Alidade

I mentioned the met station in Estevan. Once upon a time there was an FSS there, and a pilot could go up the stairs to get a weather briefing or look at weather charts, faxed from Edmonton, I think, and hand-coloured by the local briefers. Or you could phone the FSS directly to get the latest weather. You just had to be careful not to call them at the top of the hour, or else they would be outside taking the hourly weather observations, or busy coding the observation. About eight years ago they they separated the task of observer and briefer, moving the briefers into consolidated regional Flight Service Stations and leaving the lonely observers behind, in some cases literally building a solid wall across the door the pilots used to enter for briefings.

We first noticed the Nav Canada met observer when my passenger mistook him for a Transport Canada official--I guess he has a sharp eye for aviation functionaries. Late that day we spotted this device just outside the airport fence.

As you can see if you click and enlarge the photo, it consists of a ninety degree arc, fixed firmly to a vertical column and calibrated on a non-linear scale in "metres" and "coded vales." There is also a sight, pivoted so that it can swing through the arc. Its location, between the met office and the outdoor weather sensor array, plus the words Atmospheric Environment Service indicated that it had something to do with making weather observations, but what? The azimuth angle between the observer and the base of a randomly selected cloud is not a useful datum for a pilot, or for anything meteorological that I can imagine. I snapped a picture and saved it until now.

The observers at the airport where I learned to fly had a laser ceilometer, a device that bounces a short pulse of light off the cloud base and times its return. Speed times time equals distance, so you take the time it took to return and divide that by the speed of light and you have the distance the pulse travelled. Divide that by two (because it had to go there and back) and you have the height of the cloud above the emitter. I get a serious thrill out of the fact that a trivial calculation whose results I use daily involves the speed of light, so we must now pause for a big grin.

In mountainous areas, weather observers use the known height of nearby peaks to determine cloud heights by looking at where the cloud hits the mountains, but I hadn't until today given any thought to the pre-ceilometer technique in this notoriously non-mountainous province. (I made a two hour flight in Saskatchewan the other day and the elevation of the landing airport differed by one foot from my take-off point). The alidade and ceiling projector combination is the answer to the question I didn't think to ask.

The unfamiliar word alidade, describing the scale, looked eminently googleable so that's where I started. Technically the alidade is just the pointer with the sight, a part of many ancient and modern scientific instruments, but some whole instruments are called alidades by association. Fortunately for me this sort of alidade is one of the example pictures on Wikipedia, and links to the article on the ceiling projector with which it is used.

According to the Wikipedia article, the ceiling projector is a bright light that shines straight up at a known distance from and the same level as the alidade, and then the spot where the light reflects from the base of the cloud is observed with the alidade. The scale on the alidade is simply the solution to the resulting trigonometry problem. How beautiful is that? (If you won't at least pretend to appreciate the beauty of trigonometry you aren't allowed to read my blog anymore).

In my diagram, y is the height above ground of the alidade and the projector (relatively negligible for the ground-mounted instrument, but some of these are probably on rooftops), d is the distance between the alidade and projector, and A is the angle above horizontal of the observed light spot on the base of the cloud. You put your eye where the red dot is and raise the sight along the scale until the light spot lines up with both rings of the sight. The unknown x is the distance from the projector to the cloud base. Using trigonometric ratios, d tan A = x and then x + y gives you the cloud height above ground level. The scale is marked along the arc as the solution to the equation, instead of being in degrees, so the observer doesn't actually get the privilege of doing the math.

That's two ways to get the same information, one requiring knowledge of the speed of light and the ability to produce and monitor coherent nanosecond light pulses, and the other requiring only tools and skills known and available to ancients. If that's not the coolest thing you've seen all day then you are obligated to click on the comments and tell us all about your even cooler thing.

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For a scientific experiment of a different kind, there's Boobquake, a US college student's response to an Iranian cleric's assertion that earthquakes are caused by by women dressing immodestly. I'll join the effort and show some cleavage on Monday, provided that it's not snowing.

Impending Snow

We have about four hours worth of work to do and the weather forecast offers us a a little window in which to do it. We're west of Fort Nelson, near where the highway starts to enter the mountains just south of the Yukon border. There is clearly more snow visible on the ground than there was on the last flight, and there is a fine veil of altostratus cloud over our heads. Winds are strong but there is only a little turbulence

Within an hour the mid-level cloud layer is noticeably thicker and lower, and cloud continues to waft in as we fly. A large mass warm air--well not very warm, but warmer than the air mass that's over us now--is slowly travelling east. It abuts the cold bubble of air currently covering the Yukon, and northern BC and Alberta, and as it is warmer it rides up over the present air mass. Same old story: rising air expands; expansion causes cooling; and cooling causes the formation of liquid water droplets, otherwise known as clouds. The incoming warm air mass contains a lot of moisture because it formed over the Pacific Ocean, so there is the potential for a lot of cloud.

Over the course of our four hour flight the ceiling drops three or four thousand feet. The highest mountain peaks to the west have disappeared and we can see spots in the mountains where it's already snowing. The local weather forecast calls for three solid days of serious snow.

We return to the airport for landing and call one last time to refuel. We're scheduled to fly to our next job tomorrow, after the client is certain they no longer need our services here. Of course we're not yet certain where the next job will be, except that it has to be south and east of here. We asked to be released today, so we're not trapped by the incoming snowstorm, but the answer was no, they won't give up their airplane until they know they don't need it anymore. They will let us know that at 7 a.m. tomorrow. We let the hangar people know this, so they can stack our airplane near the front, and we give them instructions on sending the hangar space bill (almost as much as my hotel bill) plus a offer a forwarding address should the wing covers ever arrive. And then in walks someone with a package for us. Just in time. We put it in the airplane and then go back to the hotel to pack up for the move.

On Again

A new day, a new decision. We'll fly to Fairbanks, clear customs, and then take commercial flights home while maintenance does a phase check and the replacement crew flies in. Someone must have factored in the airfares and that shifted the math. Airfares are relatively cheap in and out of Fairbanks, as they are for most big-city American destinations. We won't get to see the remote Alaskan spot we had hoped to visit, but the novelty would probably have run out before the work, and we will get to cross northwest into Alaska.

After I get that news, I go flying. If all goes well, this will be our last flight returning to this base -- although my Alaska charts still haven't arrived. Apparently this place has some sort of repellent force that prevents newspapers, courier deliveries, mail and airline passengers from arriving on time.

The last three METARs report cloud bases at 5900', 5800', and 5700', which is odd because I asked the specialist earlier and he said they didn't have a ceilometer. All cloud heights are estimates. How would they, in the absence of nearby mountains, determine a hundred foot difference just by estimation? On other days the bases have been given to the nearest thousand feet. I ask about it.

The answer is that today, because the clouds are all cumulus, they are reporting the bases not by visual estimation, but according to a formula. I understand. I know that formula. I didn't know anyone used it for anything other than written pilot examinations. Here's how it works: cumulus clouds are formed when rising air is cooled to its dewpoint by expansion. Rising air cools through adiabatic expansion at a rate of 3 degrees Celsius per thousand feet. The dewpoint decreases by about 0.5 degrees per thousand feet. Therefore the temperature approaches the dewpoint at a rate of 2.5 degrees per thousand feet. The reported surface temperature is 21 degrees and the surface dewpoint is 5.2 degrees. So at the surface the temperature is 21 - 5.2 = 15.8 degrees above the dewpoint. The temperature will reach the dewpoint 15.8/2.5 = 6.3 thousand feet above the aerodrome. (This doesn't match the earlier report because they have given me current temperature and dewpoint values and the temperature-dewpoint spread has evidently increased by a degree since they did their last calculation. Drop the temperature to 20 and do the same math and you get 5900'). METAR altitudes are given above aerodrome elevation, which is 1250', so I can expect bases to be at about 7500' above sea level, as displayed on my altimeter.

Another pilot hearing this conversation calls in to report bases at 7500'. Science! It works. Also don't you love Canada? We have formulae that combine degrees C with Imperial feet and we think it's just fine that way.

In Canada either a flight plan or a flight itinerary must be filed for a VFR flight of more than 25 nm. A flight plan is filed with Nav Canada but a flight itinerary can be filed with anyone responsible enough to report you missing. Commercial operations like mine generally file with company and when we inform ATC of this we say we are "on a company note." That shortcuts ATC asking if we want them to open our flight plan. In imitation of this practice, a small aircraft pilot taxiing out calls the FSS and says he is "VFR to Helmut on a note." He then decides that this needs more explanation, as the FSS knows he is a private owner and not a company, so he adds, "My wife."

Another pilot calls up "Looking for the airways to Ft. St. John." That's a non-standard way to ask for an IFR clearance, but seeing as the FSS probably has his clearance sitting there and is is waiting for him to call to ask for it, he could probably have said anything he liked and still received it. If you don't ask for it, they offer it, as when the tower says to a airline pilot after he makes a taxi call. "Grab your pen."

Then it's my turn. The wind is straight down runway 26 at 10 knots, so it's an easy taxi and take-off. As the airplane waddles up to altitude in the hot weather I call to report airborne, and my turn. The FSS does a shift change. It's a woman with an unfamiliar voice, and she comments to someone on air that she's just back after two weeks. I guess she's out of practice, because she keeps getting confused, forgetting which airplane is where, scrambling our types and needing constant position reports. I am ready with a position report whenever she calls me for one and make sure I am keeping track myself of where the other aircraft are.

From the Flight Deck

You all know that I am a commercial pilot who would like to be an airline pilot. So, you would probably guess that when an airline in-flight magazine has a column written by one of the pilots, I read it with a mixture of admiration and ambition, imagining that I could be doing that someday. That's half the reason they put that column in there, isn't it? Almost everyone wants to talk to the pilot, to be reassured that he or she seems competent and caring, or just to get a touch of that stardust that makes these huge airplanes fly. For me the column counts double, because not only do I want to be the one flying the plane, I'm already one of those doing the writing. I love to write and to explain my job to people.

On Air Canada, my country's flag carrier, the pilot who writes the From the Flight Deck column is Captain Doug Morris. The other day, completely out of the blue, I received e-mail from Doug Morris. Not just any old e-mail, but e-mail thanking me for the piloting resources I put on the Internet. I was ten feet tall for the rest of the day. It's comparable to a struggling writer getting e-mail from Margaret Atwood. (Or perhaps from John Irving, to put that in American translation). So of course I wrote back, and he wrote back, etcetera. Captain Morris shares my interest not only in flying and in writing about aviation, but about aviation weather. He even has a degree in meteorology. (I'm self-taught). As many of you who have started private e-mail conversations with me know, it ends in us telling each other stories. Doug tells good stories.

He has recently published a book which is a compilation of his From the Flight Deck columns. Sample his style on his blog where he posts some of his magazine columns. It's a real blog though, including not-published-elsewhere original entries such as this detailed account of his latest recurrent sim training. You have to be able to take off in the sim, knowing that all manner of things are going to go wrong and that you will handle them correctly in order to keep your job. That demonstrates your ability to take off in real life, never knowing what might go wrong, and being able to handle that correctly so you and your passengers can stay alive.

En route now uses a question and answer format for the From the Flight Deck column, so Doug is currently soliciting questions to use in a similar context on his blog. I suggest you click over there and ask your airline flying questions now, before he has too many to answer. I expect that fans of my blog or of Dave's Flight Level 390 will make From the Flight Deck a regular read.

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.

Endless Supply of North Winds

Next morning there is no respite on the winds, but the snow has swept through and left Alberta clear. It's threatening to drop the visibility here now, but that's something I can work around. We get a cab back to the airport in early morning darkness. There is no one at the FBO, but they have fuelled the airplane as requested, tied it down and chocked it against the wind. Loaded to the gills, it's still jumping around like a kite. I put my passenger on board and close the door while I do the walkaround with a flashlight, checking especially to make sure that nothing has has blown into the engine cowlings or other important apertures during the night. I untie the ropes, but leave the chocks behind the wheels so it doesn't go anywhere while I'm starting the engines. It's that windy.

The saving grace is that the wind is now blowing straight down the runway, so that all its force will go to shortening my take-off roll and none to trying to push me sideways off the runway. The only challenge is getting to the runway. I taxi very slowly, turning my ailerons at each turn so as to spoil the lift on the into-wind wing. It's a skill I learned as I first learned to taxi in a light little two-place airplane. I don't always do it in this airplane, but today it is needed. I wonder if there is a size of airplane at which you can just leave the ailerons neutral through taxi in any conditions. I wonder if Airbus 380 pilots turn their ailerons for wind anyway, because that's what you do.

I position at the very end of the runway, because that's what you do, complete my pre-takeoff checks and set power. It's an amusingly short take-off roll. I estimate I had seventy percent of rotation speed before I even released the brakes, and there's very little rolling resistance to acceleration at low groundspeed. I climb, but not too high, because this wind is forecast to be even stronger at higher altitudes. My initial heading is due west, because the area of the worst turbulence and of low visibility and snow is approaching from the north. I've chosen a point based on forecasts at which I should be able to head northwest and be clear of the weather.

It works like a dream. I give thanks aloud for modern weather forecasting technology. The passenger is unimpressed and I resolve to shut up about that sort of thing in future. Progress is unbelievably slow. I file a PIREP reporting minimal turbulence and sixty-five knots on the nose. Just as forecast. I watch the scenery go by very slowly. I file another PIREP because I know there will be a lot of people looking at weather reports and forecasts today, wondering if they should venture out.

The frequency on which one files a PIREP up here is 126.7, the same frequency as one makes air-to-air position reports for the benefit of other traffic. Another pilot on frequency recognizes my voice and checks to confirm that it's me. It turns out to be the fueller from one of the places I've stopped recently. He's working on his commercial pilot licence and is on his way back home after a cross-country trip. He too was grounded yesterday. He's headed in the same direction as I am. I shudder to think what his ground speed might be. He's in a C172. He might get to his destination faster by driving. I respect him for being up here at all, with obviously limited experience.

Total flight time for the two legs of the trip, not including time spent on the ground at my planned fuel stop, was six hours, thirty minutes. And I took a more direct route than the 4h 07 southeastbound flight. I once had a student ask, "so where does all that wind COME FROM? Isn't the north going to run out of air?" There's actually a circulation, such that there is a polar high, constantly subsiding and being resupplied by air that circles aloft at the subpolar low.

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.

The Season of Ice

It's official almost summer, so for me this can only mean one thing: annual ice awareness training. Yeah, my company training is out of sync with the real world. It's just the way it is. Instead of just having me watch the Transport Canada When in Doubt video on deicing again, they had me do this NASA video course on icing, too. It's excellent. Canadians who only fly in Canada will find some of the weather products unfamiliar, but you can skip those parts. If you do, or might fly in the US, it's a good how-to-use for their icing products, which are different from the Canadian GFAs. And the advice is very practical, not just recitation of examination factoids, like "contamination the texture and thickness of sandpaper decreases lift by 30% and increases drag by 40%."

Here are some examples of academic information transformed to simple, practical advice.

Weather basics tell us that stratiform clouds form in layers, usually not more than a few thousand feet thick, and and cumulus clouds develop vertically. The NASA course says to escape ice in stratiform clouds, change altitude by 3000', but navigate around cumulus clouds.

Weather theory tells us that an air mass loses moisture from crossing a mountain range, and that all else being equal, more moisture in a cloud means more icing. So NASA says, plan to fly on the leeward side of a mountain range.

Fronts are the places where air masses meet, named according to which air mass is advancing. At a warm front, warm air is advancing over a wedge of cold air. At a cold front, a wedge of cold air is forcing warm air up. So the NASA advice is to fly on the warm air side of a front, i.e. traverse a warm front perpendicular to frontal movement and behind the front and traverse a classic cold front perpendicular to movement and in front of it. This is probably good advice everywhere in the US but Alaska, but I'd have to reverse it for the north in the winter, because then the cold air is down below minus thirty, too cold for icing, but if the warm air is up around minus ten it will contain the more dangerous icing.

We already do this for engine failures and depressurization events, so why not develop an icing escape plan for each point along your route.

The NASA course does treat North America wide phenomena. They mapped of the continent for icing potential, illustrating that icing is more common around large bodies of water. At first I blinked at it. Why did it not show massive icing around the northern lakes and bay? Then I noticed the caption "November to March." The northern bodies of water are frozen solid then, so are not a source of moisture for icing until spring.

They acknowledge that their audience already knows things about icing, some of which are myths. NASA is wonderful because they don't just pool the advice of experts and regurgitate it back. They go out and do research, some of which involves flying actual airplanes around in actual icing conditions, and can back up what they say.

Pilots who have flown regularly in ice have all looked out the window and confidently assured their terrified copilots that they've seen worse. NASA research, and you get to trust them more than the grizzled captain because they really do research what they write about, reminds us that it is nearly impossible for a pilot to visually distinguish between an accretion that is flyable and one that threatens survival. I am reminded every time I preflight of how little it takes to change the flyability of an airplane. That's because my airplane has a vortex kit, eighty-eight little metal tabs on the wings and tail. They're tiny, each about the thickness of a dime and the area of the first joint of my thumb. But lose three and the maximum gross weight of my airplane drops by six per cent. I mentally compare that to how an ice accretion could affect airflow over the wing. By how much will it decrease the maximum flyable weight of the airplane?

And no icing video is complete without a nod to the old advice on deicing boots. In the old days, before I learned how to fly, probably before I was born, manufacturers told pilots to wait until there was a significant accretion on the wing before inflating the boots. The theory back them was that if the boots were inflated when the ice was very thin, inflation of the boots could result in bridging, creating a boot-sized airspace surrounded by ice the boots couldn't meet. NASA points out that while this may have been possible with the old, slower inflation boots, it is not with modern boots and that no one has ever shown ice bridging to occur with any boots. "Ho hum," I thought at that part of the presentation. Old news. Everyone has heard that debunked by now. The pilots I have flown with who advocate waiting are not delaying activation to avoid ice-bridging, which they don't believe in either. They are delaying it because they are waiting for the "skin" of ice to be thick enough to pull off the runback ice as the boots pop. This video, however, addresses this too. It admits that studies confirm that larger amounts do shed more cleanly with one inflation. But that research shows that although ice may remain between cycles, the ice will ultimately clear as well as it would have, had you waited for a large build up, and by activating the boots as soon as you have ice, you reduce the maximum amount of ice you are ever carrying. This makes more sense than decreasing the minimum amount.

I have notes on a couple of things I hadn't met before, too.

Freezing drizzle (small supercooled raindrops that freeze on contact with your airframe) is according to NASA most commonly formed by collision coalescence. That is, by even smaller supercooled water droplets bashing into one another and becoming big enough to drift down as drizzle. I had thought that it was usually formed the way textbook freezing rain is formed at a warm front, with rain falling out of the warm air aloft, and becoming supercooled by its passage through the colder air below. I can diagram it and everything. Useful as my original knowledge is for passing Transport Canada exams, there's an important difference: as the NASA course points out, I should not assume that the presence of freezing drizzle indicates warmer air above, into which I could climb to escape the ice.

The other new-to-me thing I remember is electro-expulsive deicing where the shape of the actual wing is mechanically altered to dislodge ice, like flexing an ice cube tray.

It's very usable, multi-media with exercises to do, and so many videos available to watch you can miss a lot of them by not looking at the 'related information' section. The only spelling error that distracted me was the term "full blow thunderstorms" where I would have expected "full blown," but perhaps that's a regionalism. The course is something you have to dedicate a day to. There's that much information.

Adiabatic Lapse Rates

A few weeks ago I discussed expansion cooling, and the relationship of the stability of an air mass to its environmental lapse rate. You can pretend you remember that if the temperature decrease with altitude is less than the rate of cooling of a parcel of raised air, then the air is stable, but if the atmosphere decreases in temperature faster than raised air cools by expansion, then the air is unstable. I said that raised air always cools through expansion at three degrees celsius per thousand feet. But I hinted that I was leaving something out. I was.

I left out the consequences of condensation. Once air cools to its dewpoint, water vapour starts to condense into water droplets. These droplets make up clouds or mist, and as I explained in a different earlier post, the act of transforming from vapour to liquid actually releases heat. This release of heat partially cancels out the cooling by expansion, such that once visible moisture starts to form, raised air cools at an effective rate of one and a half degrees per thousand feet. You can think of it continuing to cool at 3 degrees/1000 ft, but then warming back up again by a degree and a half, for a net decrease of 1.5 degrees/1000 ft. Apply that knowledge to the idea of stability and instability and you see that moist air will be unstable with a shallower lapse rate than dry air.

Air that has an environmental lapse rate of more than three degrees per thousand feet is going to be unstable. As rising air is cooled to its dewpoint the temperature differential between raised air and its environment will increase even faster, and that's how giant cumulus clouds form. Air that has an environmental lapse rate of between one and a half and three degrees will be stable if the air is dry, but unstable if the air is saturated. so the air is termed conditionally stable. Or damn, maybe it's conditionally unstable. One of the two. But if the environmental lapse rate is less than one and a half degrees per thousand feet, the air is stable regardless.

Stable air is smoother to fly in and the clouds that form are flat and featureless. Unless flat is a feature. Rain falling from flat clouds is steady. Flat clouds are called stratus clouds, because everything is always cooler in Latin, so you get STable air with STratus clouds and STeady precipitation.

Unstable air is bumpy. And I suddenly remember an elementary school class in which Caroline, asked to provide an adjective to describe a cloud, supplied bouncy and refused to back down when the teacher rejected it. Caroline, you were right all along, and if I ever see that teacher I will take him for an airplane ride through a bouncy cumulus cloud. Cumulus is Latin for heap, I think. You can remember that the cloud acCUMULates upward. Precipitation falling from cumulus clouds is showery, meaning intermittent, not steady, but probably heavier than the precipitation from stratus clouds.

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.

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.

Hot Air Rises

In order to continue explaining air masses I must first explain density and pressure. Density is a measurement of how close together the air molecules are, or, put another way, a measure of what weight of air exists in any given volume. More weight per volume is the same thing as higher density.

If you pack socks into a box there are two ways to maximize the sock density. One is to use thin, non-fluffy socks and the other is to cram your socks in as hard as you can, stomping on them to get them to fit before you seal the box. (You know I just moved house, right?) These two factors are the same as the ones that affect air density.

'Fluffiness' of the air corresponds of temperature. The higher the temperature, the greater the volume the air wants to occupy. It's actually because the speed of the air molecules is greater at high temperature, but you can think of the volume they thus occupy as them being all fluffed up hot out of the tumble drier. All else being equal, warm air occupies more volume than the same weight of cold air. Therefore, all else being equal, an equivalent volume of cold air weighs more than warm air.

Stomping on the pile of socks corresponds to pressure. Is pressure, really. Pressure is defined as force per unit area, and in the atmosphere it is a result of all the air stacked up on top of the bit of air you're considering. Imagine you're looking at a cubic litre of air(*). Its pressure is equivalent to the weight of all the ten by ten by ten cubes of air that are stacked on top of it, all the way up to the top of the atmosphere. Sure, one little cube of air doesn't weigh much, but stack enough up and it adds up. So air down near the bottom of the atmosphere is at a higher pressure than air further up in the atmosphere, where it has fewer boxes stacked on top of it. Kind of like the box of drinking glasses underneath four boxes of aviation textbooks is under more pressure than the one that is ony one box into a pile.

You can see pressure and temperature kind of work against one another with respect to density. If the pressure increases and the temperture stays the same, the density will increase. If the temperature increases and the pressure stays the same, the density will decrease. Cold air at a high altitude is less dense than warm air at a low altitude, because the effect of the low pressure at high altitude more than balances the effect of the temperature difference. There's even a formula:
  P x V = T x constant
P = pressure, T = temperature, V = volume.

So, we have a bunch of air hanging about. Air in the same vicinity within the same air mass is pretty much interchangeable. It's all mostly nitrogen, and contains some amount of moisture, and at the same pressure because its under the same pile of air. And its at the same temperature. If some of it were to be heated up to be warmer than the surrounding air, look at what would happen. Firstly, it doesn't warm the air around it. If air were good at sharing its heat with the molecules around it, down-filled parkas wouldn't be such treasured possessions in the north. (The puffy feathers create little air pockets and heat doesn't travel well through air, so that keeps me warm.) If a little bit of air is warmer than the air around it then it is also less dense than the air around it. And that means that the gravitational force holding it down isn't as great as the pressure differential between the air above it and the air below it, so it is pushed up, and rises.

And yes, I did just spend six paragraphs explaining that hot air rises. Just think of it as a demonstration of hot air. The reason I did it that way is that warm air doesn't always rise. If that were true it would be warmer at the tops of mountains than at the bottom. Air rises if it is less dense than the air around it. If the pressure is the same, then temperature determines density. So it rises if it is warmer than the air around it, sinks if it is colder than the air around it, and stays in the same place if it is the same temperature as the air around it.

There's one further trick to the rising air, as it rises, the pressure around it decreases, so according to the formula, if the temperature stays the same, the volume has to increase. And it does. The rising air expands. It actually also cools as it expands, so the result is that the same air occupies a greater volume at a lower pressure and temperature.

What it does next is for next time this multi-threaded blog returns to weather theory.

(*)If you didn't go to elementary school in Canada after metrification you missed out on carefully measuring ten centimetres by ten centimetres by ten centimetres and building a little cardboard box. That's about four inches cubed, for the aggressively non-metric. Once you'd built and folded your cardboard cube, and mended any measuring or folding errors with vast quantities of cellophane tape, you had a concrete way to visualize a ten centimetre length, one thousand cubic centimetres, one litre capacity, and, if you imagined what your cube would feel like if it were filled with water, one kilogram. I'm not making this up. They used to hand these things out at fairgrounds, in modern 1970s colours like pink, yellow and lime green. Someone back me up here. I'll trade you a working flashlight for a genuine 1970s MetriCube.

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.