Water can only really freeze by bonding together with other water molecules. A lone water molecule can’t “freeze”. So, there can be some water in the air, at a level where bonding with other water molecules is rare.
Liquid water cannot exist at sub zero temperatures (except for some extreme cases), but water vapour can. Phase transitions are not that simple and temperature is not the only factor – the other factor is pressure.
Temperature doesn’t tell you the full story when it comes to states of matter. We’re used to thinking about them as a fixed thing, like water always boiling at 100ºC and freezing at 0ºC, but that’s only true at a very specific atmospheric pressure (roughly the average pressure at sea level).
A more accurate way to predict the state of matter of water in any given circumstance is a phase diagram, which includes temperature and pressure.
Now, the key to your question is to look at the gas part of the diagram. Water vapour can exist in a stable manner all the way down to about -60ºC if you drop the pressure low enough. Since the pressure we care about is specifically the pressure of water vapour in the air and not the air itself, if we have completely dry air at 0% humidity, meaning the water vapour pressure is 0, any bit of ice we have will start to sublimate and create vapour. That raises the vapour pressure up until it finds an equilibrium somewhere on that solid-gas line.
Temperature is just the average energy of the molecules. Eventually some of them can gain enough energy to escape their bonds to the neighbouring molecules to the air. This happens more often at higher temperatures but the possibility is never zero above 0 Kelvin.
When the air temperature reaches ~-20F, relative humidity reaches zero (or pretty close to it). The colder the air, the more likely that your skin will get “chapped” when exposed to it – have moisture sucked out of it by the air. Pretty dangerous. People in areas where it can get that cold know to protect their exposed skin and limit that exposure to prevent damage.
You should read up on the ‘adiobatic’ effect. It can describe how humidity is changed by both altitude/air pressure and temperature better than me. But at it’s most simple, it means humidity needs water molecules to be tossed around by sufficient air particles (pressure) at a fast enough speed (temperature) to keep them suspended in a non-liquid state. As those numbers change, so can the minimum requirement for the other.
But are you talking zero celcius or 32° fahrenheit? Also, most humidity is reported as relative humidity. You can be told you’re at 90% relative humidity, but it can still be done dry because that number is based on the presumed physical maximum amount of water the air can hold.
I’d bet 10% humidity in a swamp is wetter than 90% atop a mountain.
Comments
Water can only really freeze by bonding together with other water molecules. A lone water molecule can’t “freeze”. So, there can be some water in the air, at a level where bonding with other water molecules is rare.
Liquid water cannot exist at sub zero temperatures (except for some extreme cases), but water vapour can. Phase transitions are not that simple and temperature is not the only factor – the other factor is pressure.
Temperature doesn’t tell you the full story when it comes to states of matter. We’re used to thinking about them as a fixed thing, like water always boiling at 100ºC and freezing at 0ºC, but that’s only true at a very specific atmospheric pressure (roughly the average pressure at sea level).
A more accurate way to predict the state of matter of water in any given circumstance is a phase diagram, which includes temperature and pressure.
Now, the key to your question is to look at the gas part of the diagram. Water vapour can exist in a stable manner all the way down to about -60ºC if you drop the pressure low enough. Since the pressure we care about is specifically the pressure of water vapour in the air and not the air itself, if we have completely dry air at 0% humidity, meaning the water vapour pressure is 0, any bit of ice we have will start to sublimate and create vapour. That raises the vapour pressure up until it finds an equilibrium somewhere on that solid-gas line.
Temperature is just the average energy of the molecules. Eventually some of them can gain enough energy to escape their bonds to the neighbouring molecules to the air. This happens more often at higher temperatures but the possibility is never zero above 0 Kelvin.
Just to chime in, freezing fog is a thing and happens a lot during winter in the PNW.
When the air temperature reaches ~-20F, relative humidity reaches zero (or pretty close to it). The colder the air, the more likely that your skin will get “chapped” when exposed to it – have moisture sucked out of it by the air. Pretty dangerous. People in areas where it can get that cold know to protect their exposed skin and limit that exposure to prevent damage.
You should read up on the ‘adiobatic’ effect. It can describe how humidity is changed by both altitude/air pressure and temperature better than me. But at it’s most simple, it means humidity needs water molecules to be tossed around by sufficient air particles (pressure) at a fast enough speed (temperature) to keep them suspended in a non-liquid state. As those numbers change, so can the minimum requirement for the other.
But are you talking zero celcius or 32° fahrenheit? Also, most humidity is reported as relative humidity. You can be told you’re at 90% relative humidity, but it can still be done dry because that number is based on the presumed physical maximum amount of water the air can hold.
I’d bet 10% humidity in a swamp is wetter than 90% atop a mountain.