Thickness levels and surface temperatures

[Rocky Rascovich]

I guess everyone is entitled to one dumb question every now and then. The NAM especially has been overdoing the thicknesses on the warm side with this ice event here in Okla. By 18Z today, thicknesses are progged to be near 550 over cntrl OK. Current temps here at the farm as of 0915 are aprox. 23F. I doubt we'll be much above 27F for today.

The question is.. isn't 540 the magic number for ice/snow? Does that mean 32 deg. at the surface or at a certain level aloft? I always thought 540 was the freezing point at the surface.

You can PM me privately about this question. Thanks!
Rocky&family

 

[Donald Giuliano]

Rocky,

That's not a dumb question at all as the 540 line can be very deceptive without knowing a little more about it.

The 540 thickness is a "rule of thumb" used by forecasters east of the Rockies to estimate whether or not the entire column of air between 1000-500 mb is approximately below freezing (including, but not limited to the surface). This works because the thickness of that layer is a function of the total temperature profile in that layer (warmer = thicker), and 540 just happens to be the thickness that "typically" means the whole profile is below freezing (thereby yielding snow as the dominant precipitation type). However, it's important to note that this is an approximation that is dependent on local altitude and climate, and often does not hold true in reality. That said, it's a good place to start when determining possible precip types.

If you would like more detail, just reply. I'll be happy to post a couple of example soundings showing how this looks in real life when I get the chance...unless someone else beats me to it.

 

[rdale]

The 540 really isn't used anymore... It's nice to give as a first guess, but with the availability of model soundings -- that's really what you use to analyze precip type.

http://www.wdtb.noaa.gov/courses/winterawoc/IC6/index.html

 

[Steve Mirsky]

If the cold air is shallow and warm air is rising above it (over running) then the cold air is trapped at the surface, thickness is higher then 540 due to the warm air aloft and you have an ice scenario. 540 can be the snow-rain line and not the ice line.