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Interpreting Freezing Level Chart

Asked by: 1005 views Weather

Okay, this might be a dumb question: how do you interpret the areas that are not shaded in nor included within the freezing level lines? I'm thinking latitudes higher than the boundary between freezing levels and unshaded areas imply the FL is at the surface. But there is a small portion that is specifically shaded to show surface FL (why not shade everything?). Sometimes, the layer between the boundaries is not a FL at the surface but at some higher altitude, which would mean there's a sudden jump in FL. From all this, I gather I'm interpreting it wrong. Thank you.

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5 Answers



  1. Mark Kolber on Jan 08, 2017

    Assuming you are talking about the multi-color chart at https://www.aviationweather.gov/icing/frzlvl, it would be nice is there was a nice box around the white area just to the left of the violet, huh.

    You are interpreting it correctly. The white unshaded areas are where it is freezing or below at the surface. The violet is telling you where the freezing level is somewhere between the surface and 1,000 AGL. Just above freezing on the ground becoming freezing as you climb to pattern altitude.

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  2. Drew on Jan 08, 2017

    Yes, I’m referring to the colored freezing level chart.

    The part that confuses me is the boundary between the FL at SFC and colored areas is sometimes a sudden jump in altitude (e.g. the most recent chart showed a freezing level from 7,000′-9,000′ to a white area). Perhaps a meteorologist can chime in here to explain it hah.

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  3. Best Answer


    Mark Kolber on Jan 09, 2017

    Drew, you don’t need a meteorologist. You need to look at the chart. The very top in large bold print 😀 The altitudes on the freezing level chart are MSL, not AGL.

    So, yes, the “lowest” non-white color you will see for the freezing level in Denver, CO is green for between 5,000 and 7,000 MSL.

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  4. Drew on Jan 09, 2017

    I was initially confused by what you said, but I think I figured out what you mean. I was trying to interpret the chart as using all MSL or all AGL, when the chart actually uses both. The only AGL portion of the chart is the white area, which represents 0′ AGL, and colored area represent elevation in MSL. To mark areas where the FL is at 0′ MSL would be useless, since that would mean almost everywhere would be white. To mark SFC FLs as MSL would require different colors cluttered together and would be especially complicated near mountainous areas, where elevation changes quickly. As long as the FL is above the SFC, MSL is most helpful to a pilot when he gets up in the air, and regions with higher field elevations will naturally have a “jump” in high colored areas to white areas. Thank you, Mark!

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  5. Mark Kolber on Jan 10, 2017

    The confusion is in my first answer where I say 1,000 AGL. That should be MSL but I never figured out a way to edit a post here. Glad you figured it out. White is on the ground, colors are MSL altitudes. AGL would be useless for anything other than a flight around the traffic pattern.

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