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, 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.

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.