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Q: Freezing point of water based on wind chill. ( Answered,   4 Comments )
Question  
Subject: Freezing point of water based on wind chill.
Category: Science > Chemistry
Asked by: jwk147-ga
List Price: $3.00
Posted: 14 Dec 2005 13:03 PST
Expires: 13 Jan 2006 13:03 PST
Question ID: 605868
Help settle a bet please ...

If you place a bowl of water in 33 degrees Fahrenheit weather and
apply a brisk fan or wind, will the water chill to 32 degrees and
subsequently freeze.

I say no, but he says that the water on the surface will evaporate due
to the moving air and steal heat thus causing the surrounding water to
cool. Even if that were true, I assume conduction would keep the water
at an even 33.
Answer  
Subject: You win the bet...
Answered By: cynthia-ga on 14 Dec 2005 18:12 PST
 
Hi jwk147,

Wind chill does not express how cold the air is, it is a measurement
of how cold the air feels based on loss of body heat. I have found
several web pages that confirm this, and not one that said wind chill
can freeze water.


National Science Digital Library - An Introduction to Wind Chill
http://avc.comm.nsdlib.org/cgi-bin/wiki.pl?An_Introduction_To_Wind_Chill
(bottom of the second paragraph)
..."If the air temperature is 40 degrees F and the Wind Chill Factor
is 10 F degrees, water will not freeze..."


Wind Chill Information
http://media.mgbg.com/wrbl/images/weather/weatherwise/wind_chill.html
..."Does that mean wind chill can freeze things? No, wind chill cannot
freeze things, only the outdoor temperature can do that. No matter how
hard the wind blows, the temperature does not change. But wind does
make things cool off faster. For example, some people believe water
will freeze when the wind chill dips below freezing. But that isn?t
the case. Water will freeze only when the actual temperature dips
below freezing. But wind will help water freeze faster by removing
heat from the water, so wrap insulation around your exposed
pipes......[this was very interesting:]...There is little danger of
wind chill hurting you in our area because it?s tough to get frostbite
with windchills warmer than -20F. But if wind chills go below -70F,
skin will freeze in seconds..."


Wind chills won't freeze water
http://www.wonderquest.com/wind-chill.htm
..."Q: If the ambient temperature is 35 F with a wind chill factor of
25 F, will water [NOAA] Rochester, Minnesota blizzard in March
1966.freeze? I say no, my friend says yes. How can we test this during
this time of the year?

A: You?re right. A wind-chill factor of 25 F (- 4 C) will not freeze
water if the air temperature is 35 F (2 C).

How to test this during the summer? Tough. Your best bet is to rig up
a refrigerator with a 15 mph fan. It takes a 15 mph (24 kph) wind to
produce a wind chill factor of 25 F if the air temperature is 35 F.
..."

NOTE: The page above has a link to this experiment:

Wind-chill experiment
http://www.wonderquest.com/wind-chill-experiment.htm


Engineere Edge: Wind Chill Equation and Calculator
http://www.engineersedge.com/calculators/wind_chill.htm
..."Wind Chill temperature only will not cause water to freeze. Water
will only freeze when the waters temperature reaches the freezing
point..."


From USA Today:

Answers archive: Can wind chill alone freeze water
http://www.usatoday.com/weather/resources/askjack/2002-12-01-answers-wind-chill_x.htm
..."Q: If the temperature is 38 degrees and the wind chill is 27
degrees, will water on roads freeze?

A: No. Wind chill attempts to account for the effects of wind carrying
heat away from your body, or the body of an animal. No matter how hard
the wind blows, it doesn't change the air's temperature. You find more
on this by going to a USATODAY.com file about Wind chill applies only
to people, animals..."

The most FAQ about wind chill
http://www.shorstmeyer.com/wxfaqs/windchill/windchill.html
..."If the air temperature is 35 deg. F. and the wind chill 20 deg.
F., will water freeze?

No! Wind Chill expresses how quickly you will lose body heat. At 20
deg. F. and 20 mph, the wind chill is -10 deg. F. The index (-10 deg.
F.) is the equivalent temperature telling you that your body loses
heat as fast as if the temperature is -10 deg. F. and the wind light.

How cold a glass of water can get is dictated by the second law of
thermodynamics which says the water cannot freeze until the
temperature hits freezing, that is it can only cool to whatever the
air temperature is. The wind chill will just get it there faster..."

Please Pass the Science by dr. scott berk
http://www.cardhouse.com/berk/science96.htm

This link has technical information that mathmatically proves wind
chill won't freeze water:
http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?An_Introduction_To_Wind_Chill

This is interesting:

Brainstorm Answer: Freezing Water
http://www.weathernotebook.org/transcripts/1999/02/12.html


There's plenty of information available, use my search srting below
and peruse through the results, I got 167,000 results, almost every
link on the first few pages was about people wanting to know if wind
chill can freeze water!


~~Cynthia


Search terms used at google:
water freeze "wind chill"
Comments  
Subject: Re: Freezing point of water based on wind chill.
From: jwk147-ga on 14 Dec 2005 13:21 PST
 
Yes, I've seen this page from NOAA, but water inside a pipe would not
be given the opportunity to evaporate. (nor be affected by the wind).
The point on contention here is what effect evaporation has on water
temperature.
Subject: Re: Freezing point of water based on wind chill.
From: brix24-ga on 14 Dec 2005 20:31 PST
 
You probably win on the lack of freezing, but proving it is another matter.

You can get water at 33 degrees to freeze if you remove the water
vapor above it rapidly enough. However, the instance I know of is the
rapid removal of the water vapor with a good vacuum (see quote below).

The case at hand has water vapor being removed by wind at 33 degrees
and (I assume) very low relative humidity. This differs from the
vacuum pump in that the wind will tend to warm the surface of the
water back to 33 degrees whenever it cools by evaporation. Which
effect - cooling by water vapor removal or heating by the wind above -
predominates is something I don't know offhand. As you mention, you
also have the heating of any cooled water by the wind blowing on the
bowl. (Your friend, of course, has wisely chosen a highly insulated
bowl. With an ordinary bowl, you should win easily.)

Score one for your friend: the water will tend to cool by evaporation.
Score one or more for you: the wind will add heat back to the surface
of the water and also through the sides of the bowl.

Your friend's possible analogy re the cooling effect of evaporation:
If the temperature is 1 degree above body temperature, if you are
sweating, and if the wind is  blowing, do you cool off a little, or do
you heat up to 1 degree above body temperature? (You counter that such
effects are greater at higher temperatures because the equilibrium
vapor pressure of water is higher at these temperatures.)

Your friend then presents this quote regarding cooling by removal of water vapor:

"You really don't need any organic solvent if you have a high enough speed
vacuum pump that is well trapped.  I used to freeze water by evaporation
every day as an undergraduate summer student while doing research on the
gamma radiation of aqueous sodium thiosulfate solutions.  Water will freeze
quite quickly, but you've go to be able to pump off the water vapor at a
pretty substantial rate.  Unfortunately the small vacuum pumps used in most
high schools aren't going to do this."

http://antoine.frostburg.edu/college/chemistry/senese/chemed-l/199811/bin31aaqsxa

One of you then brings out all sorts of fancy equations on heat of
fusion, heat of vaporization, and various thermal transfers rates and
you two argue and calculate all through the night.
Subject: Re: Freezing point of water based on wind chill.
From: brix24-ga on 17 Dec 2005 05:12 PST
 
I'm going to have to revise my estimation of whether or not the water
will freeze and say I really don't know unless detailed calculations
are done.

However, I can get closer to the answer.

One, I'd like to revise the statement about cooling on a hot day to:
Suppose the outside temperature is 1 degree above body temperature,
that you work hard enough to raise your body temperature 1 degree, and
that you start sweating. You should cool off in a wind that is also 1
degree above normal body temperature, that is, the heat used in
evaporation exceeds the heat input from the warm air blowing against
your body.

Two, I did look up a little data on the equilibrium vapor pressure of
water at body temperature (37 degrees C) and at freezing (0 degrees
C). The vapor pressure of water vapor at body temperature is 47 mm HG
and 4.6 mm Hg at freezing.

http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/watvap.html

There is ten times less water vapor to remove at any one instant at 33
degrees than at body temperature. So, a completely dry wind at 33
degrees would start to cool the water by removing water vapor above
it, but the cooling effect is less than at body temperature. The
unsolved problem is how fast heat is added back to the cooling water
from the 33 degree wind by transfer through the bowl and at the
surface. This involves some calculation and depends on the specific
geometry of the bowl chosen.

Regarding actual freezing of the water (vs just cooling to 32
degrees): Much more water vapor will have to be removed to cause
freezing than just to cool the water. About 80 times as much heat must
be removed to freeze water at 32 degrees as compared to the amount of
heat that must be removed to cool water from 33 degrees to 32 degrees.

As noted before, it is possible to freeze water just by continuously
removing the water vapor above it with a good vacuum pump. Removal by
wind is more complicated because it also adds some heat that a vacuum
pump does not.
Subject: Re: Freezing point of water based on wind chill.
From: miniyazz-ga on 22 Dec 2005 08:32 PST
 
When water is cooled down by wind chill, what is happening is the wind
is temporarily supplying water molecules nearest the surface of the
water with energy. This energy makes it possible for the water
molecules to escape as a gas; thus you get water vapour. Water has a
high heat of vaporisation so, relatively speaking, a lot of energy is
required for water to evaporate, and it gets some of this energy from
the surrounding water molecules, hence some evaporation occurs even
when there is no wind. The evaporating water molecules therefore take
heat energy with them, from the liquid water, when they evaporate,
thus cooling down the water. It's therefore unlikely that by wind
speed alone the water could ever freeze, regardless of wind
temperature (unless it happens to be below or very close to zero,
which would cause the water to freeze but not through a wind chill
factor). As the temperature of the water gets closer and closer to
zero more and more energy will need to be absorbed by the evaporating
water molecules, and this energy will be harder and harder to get as
the surroudning water molecules have less and less energy. Therefore,
as long as the surrounding atmospheric temperature is more than a few
degrees above zero, it is very unlikely that the water will freeze.

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