Monday, April 30, 2007

Phases of Water

My last weather theory blog posting faded uncommented into the blogosphere so I probably went too far. These postings were precipitated by someone who e-mailed me more than once lamenting the lack of weather discussion on the blog, so I'm trying to oblige. Today will be easier to understand than last time. Today will be so easy to understand you'll wonder why I bothered, but it will all tie together in the end.

Water comes in three phases: solid, liquid and gas. Solid water can be in the form of snow or ice or frost or high altitude clouds. Liquid water is present in lakes and puddles and rain and clouds and mist and squirrels. A small percentage of the atmosphere all over Earth consists of gaseous water. And water in any phase can convert to any other.

When I arrived in the north, most of the water I saw was in the solid form. I remember being in a northern town in April and watching little kids gleefully jumping on frozen puddles to shatter the ice. I remember being on final for a runway and reflexively double checking the water under my approach path, to confirm wind direction, then laughing at myself because the water was frozen, its apparent ripples indicating perhaps the wind direction at the time of the latest snowfall on top, but not the current winds. As the weeks went on, open water appeared and I watched the transformation from solid to liquid, which any kid who has ever had an ice cream cone knows is called melting. It takes energy to melt ice, energy that can be provided by the sun, or by the alternators and brushes driving the propeller deicing system on my airplane.

Had I stayed through the fall I would surely have seen the reverse transformation, open water disappearing and the ice finally becoming solid and thick enough for the ice roads to go in. Demand for flying drops off then, but so does flyable weather. As water freezes, it gives off some heat, exactly the same amount of heat energy that will be required to melt it again. Energy is conserved in such a transformation.

Similarly. in order for liquid water to sneak out and hide inside the air (an explanation I once gave to a small child who wanted to know where the puddles had gone), it needs to absorb some energy. You're familiar with the cooling effect of evaporation from sweating or if you've ever worn wet clothes: the water takes heat from your body in order to effect its transition. That also explains why sweating or wearing wet clothes is not a very effective cooling mechanism if the air is muggy. Muggy air is saturated, with a very high relative humidity. The air contains the maximum amount of moisture it can at that temperature, so there is little tendency for sweat or moisture on your clothing to evaporate, hence no evaporative cooling.

When that water that has been sneaking around inside the air as water vapour reappears as liquid, the energy will be released again. I can't think of clearly observable examples of the heating phenomenon caused by condensation, but you can observe condensation itself as beads of water appearing on the outside of a glass of cold liquid, or on the inside of windows on a cold day.

There are two more possible transformations between different phases of water, but a lot of people have never acknowledged their existence. Lets start with the freezer compartment of a refrigerator. We'll assume that you are careful and never spill your icecube trays when you're putting them, full of water, into the freezer. Even if you did, you know you'd get a puddle at the bottom of the freezer compartment and some would run out onto the floor and some would freeze there at the bottom, permanently attaching the frozen broccoli to the freezer. So how does there get to be ice stuck to the inside top of the freezer? There's never any liquid water dripping there. The answer is that water vapour present in the freezer compartment deposits directly onto surfaces it finds there, transforming directly into the solid phase. And unless you and your party animal friends use a lot of ice cubes, you've probably noticed that ice cubes left in the freezer gradually shrink. They aren't melting: the water is going directly to vapour, called sublimation. As you might guess, it takes energy to sublimate, and the amount required is equal to the amount required to melt plus the amount required to evaporate. There's no shortcut. It's not like on the airlines where a ticket from Toronto to Halifax costs more than a ticket from Toronto to London, England.

So that's my whole point today. Water can be solid, liquid or vapour. It can transform up or down that sequence, one step at a time or two steps at once. It costs energy, taken from the environment, to go up or down that sequence, and the energy required is the same whether or not you stop off at the intermediate phase. When you go back down the sequence, the same amount of energy is returned. So even though you probably associate the formation of little droplets of water--as mist, on your plumbing, or on a cold drink--with cold things, try to remember that that little droplet of water brought a little teeny bit of heat with it as it appeared.

13 comments:

Lord Hutton said...

I am sure there must be examples of condensing energy transfer in specialised steam engines. But hey, what steam engine ever powered a plane?

nec Timide said...

I can't think of clearly observable examples of the heating phenomenon caused by condensation, but you can observe condensation itself as beads of water appearing on the outside of a glass of cold liquid, or on the inside of windows on a cold day.

Well you gave two good examples there, the glass of liquid gets warm (it would anyway, but not as fast). Better, condensation on windows is a well know indicator that excessive heat is leaving by the window. The condensation is the mechanisms of loss as well.

I like your weather posts, even if I don't always comment.

Ed said...

The air contains the maximum amount of moisture it can at that temperature...

This is commonly said but, while it is sort of right, it's a very misleading way of putting it. It gives the impression that the dry air (consisting of nitrogen, oxygen and all the rest of the non-water-vapour gases) somehow acts like a sponge and when all the little holes are filled it can't hold any more water.

Actually, the dry air has very little to do with the amount of water vapour around (more precisely, the partial pressure of water vapour). What is key is the balance between evaporation of water from any exposed surfaces and condensation back on to those surfaces. The rate of evaporation is controlled by the temperature. When the surface of the water is warm the water molecules move faster then when the surface is cold and are therefore more likely to break through the surface tension and so the partial pressure to keep equilibrium is higher.

Water surfaces include the tops of seas, lakes and puddles, of course, but also the surfaces of water droplets in the air (e.g., in clouds) and even the odd random presence of water molecules sticking, however briefly, to the surfaces of particles of dust in the air.

If the air is really clean so there are no particles for condensation to start on the relative humidity can get really high - 300% I read somewhere.

The distinction between the sponge and equilibrium views of "saturation" (even that word is a bit misleading) are pretty academic until you try to understand phenomenia like the flat bottoms of cumulus clouds. Thinking simply about relative humidity reaching 100% makes it difficult to explain how quickly water vapour turns to droplets but thinking about the effects of the reducing curvature of the surface of water droplets on surface tension as the grow makes it a bit easier to understand.

(I'm not a meterologist or physicist but I am an ex-glider pilot with more than average interest in this sort of stuff).

Ed said...

I can't think of clearly observable examples of the heating phenomenon caused by condensation...

A question commonly asked by starting-out glider pilots is "do thermals cause cumulus clouds or do the clouds cause the thermals". The answer is "yes" or, if your feeling a bit less unkind, "both".

Thermals cause cumulus clouds. As the air rises it cools to the point where water vapour starts condensing into droplets. As the water condenses it releases heat which warms the air around it causing it to rise more quickly hence increasing the effect of the thermal.

In other words, clouds suck because of the heating effect of condensation.

This is most clearly observable in the evening when the day's main heating is over. Thermal activity low down stops but cumulus clouds can continue to build. If a glider can arrange to be near cloud base as this starts it can continue to fly for quite a while but as soon as it gets low, out of contact with these "self-stoking" clouds, it'll be landing pretty soon.

A more spectacular manifestation of the release of heat from condensation is, of course, a thunderstorm. As air flows up through the growing cloud the water vapour condenses releasing heat which increases the updraft. The water falls out as rain allowing more air to be sucked in, etc. Add a bit of friction and things go bang.

Soaring Student said...

Another example is hurricanes... [I'm probably going to mangle this a bit]. A major source of energy in a hurricane is the rapid elevation of warm humid air. The air gives up its warmth, adding to the energy of the hurricane. But all that humidity condenses and then freezes with the altitude, and the release of heat from these two phase changes is one of the major sources of heat (energy) for a hurricane.

Aviatrix said...

I'm not looking for observable instances where heating occurs on condensation. I'm looking for instances when the HEATING is clearly observable. It has to be as simple and unambiguous as the cooling of water evaporating from skin.

I knew someone was going to ding me for vapour pressure vs. temperature. You know how long these things would be if I didn't cheat a little?

Icebound said...

Yes, you do not see many real-world examples of the "heating" due to change-of-state, only because the "condensation" and "freezing" changes of state are initiated by a "lowering" of temperature in the first place.

Consequently, what we DO see is that the rate of lowering is decreased, but because it is usually still "lowering"...yes, we do not see the actual "heating".

However, DEW is a simple example of the heating phenomena. Pick a clear calm day with the dewpoint is like plus 2 or plus 1, and the temperature in the single digits. Watch the hour-by-hour change in temperature as it begins to fall after sunset. As it gets near zero, it stops the decrease, because the heat released in the formation of the dew keeps warming the air sufficiently to keep it there near zero.

Anoynmous said...

I know a perfect example of heating due to condensation, but you need to be in the right field of work in order to see it. It's called "vapor phase reflow soldering". An inert chemical (usually a fluorocarbon) is boiled, and its vapor condenses on a circuit board to provide a uniform and constrained temperature increase. The heat melts solder and electrically connects the components.

Ed said...

I'm not looking for observable instances where heating occurs on condensation. I'm looking for instances when the HEATING is clearly observable.

Fair does, though the cloud bubbling up is as much a direct consequence of the heating from condensation as the feeling of cooling is a direct consequence of the cooling caused by evaporation.

I knew someone was going to ding me for vapour pressure vs. temperature. You know how long these things would be if I didn't cheat a little?

Yes, there's only so much detail that's needed. On the other hand, it could be worded less misleadingly without great expansion. Maybe something like: The air contains so much moisture that the heat released by its condensation cancels out most of the cooling effect of evaporation of sweat or moisture in your clothing. Which is actually quite a neat example of the effect of heating from condensation, now I come to think of it.

Note that even in "saturated" air evaporation from a wet surface happens pretty much as fast as it would in dry air, it's just that condensation is happening as quickly to counteract it.

Aviatrix said...

Note that even in "saturated" air evaporation from a wet surface happens pretty much as fast as it would in dry air, it's just that condensation is happening as quickly to counteract it.

It would be interesting to test that exchange somehow. Perhaps samples could be saturated with D2O (heavy water) and analyzed at intervals in a high humidity environment You'd have to correct for the presence of D2O vapour and control for any differential in the evaporation rates, but I think an experiment could be designed to show it.

sydost said...

My last weather theory blog posting faded uncommented into the blogosphere so I probably went too far.

No... not too far at all. I think that you didn't go far enough. Your previous weather post was a very good explanation of atmospheric stability. If you happen to know more about that subject please tell us... I'd like to know.

I am still eagerly waiting for the post containing dry and adiabatic lapse rates.

janra said...

For condensation causing observable heating: steam radiators.

Hot water radiators work strictly on the difference in temperature between the water and the room, but a steam radiator is a bit different. You fill it with steam, but you don't take steam out, you take water out. The steam condenses on the walls of the radiator, transferring its heat to the radiator and from that to the room; the water removed from the radiator (if it's running at peak efficiency) should be almost exactly the same temperature as the steam coming in. (If it's cooler, the boiler has to work harder to turn it back into steam, so those things are designed to take steam at 100C and return water close to 100C.)

I don't have my steam tables here (they're at work) but the amount of heat transferred when steam condenses at constant temperature is huge compared to the amount of heat transferred when either vapour or liquid cools without condensing.

Ed said...

Yes, having written my previous comment I wondered how you'd check it and thought of using isotopes.

I don't have my steam tables here...

Wikipedia, as always: Latent heat and Specific heat capacity. Translating from their almost but not quite SI values, the latent heat of fusion (melting/freezing) is 334 kJ/kg, the specific heat capacity of liquid water is 4.1813 kJ/(kg·K) and the latent heat of vaporization (boiling/condensing) is 2272 kJ/kg.

That's a bit of an eye opener for me; I knew the latent heats were much larger than the specific heat capacities for °C temperature changes but I hadn't realised how much larger the latent heat of vaporization is than that of fusion. Condensing 1 kg of water vapour releases more heat than cooling 6 kg of liquid water from 100°C down to 20°C.