GFA question

[charrois]

I have a question about freezing levels on a GFA.

I know the contours are said to be drawn for altitudes above sea level. But that's where I feel I must be missing something. When I've looked at them in the past, from what I recall there always seems to be a SFC contour, as well as 2500 feet, etc. But living in Alberta, 2500 feet is pretty close to sea level (edit: I mean pretty close to the ground elevation), yet the contour spacing from SFC to 2500 is comparable to 2500 to 5000. What am I missing? If ground level is more than 2500 feet, do they just skip the 2500 foot contour? If so, I don't remember seeing that.

The reason I'm confused is that I know how they claim to define the contours as being above sea level, though often in practice it hasn't looked that way.

Thanks for any input!

Dan

[ahramin]

Uh ... sea level is zero everywhere, even in Alberta. 2500 above sea level in AB isn't sea level, it's 2500 above sea level.

[Beefitarian]

Much of Alberta is above 2500 ASL indeed that contour line is skipped. The contour lines have a number printed along them, it is the height of any spot along the line in feet above sea level.

Sea level is the height of the ocean. It is a reference it is very close to the same height all over the planet. Every other thing's height is compared to that. Many rulers are one foot, it's a standard just over 30cm. The altimeter in an airplane usually measures feet ASL above sea level. Some measure meters or about 3.28.

2500 feet or 762 meters is not very close.

This is american but should be a good start.

http://flighttraining.aopa.org/pdfs/Int ... ymbols.pdf

[PositiveRate27]

Think of it this way, what ever the contour line for the freezing level says on the GFA is the altitude on your altimeter that you can expect to see zero degrees Celsius (with variation obviously). If you are on the ground at an Alberta airport and the field elevation is 2500' your altimeter will show 2500' IALT, therefore it is a good chance it will be around 0 degrees at the airport. In reality the GFA will simply show "SFC" to indicate that the freezing level in the depicted area that day will be at the earth's surface. If you look at today's GFA's for the Prairie region it doesn't show any freezing level isotherms at all. This is because the entire region is well below freezing at the surface and it isn't forecast to rise above that.

PR

[photofly]

I think it's a well posed question, which nobody has answered.

Please note that the lowest contour is not marked "Sea Level". It's marked "SFC". The line at which the surface is exactly freezing quite obviously changes in height above sea level.

You would therefore expect to see the SFC contour cross the other contours. But it doesn't. This needs some explanation.

[Beefitarian]

I believe you gentlemen are mixing up the acronym, shrimp for cocktail with something else.

[charrois]

Sorry - I misspoke when I posed my original question, which has caused some confusion about what I was asking.

When I said "2500 feet is pretty close to sea level", I meant "2500 feet is pretty close to ground level, or SFC". I do understand what sea level is Photofly figured out what I was getting at. As he said, you would expect SFC to cross other contours, which it doesn't. I wish it was warmer in the prairies right now so we had meaningful freezing level lines I could reference to show what I was getting at.

As an extreme example, take in the fall or spring when on the mountaintops it is below freezing, but in the valleys it is above freezing. From what I understand, the SFC freezing level contour should encircle the mountains and higher areas, delineating the areas on the ground that are not freezing from the peaks that are. But it never does this - from what I recall, it just seems to smoothly meander through the mountains as if they weren't there.

Dan

[Beefitarian]

I don't know for sure if I am the worst when it comes to MET knowledge but I'm probably close. Won't the air mass or system be typically much larger than a mountain, in most cases basically the peak would just poke through the contour lines?

Sort of like if the mountain was submerged by water.

In an airplane I think I want to know where the differences in temperature will be. Where it's not quite below zero C but close is where the cold moisture would be and that could mean icing conditions. Yes?

[charrois]

Just in time to be relevant, it's warmed up a bit enough in the prairies to show a good example of what I'm talking about.

In the attached GFA, you'll clearly see a freezing level of 2500' marked passing directly above Lethbridge. Yet the altitude of the Lethbridge airport is 3049' MSL. The SFC line is drawn a bit north of the 2500' freezing level, and the 5000' line a bit south of the 2500' freezing level, following essentially the same path.

If the freezing levels are truly MSL as claimed, the 2500' line through Lethbridge would indicate a freezing level below ground level - i.e.: on the ground at the airport it should be slightly colder than 0 degrees. Yet the SFC line lying north of Lethbridge on its own would imply that it would be above freezing temperatures at ground level at Lethbridge. Bottom line - from this GFA, would one conclude that it is predicted to be above or below freezing at Lethbridge airport at the time the GFA is valid for? According to the SFC contour lying north of Lethbridge, it should be warmer than zero, but according to the fact that a 2500' MSL freezing level contour is lying directly over Lethbridge and Lethbridge's elevation is higher than this, it should be colder than zero.

Also, the SFC, 2500', 5000', and 7500' line meander smoothly through western Montana and northern Idaho, despite rather dramatic elevation changes in the topography through those regions (ground elevations of up to 8000' in some places). If the SFC contour indicates the point at which the temperature should be 0 degrees at ground level, I can't see how it can extend through these areas with apparent disregard to the elevation changes. If it is meant to follow the route along the surface of the Earth at which the temperature is equal to zero degrees, it should wander around more than the other contours representing MSL elevations of freezing level, favouring lower elevations at this time of year. The lines indicating 2500', 5000', and 7500' MSL should progress smoothly independent of their underlying topography, but since SFC implies consideration of topography, I can't see how it follows the same trend as the others.

Either I'm misunderstanding something basic about freezing levels on the GFA, or something's broken somewhere It almost seems as if SFC represents that the freezing level would be at 0' MSL theoretically if the ground were that low, rather than representing that the temperature at ground level in that location would be at zero degrees. And if that were the case, then freezing level on GFAs would make a lot more sense to me. But this isn't what I learned it to mean, nor what is implied by the use of the term "SFC". And if so, it is important to know the difference, as if that's the case then lying along a line marked with "SFC" doesn't imply the airport would be at zero degrees as I always assumed it did, but rather a few degrees colder than that due to the ground altitude MSL.

Dan

[digits_]

I interpret the chart as: "without mountains, the freezing level would be at XXXX". So if the freezing level is at 2500 ft, and the ground is at 8000 ft, you will be in negative temperatures.

From: http://www.flightplanning.navcanada.ca/ ... ypeDoc=gfa

Freezing Level: Freezing level contours are indicated on the ICG/TURBC/FRLVL chart by dashed lines. The height of the freezing level is measured above sea level and the contour lines for the freezing level will be at 2500-foot intervals, starting at the surface. Modifications to the freezing level such as above freezing layers aloft, temporal changes, etc., are explained in the Comments Box for that chart.

I thought this would clarify things, but it makes them even more complex. They measure above sea level, yet start at the surface. Which is weird indeed. Could it be they take the minimum/average/some other value for "SFC" that is the same for the whole chart ?

[photofly]

I interpret the chart as: "without mountains, the freezing level would be at XXXX". So if the freezing level is at 2500 ft, and the ground is at 8000 ft, you will be in negative temperatures.

What do they draw if the ground is at 5000 feet and the freezing level is at 5000 feet?

Do they draw the SFC contour or the 5000 contour :-/ ?

[Ki-ll]

I interpret the chart as: "without mountains, the freezing level would be at XXXX". So if the freezing level is at 2500 ft, and the ground is at 8000 ft, you will be in negative temperatures.

What do they draw if the ground is at 5000 feet and the freezing level is at 5000 feet?

Do they draw the SFC contour or the 5000 contour :-/ ?

I would say that GFAs do not take terrain into account whatsoever and the SFC is actually Sea Level. I think it is done because it is much easier to display data that way and most meteorological calculations are done without any relation to actual height, rather pressure levels and geopotentials.
I am not sure whom we can actually call to verify that. I doubt FSS would know.

[Beefitarian]

I just independantly found the Nav Canada thing Digits posted which makes me feel slightly less ignorant but now I'm catching up with everyone else.

That is strange. At one point I was starting to hope to find something stating they were referring to heights above ground level.

They must be claiming surface level is below 2500 along the line. Anywhere the freezing level is below the surface is not even relevant since I can't fly there.

One question. Does the line indicate the aproximate height of zero degrees celcius? If so would the air be frozen at the surface along the SFC line and north regardless of actual ground temperature. ( even though one would think that the temperature of the ground must be at least zero )

[trampbike]

the SFC is actually Sea Level.

Exactly.

[digits_]

I interpret the chart as: "without mountains, the freezing level would be at XXXX". So if the freezing level is at 2500 ft, and the ground is at 8000 ft, you will be in negative temperatures.

What do they draw if the ground is at 5000 feet and the freezing level is at 5000 feet?

Do they draw the SFC contour or the 5000 contour :-/ ?

It would seem they'd draw the 5000 contour. They make an error somewhere, either in their definition of the terminology they use or in their analysis. The suggested SFC = MSL seems like the most likely at the moment.

[charrois]

The more examples I look at, the more it really does appear as though their depiction of a freezing level marked SFC actually is a contour drawn for a theoretical freezing level at 0' MSL, or sea level. To me, and apparently at least a few others here, it seems to depict where the freezing level would be without the pesky mountains or ground in the way.

The quote from NavCanada of "The height of the freezing level is measured above sea level and the contour lines for the freezing level will be at 2500-foot intervals, starting at the surface" doesn't seem to be valid for how they end up in actuality. As evidenced by the example GFA I posted in this forum, the surface at Lethbridge is 3049' MSL, so the first interval over Lethbridge should be 5000' if they were to follow the guidelines stated, but they have a 2500' freezing level contour drawn directly over the airport. Also, the distance between the 5000' and 2500' contours in the area is approximately the same as the distance between the 2500' and SFC contours. Assuming the freezing level changes roughly uniformly in the area, that would be impossible if SFC represented a freezing level at ground elevation.

It would be nice to clarify this with someone "official". Someone mentioned it doubtful that a call to FSS would resolve things, and that's entirely possible. I wonder if we can track down the actual department responsible for making these things in the first place...

Until I started to think about this critically a couple of days ago, I had always assumed that a SFC contour around my airport meant that the temperature would be close to zero on the ground. If SFC really does represent 0' MSL, that's a bit of a wake up call, as then the prediction would imply that the temperature at the ground at a higher elevation than sea level would be colder than that. In either case, I'm curious to how this is resolved... though I personally hope that SFC does imply a freezing level at 0' MSL, since it would be a straightforward explanation, and that particular part of the GFAs would make a heck of a lot more sense. Even the notion of a freezing level below ground elevation would be useful - it helps to imply what the temperature at ground elevation is likely to be.

Dan

[dirtdr]

Please note that this gfa covers a lot of ground.

The Lethbridge area you are referring to is the "oddball" of the area in consideration of the surface elevation and terrain.

Better than 95% of Alberta, Sask, and Manitoba are under 2500ft asl.

And my take on the gfa is that it is not meant to be a topo map, but a representation of what the air mass is doing over a VERY large area.

[AuxBatOn]

Does it have an impact on anything? No... Your airport is at 3000'MSL? Any freezing level below that above your airport means that temp will be 0 or below at your airport. Same as SFC... It doesn't mean it's close to 0, just that temp is below 0 on the ground (or sea level perhaps). Should we get them to spend money for semantics? Absolutely not.

[photofly]

I'm not sure that it's very much more expensive to put "SLVL" on a chart instead of "SFC", especially when the chart is online.

[AuxBatOn]

I'm not sure that it's very much more expensive to put "SLVL" on a chart instead of "SFC", especially when the chart is online.

Being involved in then change process for some of our products, I can tell you it probably is very, very expensive.