[snowflake]

Why is it that on the 500mb thickness charts there is a pronounced demarcation at the 552 value (a change from yellow to green and a thick black line). It isn't Evan a 1000-500 value. It cant be where the polar front or jet stream live, all I can think of is that it gives a rough guide to boundary between the warm and cool air masses (polar front again?) 

[Dave W]

I dont think it means anything especially..

[PaulKn]

I agree - I think it's just an arbitrary value taken as the swap-over for the different colour schemes...I suppose you could say that typically colder airmasses are below that value, and warmer ones above, but you could equally well pick a different one!

[snowflake]

Thanks Dave and Paul
It just seemed to my untrained eye that it signified something that I was missing, thanks again