The local altimeter measures the weight of the air above it. When
temperature is higher, the air expands (less density), but it's the
same amount of air in a taller column. The atmosphere expands you see
and is not bound tightly by a blocade above (it is bound by gravity
actually). Anyway another item of interest is what you are setting the
altimeter to is "sealevel" barometric pressure. Imagine a hole in the
ground and the altimeter is lowered to the bottom of the hole (at your
airport). Of course they dont actually have a hole, they have another
way of adjusting it. Standard barometric pressure at sealevel is about
29.92. If you fly an airplane with a manifold pressure guage, it gives
the local "absolute" pressure (non-sealevel adjusted). Anyway consider
these principals and your knowledge will expand.