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.