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