-10 C isotherm in AFD

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Carl Sidor
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Joined: Sun Nov 30, 2003 1:26 am

-10 C isotherm in AFD

Post by Carl Sidor »

I was recently checking out the AFD, and came across a phrase that left me wondering as to it's exact meaning. The forecaster stated that the freezing rain would rapidly change to freezing drizzle as the -10C isotherm unsaturated. I assume that they are speaking of a sounding isotherm, and and not an isotherm on a mandatory level plot. Am I correct in my assumption? I recently e-mailed the FO to clear up my understanding of the concept of delta -T, and I would hate to bother them again with my ignorance. I don't mind appearing ignorant before you folks!

MarcC
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Post by MarcC »

Hi Carl,

Well, I'm probably just about as ignorant as you are here, but I would guess that the forecaster is speaking about the level of the atmosphere at which the temp was -10C. He's obviously referring to the drying of the atmosphere as a reason why precipitation would be in the form of drizzle, but it seems rather strange that he would refer to a temperature surface rather than a pressure surface. Anyone else have some thoughts on this matter?

Marc

Carl Sidor
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Joined: Sun Nov 30, 2003 1:26 am

-10 C isotherm

Post by Carl Sidor »

Dear Marc,

Thank you for your reply. I think you may be correct in your interpretation. I was looking at a sounding for KGRB in my RAOB program and paid close attention to the data display as I moved the cursor up. Sure enough, the -10C level showed a RH of 78%. At -12C, the RH had dropped. This, I believe, was at about 850mb more or less. Nothing like having the answer right in front of you all the time is there! Thanks again. :roll:

miseryhawk
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Post by miseryhawk »

Check out the following presentation link:

http://www.crh.noaa.gov/lsx/science/gary/sld001.htm

If the atmosphere is saturated at a temperature of -10C or cooler...then there is a high chance of ice crystal formation. If the temperature is warmer than -10C the chance of ice crystals in clouds is less...therefore supercooled water is more likely(i.e. freezing drizzle or drizzle)

Carl Sidor
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-10C isotherm reply

Post by Carl Sidor »

Dear miseryhawk;

Excellent link! It was quite helpful, though the graphics do leave something to be desired. Thank you very much for the reply. Weather people are some of the most helpful people I have ever met. :D

Cordially,

Carl Sidor

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