Air in Water lines

A friend of mine said his power went off a week or so back, and now he's got air in his water lines to his house— He is on a well. I told him before I put a oneway valve on my house, with the well 200 feet down the hill, if I didn't "catch" the problem in time my whole house, (including the hot water tank) would drain down, but after a few days the air was gone...

So, what are the many possible ways for air to get in your water lines?

Thank you in advance!

The usual/primary cause is a leaking check/foot valve in the well; all submersible pumps have one in/on the outlet. Adding one at the pressure tank hides the symptoms of that or other leaks between the additional check valve and the one in/on the submersible pump.