Almost. Your on the right track. Thickness below 540 means the air
column IS cold enough to support frozen precipitation such as snow.
The 540 isopleth on a 1000-500 mb chart is considered the "snow line".
As the value decreases, the colder the air.

On Tue, 19 Aug 2008 00:11:06 -0700 (PDT), A1C Watkins
wrote:

On Aug 16, 3:41*pm, wrote:
Why is the 540 isopleth significant on a 1000-500 thickness chart?

Thickness below 540 generally indicates a zero percent chance of
frozen precipitation (i.e. snow) so that is pretty significant.