Pasted from "Cigar Smokers FAQ":

Must cigars be stored horizontally?
Cigars are stored horizontally because in a completely still environment moist air is very slow to mix with drier air. It obviously does stabilize eventually, but with a humidor's frequent openings and closings, this "layering effect" may make your stogies burn irregularly. You can minimize the effect by storing cigars horizontally (and not opening your box too often). I'd recommend a tupperdor over a jar for this reason.

Sliding cigars in and out of a cigar jar can often damage the ends of their delicate wrappers - it's best to subject your cigars to as little handling as possible. A shoebox size tupperdor hardly costs more than a single good cigar, and you'll be able to store more cigars (and longer ones). Picture an 8" tall jar (big enough for larger 7 1/2" sizes) with a few 4 1/2" robustos mixed in. You'll have to dump them all out to get at the shorter ones!

I too tried a jar some time back. It was very hard to keep the RH even. When I would first refill the little hygro the RH would be way too high, then after a few openings it would quickly drop too low.