Consider a situation with high pressure at the surface and low pressure at a higher level (18000 ft or above), as for example in a cold high. 

Which isobaric vertical cross-section represents the correct shape of the pressure levels?

（含图）

Consider a situation with low pressure at the surface and low pressure at a higher level (18000 ft or above), as for example in a polar front low. 

Which isobaric vertical cross-section represents the correct shape of the pressure levels?

（含图）

The air density...

When...

"Transition altitude"...