Freezing point of Water

Jaybird180

Jaybird180

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I was explaining to my wife, why knowing the temperature is important in aviaiton. I have no idea why my car has an alert at 37degF. But in the discussion, I explained that the boiling point of water increases when water is in a sealed container due the increase in pressure. However, I have no idea what happens to the freezing point of water as pressure is reduced.

So, take me in a flight in an imaginary parcel of air. it is 33degF at the surface and at 3,000 feet the temperature is 32. There is no visible moisture. As you continue the climb, the temperature remains exactly 32degF and a cloud layer is at 5,000ft with tops at 10,000 feet. As the barometric pressure reduces what happens to the ice accumulating on the airframe?
 
My car alarms at air temps of 39F. The owners manual states that the reason for that alarm is that ice can form on road surface starting at that air temperature.
 
the freezing point of water increases with a decrease in the pressure applied to it. Hence the freezing point of water will be less at high altitude and low pressure. But this effect is small. The freezing point of water rises a mere hundredth of a degree per atmosphere of decrease in pressure.
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http://www.newton.dep.anl.gov/askasci/env99/env017.htm

However, if you get high enough (low enough air pressure), the ice on the plane may sublimate directly to water vapor and just disappear. In addition, radiation heating from the sun once you pop out of the tops may also heat the icy wing enough to melt the ice. Finally, ram heating can heat the wing enough to melt the ice -- you won't see ice form above about 400 KTAS for that reason. Finally, those effects can combine to accelerate the process.
 
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Jaybird180 said:
I have no idea why my car has an alert at 37degF.
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if it's anything like my car it's always reading hotter than it really is, the actual air temp might be 32 but at the location of the thermometer it's warmer.
 
Taken from Wikipedia:

1000px-Phase_diagram_of_water.svg.png
 
You'll note from the phase diagram that the line between liquid and solid phases is essentially constant for pressures from 1 kPa to 10 MPa. So the freezing point for water does not change much in the regions of the atmosphere that aircraft fly in (5 kPa to 100 kPa.)
 
Water is strange stuff. It can remain liquid down to -20°C if undisturbed. I don't know if that applies to just small droplets (cloud or rain droplets) or to any volume of liquid water. And it expands when frozen or nearly so. If it didn't it would sink, and oceans and lakes would gradually fill with ice.

Dan
 
Jim Logajan said:
You'll note from the phase diagram that the line between liquid and solid phases is essentially constant for pressures from 1 kPa to 10 MPa. So the freezing point for water does not change much in the regions of the atmosphere that aircraft fly in (5 kPa to 100 kPa.)
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So what does it do in outer space? Sarcastic, but serious Q.
 
Jaybird180 said:
So what does it do in outer space? Sarcastic, but serious Q.
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Either the pressure is so low that any ice just sublimates directly to the vapor state, or the temperature is so low that it can't leave the solid state. Depends on shadows and proximity to stars.
 
Jaybird180 said:
So what does it do in outer space? Sarcastic, but serious Q.
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Extrapolating from the phase diagram, it seems likely water sublimates between solid and gas phases somewhere around -150 degree C.
 
Surface temperatures can be lower than air temperatures due to radiative heat loss to the cold temperatures of sky. It is not unusual to observe frost on a windshield with a clear view of the sky when the air temperature is a couple of degrees above freezing. At the same time, a neighbor's car parked under a tree will have no frost.
 
You can also get freezing at air temperatures slightly above freezing with evaporative cooling. I've seen water freeze on my car's radio antenna while driving down the interstate due the the evaporation caused by the relative wind. An example of evaporative cooling is the way rubbing alcohol cools your skin.
 
scottd said:
You are probably not seeing the effect of evaporative cooling. The static temperature was likely colder than you believed it to be. Yes, it does occur, but it's effect is minimal especially when the atmosphere is saturated or nearly so. The more likely situation is called adiabatic compression which tends to warm the surface especially the immediate leading edges of surfaces.

Balloon-borne instruments called rawinsondes do suffer from evaporative cooling if the probe enters a supercooled liquid cloud and pops out on top with very dry conditions aloft. This causes the probe to report temperatures a few degrees colder than what is really occurring.
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I remember reading about that a while back. I was wondering if I understand this correctly, also....

Adiabatic compression is the type of effect that occurs when a moist parcel of of air travels down a mountain. As the water vapor condenses, the latent heat of condensation heats up the surrounding area. In this case the water accumulating on the antenna was giving off enough heat to cause the water parcel to reach the freezing point while heating the surrounding air slightly? Like adiabatic compression can cause the leading edge of a wing to heat up slightly - and possibly to a temperature more conducive to ice accretion?

I think I'm off by a bit :redface:
 
The phase diagram of water is somewhat of a simplification. The melting point of water is 0C, and the point it spontaneously freezes is around -40C. In between is a region where water can happily be liquid, but if individual water molecules align they have insufficient energy to escape the lattice and they freeze.

The atmosphere likes to hold liquid water in that range, and as that water hits an airplane with ice some of the liquid water comes into alignment and crystals grow larger and larger. It's a runaway process once an ice crystal to build onto and liquid water below 0C are both available in contact.

The phase diagram suggests that below 0C all of the water would already be ice. I suspect if that were true, the runaway process wouldn't exist and we might not even have the concept of aircraft icing.
 
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