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.

Thursday, April 19, 2007


As discussed earlier, huge lumps of air roam freely over the surface of the earth. Some lumps are warmer, some are colder. Some are wetter, some are drier, and some are piled higher than others. And they are the way they are because of where they formed. Let an air mass sit over a warm ocean and you'll get a warm, moist air mass. Of course even an air mass that is a tropical thirty-five degrees at the surface is colder aloft, with the temperature decreasing by anywhere from about one to five degrees celsius for every thousand feet you go up. The rate of temperature decrease is called the lapse rate. There can be odd local variations in lapse rate, but by the time you reach the tropopause (the end of the first layer of air) at 30,000-60,000 feet, the temperature is -56C. At any one altitude within the same air mass, the temperature is about the same.

In addition to temperature and moisture, we are interested in the stability of an air mass. Stability is not with regards to lateral motion of the air mass, but rather to vertical motion within the air mass. If an air mass is stable then air displaced vertically tends to return to where it was, while in an unstable air mass, vertical displacement results in continued vertical motion. Kind of like a stable person who goes to Mexico for a vacation goes home and back to work, while an unstable one might get a new job as a llama herder and end up six months later calling you from Tierra del Fuego, asking you to wire money. Well maybe not much like that. But that's the terminology. I'll be using it in a few paragraphs.

Air within air masses is getting displaced all the time. As the air mass moves over uneven ground, some of the air is displaced upwards. An airplane flies by, swirling the air around. Some of the air is heated, becomes less dense and thus starts to rise above the denser air around it. There are lots of reasons for air to move.

As soon as some amount of air, some textbooks call it a "parcel," moves upward, it is in a new location. The air newly surrounding it is different than the air in its old neighbourhood. For starters, the pressure is lower. The only thing that was keeping the parcel of air at a higher pressure was the presence of air at that pressure all around it, so as it rises and the pressure around it drops, it is no longer as contained and it expands until its pressure matches the pressure around it. That expansion results in cooling, as I mentioned last time. Thus the raised air parcel has a lower pressure, a greater volume, and a lower temperature. The surrounding air hasn't changed as a result of the move, but the temperature of the surrounding air is going to be less than the temperature of the air that surrounded the parcel at its old altitude, simply because the atmosphere is colder at a higher altitude.

So which is colder, the parcel of air that has been raised, or the air that now surrounds it? They are both colder than the old temperature of the air parcel: the parcel of air cooled off as a result of expansion when it moved upward, and the surrounding air just happens to be colder than the air that surrounded the original parcel. The answer is, it depends on whether cooling by expansion was greater or less than the lapse rate, the change in temperature with altitude.

The trick is, cooling through expansion is predictable. A parcel of air that is raised one thousand feet will cool by three degrees. Done deal. So you need only look at the lapse rate of the surrounding air to predict whether the raised parcel will be warmer or cooler than the air in its new environment. If the lapse rate is steeper (i.e. greater) than three degrees per thousand feet, then the surrounding air will be cooler than the raised parcel. If the lapse rate is shallower than three degrees per thousand feet then the the raised parcel will be cooler than the surrounding air. (There's an exception to that last sentence, but I will explain it later).

Next question, why have I spent so many words wrangling with whether one bit of air is warmer or colder than another bit? Well what happens when a parcel of warm air is surrounded by colder air? (Hint: see the title of the last weather theory post). The warmer air rises. So if a parcel of air is disturbed in surrounding air that has a steep lapse rate, the parcel will continue to be warmer than the surrounding air and will continue to rise. If the lapse rate of the surrounding air is shallow, the parcel soon cools below the temperature of the surrounding air, and sinks back to its original level.

And now you can see that if the lapse rate of the surrounding air (known as the environmental lapse rate) is less than the rate of cooling with expansion of lifted air (known as the adiabatic lapse rate) then the air is stable. If the environmental lapse rate is greater than the adiabatic lapse rate, then the air is unstable.

And on that terribly technical-sounding but somewhat simplified sentence I will end this blog entry. If you know about the dry and saturated adiabatic lapse rate don't complain that I didn't mention them, I'm getting there, I promise.