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Thickness… Its meaning?

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Reading a discussion on
http://forum.weatherzone.com.au/ubbthreads.php/topics/1270374/4

and thanks to contributor ‘inversion’ here is a start to answering this question
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I’d like to know what the thickness lines mean/represent?

thickness diagram
source
http://www.bom.gov.au/australia/charts/viewer/index.shtml?type=mslp-thick&tz=AEDT&area=Au&model=CG&chartSubmit=Refresh+View
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‘Inversion’ says
“‘Thickness’ is a measure of how warm or cold a layer of the atmosphere is, usually a layer in the lowest 5 km of the troposphere; high values mean warm air, and low values mean cold air. Some approximate reference levels for freezing level are:
5240 – 300m (snow to sea level in parts of Vic (10 Aug 2005)
5280 – 600m
5340 – 1000m
5400 – 1400m

Last night neared 5280, with Melbourne sounding showing freezing level at around 800-900m)

Doesn’t have to be zero for it to snow. Snow flakes may fall through sub-zero temps all the way to the last few hundred metres, when a warm layer melts them. The temp and height of this melting layer is the key. Yesterday (last night) this layer was 5 deg up to 300m, 3 deg at 500 and freezing near 800-900m. This was enough to allow snow to fall in places around 500-600m, but generally melted lower than this.

The other key factor is down-drafts from heavy showers/storms which can locally bring freezing air right down to the surface, allowing snow. I’ve witnessed this at Creswick in 2000 with an intense down-draft rapidly plummeting the temp from 7 deg to zero in a matter of minutes, allowing snow, then it went back up to 7deg 20 minutes later.”
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2 comments on “Thickness… Its meaning?

  1. Ken kato has posted a nice explanation of the term

    thickness

    “Thickness is basically the depth (in metres or decametres) of the atmosphere between two pressure heights, typically the 1000 and 500hpa levels i.e. how thick or thin that layer between the two levels is.

    It indicates how cool or warm that layer of atmosphere is. The cooler the layer, the more “squashed” that layer will be and the lower the thickness value. Vice versa for a warm layer. 1000 to 500hpa thickness values of 5400m (540dm) often correspond to snow potential on terrain above around 1000m but that’s only a very rough rule of thumb because other factors such as moisture, any cold/warm layers near the surface or aloft, etc also have an effect.”
    source
    http://forum.weatherzone.com.au/ubbthreads.php/topics/1336739/Re_2015_Australian_Snow_Winter#Post1336739

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