Umm geez guys google LN2 and letthe teachers show you (I watched a bunch of video from the science teachers) and the thing is (and why you can reach your hand into a tube with LN2)

The cold is so cold it actually stays at teh bottom.

Coreolis effect anyone? (however you spell it) simple physics.

Temp at hte top of a half empty dewar is hugely different than the bootom... and air is so nirogren rich I guess... no thats not right...

geez dunno why LN2 does not boil away in a semi enclosed container.

My guess as to why it doesn't, would be a very low level of entropy. Heat rises, cold sinks, so the gas that comes off the top is still colder than surrounding air, and is thus sinking below the air, and sort of creating a vacuum seal of it's own (like forcing an upside down cup into a aquarium).

My dewar operates at 12psi (the heater on it auto-shuts off once 12 psi is reached, not that i've gotten to use the dang thing) but that's just mine.. they make the same model with a slightly different manifold and it goes to whatever psi you dial in. More manifolds need a feature i didn't even realize i was taking for granted, a fill level indicator. Even after months (possibly years) of storage my dewar still has "something" making it read not quite empty and shoots out gas if i open the vent.

No clue mate since I never had a leaking one. I mean you could test it yourself by purging it with pressured air. Make up rig that fits the top with a gauge and a ball valve and than pressurize this thing.