Rain at the End of the Rainbow

We're supposed to go to Rainbow Lake today, but the customer doesn't want to go if we can't get back. Rainbow Lake, despite the lovely name, is a less hospitable place to spend ones free time than Moose Jaw. Before I understood the customer's priority I said cheerfully that it wouldn't kill me to spend the night in Rainbow Lake, and was rewarded with a look of uncertainty. Perhaps vampires roam the streets of Rainbow Lake after nightfall. Popular culture would imply that vampires were a little trendier than that, though. The single eating/drinking establishment there is described to me as having tables made out of cable spools, with cigarette-burned tablecloths stapled to them. I accept their priorities and study the weather forecast.

There is a front moving in from the west that will drop ceilings, visibility and eventually snow. It is forecast to reach Fort Nelson just as I'm scheduled to be returning there, but should remain within VFR limits for a couple of hours afterwards. The progress of the front has not been out of line with the forecast, but that sort of thing is difficult to gauge across the BC mountain ranges. I use my piloty skills and experience to say yes, we can go to Rainbow Lake and return before the weather cuts off our return.

Rainbow Lake, despite the fact that its airport identifier resembles a Russian obscenity, has quite a nice airport. The runway is paved, wide, fairly level, and has a large apron. The trip out there is uneventful and the weather stays good at Zama Lake too.

I use the phone in the fuel shed to all flight services for a weather update while I wait for the on call fueller. The front is still moving pretty much as forecast, so the weather I'm copying down as the briefer reads it over the phone is pretty much what I was expecting. The current weather is better than the earlier forecast and the forecast is almost symbol for symbol identical to the earlier one, but then the briefer continues "... and from 22Z to 24Z a 40 percent probability of freezing rain." It's 2145Z now.

"What?" I say, even though I heard him perfectly. I really should have expected it. The approaching warm air mass is a little warmer than expected, so that instead of snow it might produce rain as it overlaps the cold air mass that is present now. But when rain falls through cold air it turns cold, and can be chilled below zero celsius without turning to ice. That is freezing rain, and when you fly into it, it builds up on the airframe, causing severe icing. I do not want to be flying in freezing rain. I ask the briefer a few more questions to get a picture of speed and direction and options.

"Is there anything else I can do for you?" he asks.

"Make it not be freezing rain?" I suggest, but he can't help me there.

I have the fueler put on enough fuel to get me back here if I have to return, but not ful tanks, to save time and weight. We load quickly and I think there's less than a minute on my watch between engine start and take-off. I'm flying west, conscious that I'm peering intently into the sky ahead as if I could see ahead a hundred miles and forty minutes to know what I will encounter.

The ceiling comes down a bit as I approach destination, but there is no precipitation and then I hear a radio call from a pilot doing a practice hold at Fort Nelson, in the same aircraft type as I am flying. No pilot would be out in one of these in freezing rain if he had a choice, and a training flight is clearly a choice, so I can relax a little. The FSS gives me traffic information that allows me to merge efficiently with other incoming aircraft.

The rain is just starting as I am putting the covers on the wings. They are going to be needed tonight. I'm happy that I came through for my customer with an accurate prediction of our ability to do the work, and I'm happy not to be in Rainbow Lake tonight.

I Pronounce Me Here

They've been telling me we are going to Nevada next. I was given a day off in which I didn't do much but swim laps in the (big! outdoor!) hotel pool, and draft flight plans for Reno and Las Vegas. There are other places in Nevada, but I figure if I know how to get to the big ones, I'll be able to figure out the way to Winnemucca. (The problem with that would be pronouncing it when I get there. Did you know I managed to pronounce Birmingham wrong!) And then they told me to fly to Salt Lake City. At least I can pronounce that.

That meant a two leg trip, one flat, one with mountains. The flat leg of the trip was the more difficult one, however, because the flat bits of the US are all about thunderstorms this time of year, and these ones had SIGMETs for hail. Unfortunately I can't find the bit of paper on which I scribbled that SIGMET because it didn't just slap down the symbol GR for hail, it spelled out the diameter in inches of the expected hail, and I don't want to exaggerate when I report the size, because it was a number that was sufficiently foreboding without exaggeration, but I can't remember it. It's a little like if someone told you there was a dangerous, angry poisonous spider in your bed, you might forget exactly how big they said it was. Most thunderstorms do have hail; it just doesn't always reach the ground. And thunderstorms have enough nasty components without hail. But when you consider that hail can be thrown out of the storm ten miles away from where the storm seems to be, and hail like that could damage my aircraft even without gusty winds, downdraughts and severe turbulence, you can see that was a no go. The thunderstorms are not air mass thunderstorms, formed pretty much daily over large flat hot areas with available water, but frontal thunderstorms. They are being caused by a fast-moving cold front that is lifting the hot moist air. That makes for a long, impassable line of storms. I have to go around. Not too far around, however, because there are air mass thunderstorms in the area, already topped to 60,000', about as high as they go at this latitude, to the south, and they are moving north. I'm running the gauntlet between two storm areas.

I pick some VORs as follow-the-dots points for a routing and file my flight plan, IFR because there's a lot of moisture associated with the cold front, even away from the convective areas. I took a few pictures of distant convective cloud, but they aren't very impressive because there's no sense of scale.

The flight goes well. The air traffic controllers are friendly and cooperative, offering me more direct routings and giving me information about the convection and rainfall they can see on their scopes. I cut the corner a little on my planned routing. It was conservative, allowing for the front to slow down, but it moved as forecast and I'm able to curl in behind it, going north in the wake of the storms. There is a lot of moisture left and one controller calls me to tell me he sees about fifty miles of light precipitation ahead on my route. I acknowledge that, I've just entered cloud, and tell him that it is smooth. He then comes right back and says "and then the fifty miles after that is moderate precipitation, let me know if you need any deviations." He must be practicing his comedy routine. I found it really funny that he told me about the light stuff, waited for my response and then told me there was heavier stuff to follow. He was pretty much exactly right, then the clouds thinned rapidly after the hundred miles had passed and the skies were clear for my landing in the middle of the country.

The FBO lends me a courtesy car that is parked on the inside of the security gate. I drive up to the gate, pick up a phone and tell them I want out. The stop sign immediately after the gate says not STOP but STOP HERE AND WAIT UNTIL GATE HAS CLOSED COMPLETELY. It's a smaller point size than your regular stop sign. Gas, washroom, food, weather and go: VFR this time because this leg will put me over my eight hours max IFR, and the weather is clear all the way to Salt Lake City. Yippee-kay-yay! On takeoff I'm cleared on course and to me requested altitude right away.

I put on the autopilot while I check out the scenery. The land is still flat, but not level. It's sloping up towards the continental divide. The GPS tells me that I'm crossing the Canadian River. I wonder what's Canadian about it. There's a North Canadian River, too. I have a notebook in which I write down things I want to blog about later, and I see that last month I crossed the Choctawhatchee River. You can't make up names like that.

I've been flying westbound at 8500' watching the ground get closer as it smoothly slopes up. I've also been kicked by light turbulence, chop as I call it in the PIREP I file after 50 miles of it. I wouldn't ordinarily file a PIREP for turbulence that was only light: I'm doing it because there was an AIRMET for moderate turbulence along my route. My PIREP might help the forecasters, or help someone make a decision to fly.

I could possibly reduce the turbulence by climbing higher, but I don't really want to. Eventually the time comes, however, to suck it up and climb. The upsucking is quite literal. Although US rules allow a pilot to fly without oxygen for unlimited periods up to 12,500, I have to obey the more stringent Canadian ones and suck oxygen through a tube for the entire duration over 10,000', if I'm to be there for more than 30 minutes. I can go high enough to go straight up and over all the mountains, but the power of one engine alone wouldn't give me a good safety margin if the other engine quit. I would keep flying on one, but I would drift down, and there might be no escape, no where to drift to, over these mountains. I will follow the line of a pass rather than skimming over the peaks. This route will also give me a better approach into Salt Lake than trying to dive bomb the city.

At this time let it be said that I hate this oxygen mask. It ought to be cool at fourteen thousand feet above sea level, but the OAT is 12 and with the sun blazing into the cockpit from clear skies it's like a greenhouse in here. Clamping a rubber mask over my nose and mouth does not improve the situation, nor does the transit through plastic hoses and rebreathing bag improve the flavour of the bottled oxygen. It's ironic that up here where the mountain air is probably the freshest in the country, I get to breathe out of a bag.

I also get to talk through a bag, with ATC having trouble understanding my routing because I sound like Darth Vader. I wonder idly if I'm suffering from carbon dioxide poisoning, or just going mad. I should buy one of those ear-clip things to see if I'm sufficiently oxygenated. I wonder if they have to poke you, in order to get to your actual blood. If so, I'd bleed all over it. Or do they just look at the colour of your ear, in which case they wouldn't work on dark-skinned people.

Here's me heading for an invisible pass in the mountains. You can't see it from this angle, but the road goes through it, and so did I. Wow, I didn't notice until I saw these pictures side by side in the preview how much clearer the air is for the mountain picture. The windshield has not been cleaned between the Canadian River picture and this one: it's the same flight.

After the pass the land became flatter, but not much lower. I overflew one ten-thousand foot plateau, wind-packed snow berms still visible along the top. The visibility is decreasing, but this is a good sign when your destination is at a lake. When the drier mountain air gives way to moister, cloudier air, that means you're almost at the lake. I fly into SLC from the south following ATC instructions to overfly the interstate. I've studied the airport diagram ahead of time and based on the ATIS and the runway lengths, I guess that I will be given runway 35 to keep me out of the way of the faster airliners. I can see the airport ahead. The freeway passes to the east of it. The controller tells me to follow the freeway and maintain 6000'. He will turn me for the runway about three miles back.

It is less than three miles diagonally to the threshold when I'm allowed to turn to land. I've already cooled my engines and slowed to the first stage of flaps, so now I chop the power and pull up the nose to get the speed down so I can dump gear and the rest of the flaps and plummet to the runway. High density altitude gives me a high groundspeed, but also a high descent rate, so I make it down to runway level as I reach the big 35. I've arrived.

Stability

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.

Great Lumps of Air

I keep promising weather theory, but I get distracted. It's also hard to start in the middle as I have to assume something. So I'll start at the beginning and weave more weather into the continuing story, continuing to be distracted on and off. My life is alrady a soap opera, so now I'll run multiple story lines. In any one week everyone should be able to find something of interest. And that will distract you from the fact that I haven't confessed what I'm doing yet. Today you have my take on some of the basic components of weather.

The Earth, as those of you who breathe regularly will have noticed, is surrounded by air. All the air contains the same gases: nitrogen, oxygen, argon, water vapour and a number of lesser components like carbon dioxide, helium, and even krypton (no, it's not green). The proportions of the non-water gases are almost completely uniform from place to place, so in dry air, that's 78% nitrogen, 21% oxygen and 1% argon (the other gases are present in a few parts per million, sharing that one percent with the argon). Air temperature varies from place to place, both horizontally and vertically. Plus the air is not distributed perfectly uniformly about the earth. There are bigger piles of it some places than others.

Some of you won't believe me about the bigger piles thing, thinking that making a bigger pile of air would be like making a bigger pile of water, and that differences in pressure thus created would fill in the gaps and and even out the piles. Of course that does happen, and that plus the behaviour of the water vapour makes weather.

I didn't mention water vapour yet, because its variation would have made it awkward to include in the general composition of air, and it's important enough to merit its own paragraph. Water vapour is the gas form of the wet stuff we normally call water. It is a colourless, invisible gas. (The steam you see coming out of the kettle is not actually water vapour, it is liquid water droplets. If you want to 'see' water vapour, crouch down to eye level with the spout of the boiling kettle. Be careful not to burn your nose, and you will be able to observe a space in the first centimetre above the spout in which there is no appearance of steam. That's air with a high concentration of water vapour, rising from the kettle. As the liquid water boils, it turns to hot vapour and rises. As it leaves the spout of your kettle it mixes with the much cooler air of your kitchen and condenses, turning back into liquid water. Because the liquid water is in the form of very small droplets, the warm rising air can support its weight and it continues to rise as steam. Until it condenses on the underside of your cupboards and makes them all soggy so they won't hold plates anymore. But I digress.) So there is water vapour in the air all around you, but you can't see it any more than you can see the nitrogen. The proportion of water vapour may be up to about 4% of the total air, but can be 1% or less. So where you sit right now the actual proportion of gases in the air might be something like 76% nitrogen, 20% oxygen, 3% water vapour and 1% argon and other.

So we have these great piles of air. Each pile, called an air mass, starts at the surface and wherever it ends, somewhere between around 30,000' and 60,000' up, is called the tropopause. There's more air above the tropopause, but that's called the stratosphere and stratospheric weather is a different subject. Air masses are formed by air lying around in one place for a while. Air masses are big, so by "one place" I mean "Antarctica," "subtropical Africa," "the Pacific Ocean" or "the far north of Canada." The air takes on the relative characteristics of the place it hangs out. Well not all of them. We don't get pointy air or high-crime air or fundamentalist Christian air. We just get moist air versus dry air and cold air versus warm air. It's all relative, so an air mass that forms over the Canadian prairies/American midwest in winter is cold compared to the air mass that formed over the southern states, but warm compared to the one that formed over the bleak arctic tundra and frozen seas. Yes, frozen seas. But it's a dry cold.

Of course everything has special names so that you don't think this weather stuff is easy. Moist air masses, like the kind that form over non-frozen seas, lakes, and jungles is called maritime, and dry air, like the kind formed over deserts or frozen landscapes is called continental. If you've ever had a "continental breakfast" at a Holiday Inn you can remember this by the dry, cellophane wrapped pastry. Or you can just remember it, because continents that don't have the Great Lakes and Michigan/Manitoba in the middle of them tend to be drier in the middle and wetter at the (maritime) coasts. That second way would really be a better way to remember it, because it is actually true, but isn't as funny as Holiday Inn breakfasts.

The cold and warm air masses mostly just go by "cold" and "warm" but they do have fancy-schmancy names, too. From north to south in Canada we are influenced by two different Arctic air masses, Polar, and Tropical air masses. South of the tropical air lurks an Equatorial air mass, but I must confess to being largely unfamiliar with its whims.