freezing rain and Skew-T

[NoHeat]

Freezing drizzle this morning on the ground in Iowa City.

I looked at the Skew-T just as a learning experience. It predicts 100% humidity below 2400 meters (about 7500 feet).

Zero degrees C is at about 950 meters (about 3000 feet) with above-freezing temperatures at higher altitudes - so that's where the rain is coming from.

Area forecast:
SERN...OVC010-020 TOPS 080. OCNL VIS 3-5SM BR/-DZ. 15Z OCNL VIS 3-5SM -FZDZ. 19Z OVC010-020. OTLK...IFR CIG.

Metar:

[FONT=Monospace,Courier]KCID 201452Z 03008KT 2 1/2SM -FZRA BR OVC005 M04/M06 A2983[/FONT]

Supplementary icing information :



Skew-T:

 

[NoHeat]

well, for some reason the supplementary icing product image doesn't appear, but it shows a small area of red cross-hatching at my location, which would indicate a risk of SLD (super-cooled liquid).

 

[Jaybird180]

 

[AggieMike88]

If you haven't already done so, participating in ScottD's "Mastering the Skew-T Diagram" workshop is a worthwhile thing.

 

[Ted]

Yep, that looks about right. Now the real question: would you depart? What would be your strategy?

 

[NoHeat]

Mostly I just wanted to see how well the forecasting products work, when I could observe actual freezing rain. It turns out they worked well. The Area Forecast, the TAF (which I did not show), and the supplementary icing product all explicitly warn you of freezing rain. Interpreting the Skew-T tells you this also, with some fine-grained info on the altitudes.

I would not depart or land under these conditions, with freezing rain from 3000 MSL downward.

If I were en-route, an altitude of 9000 or higher should be good, based on the Skew-T and the area forecast.

 

[ifconfig]

If you haven't already done so, participating in ScottD's "Mastering the Skew-T Diagram" workshop is a worthwhile thing.

THANK YOU! You're a treasure trove; I was just about to ask if there was a good reference for the Skew-T diagram.

Edit: just saw it's a workshop to take. While still, I'm sure, a good reference, and being lazy, any other good references on the Skew-T?

 

[EdFred]

Yep, that looks about right. Now the real question: would you depart? What would be your strategy?

Nope. Find a good looking woman, some scotch, and curl up under a blanket for a few days.

 

[AggieMike88]

THANK YOU! You're a treasure trove; I was just about to ask if there was a good reference for the Skew-T diagram.

Edit: just saw it's a workshop to take. While still, I'm sure, a good reference, and being lazy, any other good references on the Skew-T?

Scott does have a 15 minute overview of the Skew-T on YouTube. http://www.youtube.com/watch?v=CNBzdkmaAKE

 

[Ted]

Nope. Find a good looking woman, some scotch, and curl up under a blanket for a few days.

What would you do with the good looking woman besides **** her off?

 

[mondtster]

Yep, that looks about right. Now the real question: would you depart? What would be your strategy?

No way. I don't know what the runway is like in Iowa City but just a few miles north of there the runway at my home airport is a sheet of ice. I'm not even sure you could get an airplane drug out of a hangar to think about flying anywhere.

It's looking like we're going to have another day or two of crummy weather around here that isn't going to be good for flying. Because of this, my vacation is probably going to have to start a little later than planned.

Mostly I just wanted to see how well the forecasting products work, when I could observe actual freezing rain. It turns out they worked well. The Area Forecast, the TAF (which I did not show), and the supplementary icing product all explicitly warn you of freezing rain. Interpreting the Skew-T tells you this also, with some fine-grained info on the altitudes.

Was this a forecast or was it an observation? If it's an observation of course it works well. :wink2:

 

[ifconfig]

Scott does have a 15 minute overview of the Skew-T on YouTube. http://www.youtube.com/watch?v=CNBzdkmaAKE

Thanks Mike.

 

[NoHeat]

any other good references on the Skew-T?

It's remarkably hard to find one that's suitable for pilots, so here are some quick pointers:

A skew-T is basically graph paper with two curves drawn on it.

The graph paper:

• Altitude increases upward along the vertical axis.
• Temperature increases to the right on the horizontal axis. But there's a trick: the temperature lines are tilted (skewed) to the right -- they are not not up and down as in most graphs. In the graph I posted, the temperature gridlines are the olive-colored lines that slope up and to the right

The curves:

• temperature (solid red in the graph I posted)

• dewpoint (broken black in the graph I posted)

Interpretation:

Three useful things to identify in a Skew-T are:

• tops
• freezing rain conditions
• stability.

I'll tell you about the tops and freezing rain, since they are present in the graph I posted.

1. Tops. You can only do this if there are clouds with no lifting action, which means stratus clouds, as is fairly common in the winter but not so often in the summer. Assuming you have stratus conditions, pick an altitude and find the temp and dew point at that altitude:

• If the two curves (temp and dew point) coincide, i.e., lay on top of one another, then at that altitude there's a good chance (not a certainty) that you're in IMC.
• If the two curves are separated by at least 3 or 5 degrees C then at that altitude you're almost certainly in VMC.

Look at the example I provided:

• the two curves coincide from the ground up to 2399 meters, so you would expect to be in IMC at those altitudes.
• The two curves separate above that, so you would be in VMC up there.
• The altitude where the curves separate by 3 or 5 degrees C is about 3000 meters, so that's your predicted top.

2. Freezing rain.

• Rain formation is possible at some altitude if the temp is above freezing there, and the two curves coincide.
• As the rain falls, if the temp curve drops below zero C, then you would expect possible freezing rain at those altitudes.

 

[RV10flyer]

Thanks No Heat.

 

[bbchien]

Yup. the extreme rightward slope of the temperature and dew point lines with the convergence going right of the 0C line, and -4 at the surface tells you the warm air aloft is carrying TONS of moisture that will all be solid when it gets to the ground....

Tops at about 8,000.....subject to verification of the sat. picture of temp of the tops at about minus 8 C.

Similar here. It's sleeting pretty good.