have a ‘dewpoint’ - I don’t know if this has another name, I might fancifully say ‘frostpoint’.

For a great explanation of what humidity is, see
100% humidity? - a recent thread here.

The opposite of sublimation is deposition.

Thanks, Bob Seeger!

Learn something new everyday.

Thanks Doug.

I’ve definitely hear it called the “frost point” before.


Nice technical answer. Maybe this will be easier to understand, though.

You have to remember to humidity is always shown as “Relative Humidity”. The key here is the ‘relative’ part. Relative to what? – to how much water vapor the air can hold at a given temperature before becoming saturated. When the air becomes saturated, the water vapor condenses (not vapor any more). This means that there can be much more water (by mass) in the same amount of air (by volume) at different temperatures even though the relative humidity is reported to be the same percentage.

Air can actually hold more water vapor the cooler it is (subject to limits - like freezing) because there is more ‘air’ for the water vapor to dissolved into.

Your initial assumption is on the right track - when the temperature falls below freezing, the water vapor freezes. The only trick is that it never ‘all’ freezes - there’s some left behind doing other stuff.

Are you sure about that? I know with liquids you can dissolve more into a warm liquid because there is more space between the liquid molecules. I would think it’d be the same with air and water vapor.