S.G.

Hey guys can I ask a question please. When I view the 850hpa temp & SLP chart I would like to know why the High pressure 1024 is in the Atlantic when the thickness line is 10-15 but when it comes over the high pressure start to decreases when the thickness line is near to 20. When I see towards the Mediterranean the surface pressure is lower at 15-20 on the thickness line which it shows its still high pressure if it is or not but can you explain me how it does this and a site which explains it and when the high pressure move out the way the red indicating tend to move towards the north of the country.