PDA

View Full Version : Why is winter air dry?



bergenski
9 Feb 2006, 09:36 PM
Just wondering...my whole skin is itchy because of the damn dry winter air...gotta plug in our humidifier and use moisturizer...

Nemesis
9 Feb 2006, 09:38 PM
It just has to do with the fact that cold air can't hold as much moisture.

bergenski
9 Feb 2006, 09:42 PM
It just has to do with the fact that cold air can't hold as much moisture.

Thanks...any more info?

In...TP
9 Feb 2006, 09:50 PM
Ease off on the hot shower/bath.

Nemesis
9 Feb 2006, 09:53 PM
Thanks...any more info?
Water dissolves into air, and depending upon tempurature, air can only hold so much before it rains. In the winter, this amount is substantially low, which, coincidentally leads to those cloudless days of bright sunlight and alot of wind that I despise.

cryokinetic
9 Feb 2006, 09:54 PM
It just has to do with the fact that cold air can't hold as much moisture.
That's part of it... but the major reason is that, being cold outside, there isn't much water evaporating... and the water that's already in the air has a tendancy to rain/snow/etc back down because of the decreased temperature.

Nemesis
9 Feb 2006, 09:58 PM
That's part of it... but the major reason is that, being cold outside, there isn't much water evaporating... and the water that's already in the air has a tendancy to rain/snow/etc back down because of the decreased temperature.
It's not evaporation, it's dissolving. In the summer It isn't hot enough to cause the water to evaporate in to an aqueous gas, it just more readily dissolves because the hot air aids in the process for some reason.

bergenski
9 Feb 2006, 09:58 PM
That's part of it... but the major reason is that, being cold outside, there isn't much water evaporating... and the water that's already in the air has a tendancy to rain/snow/etc back down because of the decreased temperature.

Why does it snow back down because of the decreased temperature?

Nemesis
9 Feb 2006, 10:05 PM
Why does it snow back down because of the decreased temperature?
It snows because the air becomes saturated with water, it just crystalizes in the low temperature

Crazy
9 Feb 2006, 10:21 PM
One more thing, most heating units (cars, houses, office buildings) will dry out the air in the process of heating it. This is especially true of electric heaters.

SensEye
9 Feb 2006, 10:24 PM
As well, compared to summer (where the air is roughly the same temperature outside as inside, air conditioning not withstanding) the relative humidity both in your house and outside is about the same. However, in winter, when you bring that cold dry air inside and heat it up, the relative humidity in your house ends up much lower inside than outside. So it really seems dry inside even though the absolute humidity is the same as outside.

Master O
9 Feb 2006, 11:27 PM
because water in the ground doesn't vaporize and rise into the air. unless there is a warm water source. but those vapors cool off and condense very quickly and become too heavy to float.

Serotonin
10 Feb 2006, 12:15 AM
I hate the cracked, dry skin on the knuckles in winter. Sorbolene with glycerol on them every night does the trick, but damn it stings when it's soaking in!

PiccoloNamek
10 Feb 2006, 12:16 AM
cloudless days of bright sunlight and alot of wind that I despise.

That's my favorite type of day! The best day there is!

Geoff
10 Feb 2006, 12:28 AM
The dewpoint is reached more easily at a cold time of year - the point at which the water condenses into vapour, then to fall out of the sky. Without a ready source of moisture brought in (eg in the UK where it is carried on the jetstream across the atlantic as cyclones).... the air dries out quickly without it having to be hot (it is easy for the air to condense and dump water).

This will typically happen on the leeside of a mountain range - because air cools as it rises less quickly than it warms when it descends (so air travels over a ridge, warms on the other side, condenses and rains... a little further along it will therefore be dry). This is why it is cold and dry to the east of the rockies during the winter....

(my Geography is a little rusty, this goes back 15 years to school, but hopefully I havent misremembered too much ;) )

-Geoff

euterpenc
12 Feb 2006, 06:57 AM
Because all the water is going into *snow* banks.

Nemesis
12 Feb 2006, 09:13 AM
As well, compared to summer (where the air is roughly the same temperature outside as inside, air conditioning not withstanding) the relative humidity both in your house and outside is about the same. However, in winter, when you bring that cold dry air inside and heat it up, the relative humidity in your house ends up much lower inside than outside. So it really seems dry inside even though the absolute humidity is the same as outside.
True dat yo. It fucks up my violin to the point where I have to get it repared about the same time every fall when we first turn the heater on.

That's my favorite type of day! The best day there is!
Blegh.

Apostasius
12 Feb 2006, 05:19 PM
Why is winter air dry? The answer is more complicated than it might seem, and a good grasp of chemistry and physics will help. I don't claim to be an expert, but this is what I understand:

The formula of water is H2O. Each molecule of water has a high affinity with other molecules of water and will hyrdogen bond with four other water molecules. These bonds are transferred from molecule to molecule as they "bump" around. As water is heated, the electrons are excited and become more susceptible to entering the gas phase as the heat of vaporization is, in turn, lowered. Entropy increases.

Water has a density of nearly 1 at 0 degrees C but becomes less dense with an increase in temperature. Combined with atmospheric pressure and heat of vaporization values one can begin to see that evaporation (or some might prefer the term, melting point) would occur more rapidly at higher temperatures assuming there was not high humidity. Similarly, when the temperature cools, water becomes more dense and is more likely to condense as hydrogen bonding becomes stronger.

The winter cold, therefore, leads to a molecular slowdown and decreased entropy of water. It is not so much that the air cannot "hold" the water (depending on the pressure) but that the water resists entering the gas phase based on its chemical properties. Warmer air, in contrast, has a higher temperature, and given sources of surface ground water, the hydrogen bonds of water will weaken and water will vaporize.

Anyway, this is oversimplified and probably not very clear, but hopefully, one gets the idea.

Oh... and the density of water increases as temperature approaches 0 degrees C but then decreases as it begins to freeze, which is why ice floats.

Ka.avik
12 Feb 2006, 05:55 PM
Water has a density of nearly 1 at 0 degrees C but becomes less dense with an increase in temperature.
[...]
Oh... and the density of water increases as temperature approaches 0 degrees C but then decreases as it begins to freeze, which is why ice floats.

A good try but there are a few holes here I'll help patch up -- water has a specific density of 1 at 4c (or 3? correct me if I'm wrong ...). Specific density is a dimensionless number as you're comparing a thing's density to water when h20 is at it's most dense...

Due to that affinity you mentioned, ice can only form when the points are properly aligned in a crystaline shape, but before that they were so cozy bumping into each other, that in order to freeze, water has to expand first. Ice floats, but just barely -- it doesn't want to rise above the surface very much because it's only a very little bit less dense...but that expansion is why your pipes crack when they freeze ;)

I don't believe ice expands after it freezes though, or if so, not through a full degree centigrade. That means in turn that the freezing point of water is pressure-dependant. If you can squeeze the ice hard enough, you'll force the h20 into a non-crystaline arrangement, which is a liquid regardless of tempurature. But you have to push really hard :) this is how ice skates work, actually -- they glide on a lubricating layer of water.

None of this directly answers the question, and I can't be of much further use here, so I'll close now.

Birdsnest
12 Feb 2006, 08:24 PM
It depends where you are. I think Winter in the South feels colder because the air is so moist. I think the amount of humidity in the air makes it feel colder or hotter.

For instance in Lake Tahoe, the snow will feel warm even when its 35 degrees out, probably because its less humid. In the South, 35 feels much colder.

Warm air holds more moisture than cold air though.

Found a temperature converter here: http://www.srh.noaa.gov/bmx/tables/metcal_java.html

Carebear
12 Feb 2006, 11:01 PM
It depends where you are, I think winter in the South feels colder because the air is so moist even in the winter here. I think the amount of humidity in the air makes it feel colder or hotter, temperature aside.

For instance in Lake Tahoe, the snow will feel warm even when its 35 degrees out, I am guessing because its less humid. In the south, 35 feels way colder than in Lake Tahoe, at least it seems like it to me.

Humidity plays a part indeed. Swimming in water at 4 degrees C will make you much colder than running in air with the same temp. Same goes for humid air vs dry air.

Wind will also increase the thermal capacity of air, since wind takes away the air you just warmed up around the body and brings new colder air to suck your energy. Which is why we wear clothes, and why a layer of wool with a windproof jacket on top will keep you warm (since you warm up the air in the small air-pockets in the wool and keep it there).


Similarly, when the temperature cools, water becomes more dense[...]
Finally, after all these years I got the answer to a question I've subconsiously been pondering since... err.. who knows? I think it was in secondary school that I once asked a teacher if water could have different densities, and he answered that "no, water has one density, ice another and steam a third, water can't have different densities". I never quite believed that, but have for some reason never actually confronted that piece of misinformation.

And now all of a sudden I realize why heat rises. It's not really heat that rises, but the gas/liquid etc which recieves the extra energy, making it less dense, which causes it to rise to a layer of matter with equal density, right? Which explains why deep water is cold while surface water is hotter, I guess. And why air temperature drops with something like 1 degree C per 100 metres altitude. And why I never chose physics, since I've been cursed with bad physics teachers from 2nd grade and up. Thanks for finally opening my eyes, Apostasius. :shock:

(Or thanks for eventually opening my eyes if I got it all wrong the first time around. Feel free to revolutionize my world view or nitpick on the points I got wrong.)

Apostasius
13 Feb 2006, 12:10 AM
A good try but there are a few holes here I'll help patch up -- water has a specific density of 1 at 4c (or 3? correct me if I'm wrong ...). Specific density is a dimensionless number as you're comparing a thing's density to water when h20 is at it's most dense...

Due to that affinity you mentioned, ice can only form when the points are properly aligned in a crystaline shape, but before that they were so cozy bumping into each other, that in order to freeze, water has to expand first. Ice floats, but just barely -- it doesn't want to rise above the surface very much because it's only a very little bit less dense...but that expansion is why your pipes crack when they freeze ;)

I don't believe ice expands after it freezes though, or if so, not through a full degree centigrade. That means in turn that the freezing point of water is pressure-dependant. If you can squeeze the ice hard enough, you'll force the h20 into a non-crystaline arrangement, which is a liquid regardless of tempurature. But you have to push really hard :) this is how ice skates work, actually -- they glide on a lubricating layer of water.

None of this directly answers the question, and I can't be of much further use here, so I'll close now.
Yes, water has a density of 1.000 g/cm^3 at 4C. It has a density of 0.999 g/cm^3 at 0C (liquid) and 0.9150 g/cm^3 at 0C (solid).

http://www.simetric.co.uk/si_water.htm

Density does have units, but specific gravity (aka relative density) does not.

Apostasius
13 Feb 2006, 12:17 AM
http://www.lsbu.ac.uk/water/index.html

Wow. The site above has more information about water than one could reasonably want to know. It doesn't seem as if I am too far off in my explanation, but it was definitely woefully inadequate.