Russ, if you have a cheap gauge I would definitely replace that first. I had this same problem when I thought my pressures were climbing one time. Turned out to be a worn out, sticking gauge. I'd replace the gauge with a new good one. Of course this won't tell you squat about whether you had a pressure increase occur since you don't know if the old gauge was reading correctly. But if the new gauge reads something you can live with just sit tight and see if it changes over time. In the mean time do all the checks Bill mentions just to make sure you don't have something restricted. I didn't comment on this before but Bill is right that warmer air will not cause higher pressure when you are pumping it through a hose and diffuser. Warm air does increase pressure in an enclosed container but that's because it is enclosed and the air is expanding. In your aeration system warm air will also expand but will travel out of the system under the same pressure it would normally take to pump air through the pipes and out the diffuser. The pressure required to push xx gpm through your system is mostly a function of how much restistence there is to flow. It is true that a small un-measurable drop in pressure might occur since escaping air is slightly less dense and thus may slip through the diffuser a little easier. But this is very negligeable when talking the small temp changes you are experiencing from the weather.

Gotta get back to fishin!