3" of snow in Louisiana

[Dr.3D]

I don't know how you thought that would answer my question. I wasn't asking for an example of a weather report that says it happens, I was asking HOW.

The humidity is relative to the temperature. In his example, the air at that temperature was holding all of the water it could, thus 100% humidity.

 

[GunnyFreedom]

The humidity is relative to the temperature. In his example, the air at that temperature was holding all of the water it could, thus 100% humidity.

He appears to be under the impression that air below 32°F can't hold any water.

 

[Dr.3D]

He appears to be under the impression that air below 32°F can't hold any water.

I used to believe that before I got into HVAC for a living. I then learned how to depress the dew point to squeeze the water out of air and then fluff it back up by heating it to make it dryer than it was when I started.

 

[GunnyFreedom]

I used to believe that before I got into HVAC for a living. I then learned how to depress the dew point to squeeze the water out of air and then fluff it back up by heating it to make it dryer than it was when I started.

I used to believe that before I visited relatives in Milwaukee and asked them how in the heck they can have 60% relative humidity when when temperature was 5°F?

 

[Dr.3D]

I used to believe that before I visited relatives in Milwaukee and asked them how in the heck they can have 60% relative humidity when when temperature was 5°F?

Then you found out the humidity was relative to that 5°F temperature and it all made sense, right?

 

[pcosmar]

He appears to be under the impression that air below 32°F can't hold any water.

Snow and Ice will evaporate at sub zero temps.. without becoming a liquid.

It is interesting to observe..

 

[oyarde]

Snow and Ice will evaporate at sub zero temps.. without becoming a liquid.

It is interesting to observe..

Makes sense to me , it sucks the life right out of me as well . I was outside today trudging through snow about five hours , cutting , stacking wood . I am wore out from dragging myself through the new snow and it was warmer today , to turn back very cold again Sun night .

 

[69360]

This cracks me up. I haven't seen my grass or driveway since thanksgiving. Snowpack melted down a little bit last week, it's about 2 foot now. One day 2 weeks ago the daytime high was -15. I think we saw about -18 one night this winter. Last winter we had 6 foot on the ground and the piles in the driveway were about 12-15 feet high. One storm alone was 30".

I don't even wear a coat until it hits the 20s.

 

[GunnyFreedom]

But how does the humidity stay melted for temps below freezing?

a gas is not a liquid, it is a gas. sublimation occurs even well below freezing.

Snow and Ice will evaporate at sub zero temps.. without becoming a liquid.

It is interesting to observe..

sublimation FTW!

 

[oyarde]

This cracks me up. I haven't seen my grass or driveway since thanksgiving. Snowpack melted down a little bit last week, it's about 2 foot now. One day 2 weeks ago the daytime high was -15. I think we saw about -18 one night this winter. Last winter we had 6 foot on the ground and the piles in the driveway were about 12-15 feet high. One storm alone was 30".

I don't even wear a coat until it hits the 20s.

We are to be back to -19 again Mon night I think.Obviously I have had enough of that .

 

[GunnyFreedom]

Then you found out the humidity was relative to that 5°F temperature and it all made sense, right?

Aaaaactually no; I accepted it as true, but it didn't make sense until I studied thermodynamics and understood 1) water has a melting point, NOT a freezing point (contrary to common parlance) just like water has a boiling point but not a condensation point; and 2) water molecules have to give off energy to drop to a lower state of excitation to go from gas to liquid or liquid to solid, and if they are 'stuck' in a position where they cannot give off this energy they will remain liquid or gaseous no matter how cold they get.

I poked around and found a relatively decent answer for corroboration for Paul:

http://answers.yahoo.com/question/index?qid=20091231182417AAw01dJ

Michel Verheughe answered 4 years ago
This is a very interesting question and I will star it.

As Statman writes, as long as the atmospheric temperature is above the dew point one, moisture will stay in a gaseous form. But there is more to it in the world of thermodynamics.

First, water can be found in a gaseous, liquid or solid form. When it is still gaseous in an environment where it should be liquid, it is then called vapour, and when it is still liquid where it should be solid, it is called under-cooled. But why is that?

Well, first we must differentiate between absolute humidity and relative humidity. Water molecules tend to mix with air molecules all the time; even ice evaporates as gas. This is called sublimation.

Absolute humidity is the volume of water that is contained in a volume of air. The higher the temperature, the greater the volume of water it can contain. For example, at the average temperature at sea level and average pressure, of 15 C, a cubic meter of air can't contain more than 13 grams of water. When that happens, the air is saturated with moisture and the relative humidity is 100 percent. When the temperature goes down to 0 C (freezing point) the air can sustain only 5 gr/m3. The absolute humidity is then 5 gr/m3 but the relative one is still 100 percent because at saturation point, a.k.a. dew point temperature.

What happens when the temperature sinks below dew point? Well, the parcel of air has to get rid of some of its moisture by condensing it into tiny water droplets. But that can only happen by giving away energy in form of heat and the tiny molecules have virtually no mass to dissipate that heat energy. They have to touch anything like a dust, smoke particle, pollen or the bonnet of your car, to condense. Likewise, it must give heat energy to go from liquid to solid and that's why you can get tiny drops of water, in the clouds, that are under-cooled all the way down to -40 C!

Incidentally, when water goes from solid to liquid (melt) or liquid to gas (evaporates) it needs to take energy in form of heat and that's why we sweat; its evaporation cools down our skin.

So, when the temperature falls below freezing point, the water molecules in form of gas or vapour can't turn into liquid or solid before it touches something. And that's why you can observe ice rims on the branches of a tree, early in the morning, when the heat of the ground has radiated back into space, during the night, and the surface temperature has fallen under freezing point: the tiny gas molecules turn directly into ice crystal as they touch something with a mass.

This is for example very dangerous when it happens with my little aircraft. If I fly in what is called freezing fog, ice will build up on the leading edge of my wings in a matter of seconds and I will loose altitude at once. It happened to a friend who went flying during the winter and thought he could go through the freezing fog as he could see the sun shining above it. But about 200 feet over the ground the ice was already so thick that he went down and crashed in the wood ahead of the runway. Luckily there was a lot of snow over the short pine trees and he save his life but he broke both wings of his plane.

 

[FindLiberty]

...decent answer...

Nice.

 

[cajuncocoa]

More sleet and possibly snow in our forecast for Tuesday.

 

[torchbearer]

Ski resort may be a good idea.

 

[Anti Federalist]

Snow is already frozen. My point was that, if the air is so cold, then you would expect any water in the atmosphere to turn to ice. If the only water in the air is ice, is it still considered humidity? I think of humidity as steam or tiny droplets of liquid water in the atmosphere, but it seems like if it turned to ice, then it wouldn't affect people in the same way. Having little droplets of ice hit your skin doesn't have the same effect as little droplets of liquid water.

And that is where you are making a mistake.

Water vapor, which is what relative humidity is a measure of, is a colorless gas, totally invisible to the naked eye.

Steam, clouds, dew, fog, drizzle, sleet, snow or rain is all water, not vapor, that has already precipitated out of supersaturated air.

Which is why relative humidity is such an important weather forecasting tool.

When the R/H reaches 100 percent, some form of water is going to precipitate out.

 

[torchbearer]

And that is where you are making a mistake.

Water vapor, which is what relative humidity is a measure of, is a colorless gas, totally invisible to the naked eye.

Steam, clouds, dew, fog, drizzle, sleet, snow or rain is all water, not vapor, that has already precipitated out of supersaturated air.

Which is why relative humidity is such an important weather forecasting tool.

When the R/H reaches 100 percent, some form of water is going to precipitate out.

100% humidity is a norm for louisiana summer. thunderstorms form as the vapor is constantly condensing and evaperating rapidly

 

[GunnyFreedom]

Fixin to get ugly in Eastern NC over the next 48 hrs. Starting Tuesday Afternoon and going to Wednesday day. The hits are coming in this order:

~28° freezing rain 3-4hr -> sleet and ice 2-3hr -> ~20° snow 6-7hr -> ice 1hr -> temps plummet to 7°

Oh my Wednesday and Thursday 6AM jobs going to be fun...

Really glad I have access to a 4wd XTerra to running jobs...

 

[cajuncocoa]

Just got back from the grocery....it was a zoo!! Reminded me of the frenzy we see when a hurricane is a day or two away. Everything is closing again tomorrow and some things are also closed Wednesday. Ready to hunker down and wait for snow.

 

[PaulConventionWV]

Aaaaactually no; I accepted it as true, but it didn't make sense until I studied thermodynamics and understood 1) water has a melting point, NOT a freezing point (contrary to common parlance) just like water has a boiling point but not a condensation point; and 2) water molecules have to give off energy to drop to a lower state of excitation to go from gas to liquid or liquid to solid, and if they are 'stuck' in a position where they cannot give off this energy they will remain liquid or gaseous no matter how cold they get.

I poked around and found a relatively decent answer for corroboration for Paul:

http://answers.yahoo.com/question/index?qid=20091231182417AAw01dJ

That answers my question. Thanks!

 

[PaulConventionWV]

You are pretty much the only person [ETA: from a cold state] I have ever met who thinks that subfreezing air automatically renders all the water in it into ice. Suspended water vapor in air can go as cold as -40°F without being forced to precipitate out as ice.

Ok, I get that now.

I suppose you think I'm stupid. You're free to think that if you want, but I'm really just a curious soul who's constantly seeking to reaffirm unsettled truths in my mind that I can't explain. I am constantly seeking knowledge, see, so anyone who thinks I'm stupid because I ask questions would be sorely mistaken.

Genius sometimes takes unlikely forms.

I know you didn't exactly ask me to explain that, but hey, it's open discussion, so I'm not afraid of derailing the thread with my revelations about what the distinction between genius and stupidity is. I happen to think I am of the former category, but you can decide for yourself. It doesn't bother me one bit because, at this point, I'm just rambling about nothing of particular importance to anyone.