Date: October 6 2017 Time: night (as defined in pilot speak as 1 hour past sunset and 1 hour prior to sunrise) Aircraft: Piper Cherokee 140 Flight: Falcon Field Mesa, AZ - Tucson International Although it had never happened to me, I've read that if a door pops open in flight it isn't a big deal you can still fly the airplane. I found that this is true, however it may get cold and very noisy in the cockpit. I've had an issue with the top latch on my Cherokee for a while. I've always had to pull extra hard on the top of the door to get the latch to catch. This particular time it took me a while but I thought I had it latched. As I was climbing out of KFFZ, the top of the door popped open. I tried several times to get it latched but in a climb and in the dark and below class Bravo airspace, I decided I'd wait until I got up to 5,500' to trim it out and try again. No luck, so instead of turning around and landing back at FFZ, I decided to continue on to Tucson. I was VFR all the way until I hit the class C airspace so I never talked to anyone. As soon as I keyed the mic the noise was pretty bad. I tried cupping my hand over the mic but that made it worse. I tried moving my head as close to the glare shield but that didn't help either. I just lived with it and hoped the controllers didn't complain that they couldn't understand me. Surprisingly, nobody seemed to notice and I landed without incident at Tucson. The next day I broke out my service manual and found the fix. Take off the two screws that hold the strike plate and latch loop in place. Turn the latch loop one half turn counterclockwise (anti-clockwise if your reading this in Australia and other regions that use this colloquial) and reassemble. The door latched without effort. Easy two minute fix. If there ever is a next time, I'll know what to do.