Atmospheric moisture

[Jan Fiala]

It’s -20 where I am now, and only cold “a cold snap is on the way”.

But point taken. -20C or -4F. Mostly I chose -40 because it’s the same F or C, so it’s easy to discuss.

Kev

Most defogger problems are in the temp around freezing point. Less than that and the air is already dry with 0 humidity ..

 

[Ken7]

Most defogger problems are in the temp around freezing point. Less than that and the air is already dry with 0 humidity ..

What makes you think there is no moisture in sub-freezing temps? How does snow occur?

 

[ucmndd]

What makes you think there is no moisture in sub-freezing temps? How does snow occur?

I imagine theyre making the (true) point that in very cold temperatures, next to none of that moisture is present as water vapor, which is what makes humidity. Cold sub-freezing air is indeed “dry” air.

Now, I do question what difference that really makes for the defogging of a car windshield, especially when you get a couple humans in a very cold car breathing hot moist air right on the window. Instant source of water vapor.

 

[Jan Fiala]

What makes you think there is no moisture in sub-freezing temps? How does snow occur?

Lower secondary school?

 

[Ken7]

I imagine theyre making the (true) point that in very cold temperatures, next to none of that moisture is present as water vapor, which is what makes humidity. Cold sub-freezing air is indeed “dry” air.

Now, I do question what difference that really makes for the defogging of a car windshield, especially when you get a couple humans in a very cold car breathing hot moist air right on the window. Instant source of water vapor.

Although it is true that the colder air holds less moisture, there still is enough to produce considerable snowfall even in sub-freezing temperatures. So don't kid yourself, there can still be considerable moisture in cold air and that moisture is indeed in the form of water vapor. Remember the 3 states of water: water, water vapor & ice.

As that cold air is brought into the car, it's warmed and when pushed into contact with the cold windshield, the water vapor condenses on the windshield...until the windshield is warmed.

 

[ucmndd]

However that cold air is pulled into the car, warmed and the moisture content made available. That now available moisture condenses on the cold windshield...at least until the windshield warms up enough.

Veering waaay off topic now, but that “cold air pulled into the car” is very dry as almost all of the available moisture has crystallized into ice and dropped out of suspension in the form of frost, etc. simply heating it up isn’t going to magically make moisture content “available” that isn’t there to begin with.

 

[Ken7]

Veering waaay off topic now, but that “cold air pulled into the car” is very dry as almost all of the available moisture has crystallized into ice and dropped out of suspension in the form of frost, etc. simply heating it up isn’t going to magically make moisture content “available” that isn’t there to begin with.

The 'crystallized ice' is there because that's what happened to the water vapor upon hitting the freezing windshield. If the windshield had been warm, it might have simply condensed on the windshield as water. In the case you cite, it condensed as ice. The water vapor is in the air that's pulled into the car, it didn't need to be warmed to be made available. Again, cold air can contain quite a bit of moisture. For every 10" or so of snow, that's equivalent to 1" of water. Yes, cold air holds less moisture than warmer air, but it still can hold quite a bit.

 

[ucmndd]

Yes, cold air holds less moisture than warmer air, but it still can hold quite a bit.

Last post on this before we get whacked with the ruler:

Cold Air Is Dry Air

A psychrometric chart might be of use here. The moisture-bearing capability of air below freezing is dramatically less than at higher temps (about 1/10th the capacity at -10c vs 30c). Maybe we are saying the same thing but are assigning different meaning to relative scales like “quite a bit” or “very dry”, but very cold air has a very limited capacity to carry moisture at surface pressure.

 

[Ken7]

Last post on this before we get whacked with the ruler:

Cold Air Is Dry Air

A psychrometric chart might be of use here. The moisture-bearing capability of air below freezing is dramatically less than at higher temps (about 1/10th the capacity at -10c vs 30c). Maybe we are saying the same thing but are assigning different meaning to relative scales like “quite a bit” or “very dry”, but very cold air has a very limited capacity to carry moisture at surface pressure.

We are saying the same thing. My point though was that although cold air certainly does hold less moisture than warmer air, it can still hold enough moisture to dump feet of snow on an area. If it didn’t, then winters wouldn’t be such a pain in the arse.

I wasn’t going to throw this out at you, but I do have 2 degrees in meteorology, so I’m not quite speaking as a novice. I never thought we’d be discussing meteorological issues on TMC.

 

[agtdDelirium]

Although it is true that the colder air holds less moisture, there still is enough to produce considerable snowfall even in sub-freezing temperatures. So don't kid yourself, there can still be considerable moisture in cold air and that moisture is indeed in the form of water vapor. Remember the 3 states of water: water, water vapor & ice.

As that cold air is brought into the car, it's warmed and when pushed into contact with the cold windshield, the water vapor condenses on the windshield...until the windshield is warmed.

Veering waaay off topic now, but that “cold air pulled into the car” is very dry as almost all of the available moisture has crystallized into ice and dropped out of suspension in the form of frost, etc. simply heating it up isn’t going to magically make moisture content “available” that isn’t there to begin with.

The 'crystallized ice' is there because that's what happened to the water vapor upon hitting the freezing windshield. If the windshield had been warm, it might have simply condensed on the windshield as water. In the case you cite, it condensed as ice. The water vapor is in the air that's pulled into the car, it didn't need to be warmed to be made available. Again, cold air can contain quite a bit of moisture. For every 10" or so of snow, that's equivalent to 1" of water. Yes, cold air holds less moisture than warmer air, but it still can hold quite a bit.

Water vapor can be present in the atmosphere down to extremely cold temperatures (50-60 degrees below zero F). The amount of water vapor that the atmosphere can hold is directly dependent upon the temperature of the atmosphere. The humidity (or more accurate term relative humidity) is just the percent of water vapor in the air in proportion to the largest amount of water vapor that the air could hold at that temperature. It can be -30F and 100% humidity, just as it can be 100F and 100% humidity. The warmer air will contain significantly more water vapor, though.

The frost or "fogging up" of your windshield in the winter almost certainly comes from you entering your car, not the cold air coming in with you. You exhale around half a liter of water every day. It does not take much to produce condensation on your cold windshield. Your defroster works by both running air through the air conditioner system to condense as much water as it can out of the air, and then through the heater to increase the temperature of that dry air before blowing it on your windshield. The dry warm air can hold much more water vapor than cooler air and evaporates the water condensation on your windshield, whether it's in the form of liquid condensation or frost which can sublimate (go directly from ice to water vapor).

 

[Ken7]

The dry warm air can hold much more water vapor than cooler air and evaporates the water condensation on your windshield, whether it's in the form of liquid condensation or frost which can sublimate (go directly from ice to water vapor).

And sublimation is, by far, the least efficient form of evaporation. So they tell me this is a Tesla forum?