Howcome that whenever you have a change in altitude, your ears pop? And howcome my ears refuse to pop when the airplane that I am on takes off until about half an hour after the plane takes off?
Your ears are reacting to the relatively rapid change in air pressure when you go up in an airplane. During flight, the inside cabin pressure is not the same as air pressure at sea level (I think it’s equivalent to something like 6,000 ft., but don’t quote me on that). 30 minutes is probably how long it takes for an aircraft to reach cruising altitude, which explains the time-delay you experience. If you were to walk up a 6,000 ft. mountain from sea level, the pressure around you changes slowly enough for your ears to not pop (driving, on the other hand, moves you fast enough to induce ear popping, too).
A few of the many explanations found on the Web:
http://www.goaskalice.columbia.edu/1862.html
http://www.globesyndicate.com/adv/2001/062201.html
http://cbshealthwatch.medscape.com/cx/viewarticle/235128_print
Cecil’s column: What’s going on when your ears pop?
And a Staff Report: Why do my ears hurt when I fly in a pressurized jet?