I was wondering how to tell when your disk brake pads need replacing. My front seems to be not stopping as quickly as it used to, or as I would like. I know this could be road grime build up, but I am not sure the indicators for replacement time for the pads.
I change mine as soon as they don't work as expected. You usually notices this when you have to apply an unusual amount of force on he levers to make the bike brake. It really depends on how much you use the bike, it can be enough to go out in the rain and brake hard a couple of times to destroy a set of break pads.
Shimano and SRAM specify minumum pad thickness, which you can look up. Park Tool suggest at least 1mm, which you can measure with 3 business cards.
I've had disc brake pads become poor performing well before they are worn out, with no recovery possible, so I now keep a small stock of relatively cheap pads to swap out when I start to get performance issues.
Causes of poor performance? Hydraulic fluid leaking from brake cylinders (even tiny amounts you cannot see) or burning of pads by over-exuberant use before they are bedded-in. I'm getting better at both these issues!
I never change mine until I hit metal on part of the pad. Hitting metal is probably the only indicator that accurately predicts the true state of wear on the pad. All the other "feelings" that maybe the brakes aren't performing as well as they used to are generally just based on the conditions they've been subjected to over the past few days (as OP noted). I've had pads that worked like complete garbage in the rain, and after I took them on a gentle downhill in the sunshine, worked amazing in all conditions afterwards. Side note - that's also how I get the screaming banshee sound to go away on a loud set of pads.
Basically, if you're noticing a difference in braking quality at the top of the pad vs the bottom, you should be very concerned about the manufacturing process that the pad manufacturer is using. There should be no difference between the top of a sintered pad (I assume the same goes for organic) and the bottom. As long as you have some pad material left over the entire area of the pad, you're good.
And yes, I assume that the OP has replaced pads by now :)