question about weather charts

[kath]

Hi, all,

The low-level significant weather prog chart (the one with MVFR/IFR, freezing levels, etc on it)... I have a question about the freezing level lines. There are dashed (green) lines labeled "80" or "120" to indicate where the freezing level is 8000 feet and 12000 feet, and zigzaggy (blue) lines to indicate where the freezing level is to the surface. I've always thought of these as being like contour lines for the freezing level, just very widely spaced.

I'm looking at one today (for 06Z Saturday the 16th),
http://aviationweather.gov/data/products/swl/ll_06_4_cl_new.gif
...which has a "surface" line crossing both a "40" and "80" line in New England. What does that mean?

On the same chart, there is a "80" line horizontally across the northwestern US, with a "120" line across the southwestern US, and a little patch of "surface" between them. Shouldn't there be a "40" in between somewhere?

The "contour line" analogy is breaking down... is there some other way of interpreting these lines?

--kath

 

[EdFred]

Mountains.

I don't believe the freezing level is AGL, but MSL. If the freezing level is 080 and the elevation is 10,000 ft that puts the freezing level at the surface.

 

[kath]

Mountains.

Right. Duh.
Okay, I'd better go find some coffee now...

--Kath

 

[EdFred]

Don't worry about it, I ran into the same issues myself. That happens when you live under 1000' MSL.

 

[river_rat]

Hi, all,

The low-level significant weather prog chart (the one with MVFR/IFR, freezing levels, etc on it)... I have a question about the freezing level lines. There are dashed (green) lines labeled "80" or "120" to indicate where the freezing level is 8000 feet and 12000 feet, and zigzaggy (blue) lines to indicate where the freezing level is to the surface. I've always thought of these as being like contour lines for the freezing level, just very widely spaced.

I'm looking at one today (for 06Z Saturday the 16th),
http://aviationweather.gov/data/products/swl/ll_06_4_cl_new.gif
...which has a "surface" line crossing both a "40" and "80" line in New England. What does that mean?

On the same chart, there is a "80" line horizontally across the northwestern US, with a "120" line across the southwestern US, and a little patch of "surface" between them. Shouldn't there be a "40" in between somewhere?

The "contour line" analogy is breaking down... is there some other way of interpreting these lines?

--kath

"Freezing level height contours for the highest freezing level are drawn at 4,000-foot intervals. The 4,000-foot contour terminates at the 4,000-foot terrain level along the Rocky Mountains. Contours are labeled in hundreds of feet MSL The zig-zag line where the freezing level is forecast to be at the surface and is labeled "SFC." An upper freezing level contour crossing the surface/32 degree line indicates multiple freezing levels. Multiple freezing levels indicate layers of warmer air aloft. If clouds and precipitation are forecast in this area, icing hazards should be considered."
Quoted from this http://avstop.com/AC/aviationweather/sec8.html

greg

 

[bbchien]

Greg's got it....multiple freezing levels at one location mean up and down mixing of air. If there's moisture there, it's a potential icing ride....