5
$\begingroup$

When I was in Iceland, I went to a tour in a glacier and I remember the ice that was there. It looked and felt like a huge amorphous block of glass: hard, smooth, not wet, and not that cold.

Not as cold as the cold I feel when I touch the ice cubes from my freezer.

Why is that?

I'm not expert of biology, but if feel cold when I touch an object that means

  • $T_{hand} > T_{obj}$, where the temperature is measured at the surfaces which are to be put in contact
  • the ratio of thermal conductivities $\frac{k_{obj}}{k_{hand}}$ must be sufficiently high to guarantee that
    • the object can sustain an inward heatflow without rising the temperature at the surface too much (otherwise it would reach equilibrium with my hand, which means I don't feel cold anymore, right?)
    • my hand can't keep up with that, as in the temperature at the tip of my finger is at the temperature of the ice, so my hand keeps losing heat through a high gradient of temperature between the bulk of my finger and the surface

Now, based on my experience, I felt less cold when I touched the ice in the glacier than when I touch the "wet" ice from my freezer.

I guess this different perception has to do with different conductivities and/or capacities of ice and water. But how?

Most importantly, is there a dimensionless number which can express this?

$\endgroup$
2
  • 2
    $\begingroup$ Without making any statement on the physics involved, I would imagine that when you're reaching into your freezer your hand is relatively warm (and wet) compared to when you're out in the Icelandic wilderness touching a glacier, so it seems like there would be a difference in perception even if there wasn't a significant physical difference. $\endgroup$
    – J. Murray
    Commented Jan 8, 2021 at 20:29
  • $\begingroup$ That surely has an impact, but I can tell you that there was no way to get my hand wet by holding my hand on the ice of the glacier. As regards how long I could have kept my hand on it before surrendering to the cold, I can't remember exactly, unfortunately. $\endgroup$
    – Enlico
    Commented Jan 8, 2021 at 20:39

4 Answers 4

5
+250
$\begingroup$

TL;DR
enter image description here


I tend to agree with the answer by @GyroGearloose that the difference in the temperatures of ice is the crucial factor here. In the glacier situation the the ice surface and the surface of one's hand are both at the same temperature, so nearly nothing happens during a brief touch. If holding a piece of ice for a while, it begins to melt, as the hand is being constantly warmed up and loses its heat to the air and to the piece of ice that it holds.

The ice coming out from freezer is at the temperature lower than the freezing point (-18C and possibly lower.) Hence the characteristic sticky feeling when holding it - the moisture on the hand freezes when in contact with ice, and the ice begins to melt only after being out of freezer for come time.

A similar effect can be observed, if touching the glacier ice with a warmer part of one's body, e.g., with one's tongue (I suggest doing it as a though experiment, rather than actually trying it - in freezing weather one risks not being able to separate one's tongue afterwards with possible long-lasting physical damage.)

Update
In response to the comments it is worth stressing that the temperature of human body is not homogeneous. It is true that warm-blooded animals, like humans, need to sustain temperature in a certain range, to remain alive. However, the optimal temperatures depend on the tissue, and lower for the parts of the body exposed to exterior. Furthermore, the outer layers of skin are essentially dead cells, which are warmed only via contact with the still alive cells inside, but cooled by their contact with exterior.

To illustrate these points:
Distribution of temperatures in human body image source enter image description here

Optimal temperature for different parts
The fact that temperature varies around the body is taken into account in medical settings, where different temperature ranges are considered as "normal" for different parts, and certain measures are considered more reliable (i.e., subject to less fluctuations) - like rectal and oral temperature, vs. less reliable armpit:
enter image description here

(image source)

Skin in contact with cold air
Outer skin temperature approaches that of the surrounding environment:
enter image description here

(image source)

Heat capacity and size
Whether touching ice may cause residual water on fingers to freeze or whether the ice melts from the contact with fingers depends on the temperatures (of the ice and the fingers) and the amount of ice. Let $c_{ice}, c_{water}$ be the specific heat capacities of ice and water, $m_{ice}, m_{water}$ are the amounts of ice and water on the fingers, $T_{ice}, T_{water}$ their initial temperatures, and $\lambda$ the latent heat of ice.

We expect that the ice warms up from the contact with fingers, while the water on fingers cools down (in principle, we have to take into account cooling down of the skin, but the outer skin layers have 70% water content and heat capacity close to that of water - e.g., see here). If the ice didn't melt, i.e., the equilibrium temperature is below the freezing point, it is determined by: $$m_{ice}c_{ice}(T-T_{ice})=m_{water}c_{water}(T_{water}-T) + m_{water}\lambda$$ that is $$ T = \frac{m_{ice}c_{ice}T_{ice} + m_{water}c_{water}T_{water}+m_{water}\lambda} {m_{ice}c_{ice} + m_{water}c_{water}} $$ If the temperatures are measured in respect to the freezing point (it is the default for the Centigrade scale, otherwise simply replace all temperatures by $T-T_{freezing}$), then the first term is negative, and the result is negative, provided that $m_{ice}T_{ice}$ has big enough magnitude - i.e., when we deal with a large quantity of ice or when it is very cold (or both.)

If instead the ice melted, the condition is: $$m_{ice}c_{ice}(T-T_{ice})+ m_{ice}\lambda=m_{water}c_{water}(T_{water}-T)$$ that is $$ T = \frac{m_{ice}c_{ice}T_{ice} + m_{water}c_{water}T_{water}-m_{ice}\lambda} {m_{ice}c_{ice} + m_{water}c_{water}} $$ Assuring that this temperature is above freezing point requires higher temperature of fingers.

While estimating the exact amount of water on fingers, its temperature and how fast the heat propagates may be tricky, we can base our reasoning on the common experience that:

  • ice cube does not melt immediately when taken out of the freezer: the amount of heat required to melt it is substantially larger than the amount of heat in the fingers. It eventually melts because the heat is resupplied by the rest of the body, which tries to prevent temperature loss
  • ice cube taken out of a freezer is sticky, while the ice cube that has been exposed to the ambient temperature above the freezing point is not sticky - its surface has temperature that is not very different from that of our fingers, and the contact does not result in immediate freezing or melting of water.
  • likewise, at ambient temperatures below the freezing point, no sticking is observed - this time because the outer surface of fingers is cold.

Sensation of cold
The sensation of cold is not determined by a gradient of temperatures between the temperature inside and outside the, but rather by the firing rate of thermoreceptors, a particular type of sensory neurons. There are different kinds of thermoreceptors, active at different temperatures, and with temperature-dependent sensitivity:
enter image description here

(image source) Holding an ice cube coming out of freezer (-18C or lower) triggers cold-pain receptors, which are normally inactive at comfortable room temperature (about 20C). This also generates higher contrast with change in firing of cold receptors. Finally, this generates contrast with the other receptors in the body, which are far from the area in contact with ice (there are no thermoreceptors in the outer layers of skin, they are buried inside (see, e.g., this article.)

When touching glacier at lower ambient temperatures, the contrast between the firing rates of various receptors and the receptors immediately affected is not that great. Furthermore, if the ambient temperature is above -15C, the cold-pain receptors are not involved at all - whereas at lower temperatures, their signal is no different from the signals sent by the similar receptors in the rest of the body.

Related
Why does ice become sticky the colder it gets?
Why does the tongue stick to a metal pole in the winter?

$\endgroup$
8
  • $\begingroup$ the ice surface and the surface of one's hand are both at the same temperature, so nearly nothing happens during a brief touch that's a totally incorrect statement. Ice is by definition at <= 0°C (at ambient pressure), and I am a human, so there's no part of my body below 0°C. At all. So when I put my hand in contact with a slightly-below-zero ice, it's surface will quickly rise up to 0°C and then start to melt. $\endgroup$
    – Enlico
    Commented Jul 9 at 16:32
  • $\begingroup$ @Enlico you are mistaken on at least two counts: 1) that the temperature of human body is uniform - it is not, and this is even taken into account in medical setting - taking temperature on forehead, armpit and in mouth produces different results. 2) when two objects touch each other, the outcome depends on their heat capacity, size, relative temperature, etc. $\endgroup$
    – Roger V.
    Commented Jul 9 at 16:57
  • 2
    $\begingroup$ Yes, my body has not a constant temperature, but still, no part inside of my body has a temperature close to 0°C, and if a piece of my skin goes to 0°C because it's in contact with, say, ice, I will perceive as "cold" the gradient between the outermost layer of my skin and whatever the layer is were receptors are. $\endgroup$
    – Enlico
    Commented Jul 9 at 17:12
  • $\begingroup$ Capacity and size are both extensive quantities, so they have absolutely nothing to do with what happens at the contact surface of two bodies. $\endgroup$
    – Enlico
    Commented Jul 9 at 17:12
  • $\begingroup$ @Enlico full heat capacity is proportional to size, which is why it is called extensive. The greater is the heat capacity, the more heat is required to raise the temperature by 1 degree. The sensations are quite different, if you hold a cocktail ice cube vs. leaning on a huge block of ice. One cannot melt an iceberg by the body heat, and burns from very cold objects are a real thing. $\endgroup$
    – Roger V.
    Commented Jul 9 at 17:35
3
$\begingroup$

Your modern freezer keeps a temperature of -18°C or below, intended to freeze any biological matter well below the temperature where biology as we know it is possible.

A glacier only needs a temperature below the freezing point of water (depending on the pressure). There is no mechanism for the glacier to deep freeze 20 degrees below the average outside temperature, which even in Iceland, are above the freezing point.

$\endgroup$
3
  • $\begingroup$ Does this explain at all the experience? If the ice of the glacier is just below 0°C, then touching it with a body temperature of 36°C would make the layers in contact with the skin melt almost instantly, which will mean that my skin is in contact with water just above 0°C, which would absorbe my heat twice as fast as freezer ice, which does not melt immediately, if it's so much below 0°C. Or not? $\endgroup$
    – Enlico
    Commented Jul 7 at 20:11
  • $\begingroup$ @Enlico A glacier forms where the temperature is lower than the average temperature, so it might well be several degrees below zero. Surest thing would be to by an infrared thermometer and visit that glacier again. Or, visit comparable glaciers. I would doubt that the inside temperature would be below -10°C. $\endgroup$ Commented Jul 7 at 20:16
  • $\begingroup$ No, I'm not saying it is below that. I'm just saying I don't really get how that supports my observation that the freezer's ice feels much colder. Maybe I'm underestimating the importance of the difference in temperature (higher between me and the freezer than between me and the glacier) and overestimating the importance of the difference in specific heat. On the other hand, I still have the memory of the ice of the glacier feeling like glass.. as in, it didn't melt when I touched it... $\endgroup$
    – Enlico
    Commented Jul 7 at 20:21
1
$\begingroup$

Well, yes your hand is actually colder in Iceland than in your room. So $T_\text{hand}>T_\text{obj}$. But that does not justify why the sensation differs by that much, considering humans homeothermal. So, the thing of greater importance is that the ice in your freezer is wet, that is, it is melting. Latent heat of fusion of ice is 336 KJ/Kg/K. On the other hand, the ice in Iceland is not melting. So its specific heat capacity is 2.108 kJ/kg/K. So, it cannot nearly be as cold as that.

There is also another reason. In Iceland, temperature is cool enough to sustain frozen ice but it is not so in your room. When ice melts in your room, it continues to do so until it attains room temperature thus melt and take huge amount of heat. While in Iceland, the atmospheric temperature allows glaciers to be frozen, thus even on giving it a little heat, it again exchanges with the atmospheric and the entire glacier. Also, there is no dimensionless number, but you have specific heat capacity. I hope that helps.

Edit-

When you touch the glacier, heat transfer occurs from your hand to ice, resulting in temperature drop of ice but not enough to melt it because, it again dissipates the heat energy absorbed into the whole glacier which again dissipates it to the ocean.

So there is a chain of heat transfer like

Hand> Glacial Ice touched> Entire Glacier> The ocean.

On considering the entire system (Ocean + Glacier), the heat transfer is not so susceptible became it reaches a dynamic equilibrium.

In case of ice in freezer, such a chain reaction doesn't take place rather it absorbs heat from the comparatively warm atmosphere, here the chain becomes

Hand> Ice

As there is nothing to absorb heat from ice and a temperature difference is there, it doesn't reach equilibrium, rather continues losing heat in a perceptible manner. This change is much better understood. Also, if the ice begins to melt, i.e.becomes wet, it absorbs huge amount of heat energy, resulting in a very cool sensation.

$\endgroup$
11
  • $\begingroup$ Specific heat is measured in J/kg/K, I believe, no? $\endgroup$
    – Enlico
    Commented Jul 7 at 5:36
  • $\begingroup$ Yes, but therefore it is not dimension less. Hope that helps! $\endgroup$ Commented Jul 7 at 11:13
  • $\begingroup$ I was not commenting on dimensionlessness. I was just saying you've got something to fix in the answer, i.e. the units. $\endgroup$
    – Enlico
    Commented Jul 7 at 11:41
  • $\begingroup$ @Enlico I see. Actually in physics we omit that often as 1 unit in Kelvin scale equals 1unit in Celsius scale (The two most commonly used units). But that is still a mistake to omit that. So, I shall correct it. Thanks for the advice! $\endgroup$ Commented Jul 7 at 12:44
  • $\begingroup$ @Enlico Please inform me whether my answer is complete or not. I would be thanked if you upvote me! $\endgroup$ Commented Jul 7 at 17:23
0
$\begingroup$

The ambient temperature in your house combined with your hand can elevate the ice temperature, making it wet. The combined temperature of your hand and the ambient in Iceland, can not.

You already answered yourself: $T_\text{hand} > T_\text{obj}$. In your house, your hand is way warmer than in Iceland; so the difference in temperature that your skin/nerves perceive is greater in your house.

For capacities: specific heat of water: 1 cal/gºC · specific heat of ice: 0.5 cal/gºC. That means: yes, water transfer more heat.

All of that is combined in the equation: $Q = mc\Delta T$;
$Q$ for heat transfered, $m$ for mass, $c$ for specific heat and $\Delta T$ for difference of temperature between the two surfaces.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.