Why density of air decreses with altitude

Just to get to the answer to that question yourself :

Go with friends or children to the beach. Dug a very deep hole. Jump into that hole and ask the others to fill up the hole.
What do you feel increasing while they fill in the hole?

:)

In many ways, gases behave like fluids. The same as water pressure is greater at the bottom of the ocean, the weight of our atmosphere is greater at its' bottom than its' top... this difference is measured as "atmospheric pressure" and has a direct impact on density.