Temperature and pressure DO have an affect on sound speed, but its effects are more noticeable in denser media (like water or wood) and over longer distances (and other factors). Air temperature variations in your house will have very little effect on the frequencies you hear.

I'd say it's more a matter of old strings. Best way to find out is to put on a new set.

It can also be a matter of intonation. If your saddle is uncompensated, tuning may be OK, but the frequency of the string when fretted might be very different from expected. I once had a cheap Kay guitar that had a straight saddle, no compensation at all. It tuned OK, but when I would play certain chords the D or the high E would sound off tune. I'd adjust the strings to make that chord sound right and, poof, all the other chords were out of tune. Very frustrating.

Your house is only 50-60 degrees?? Are you an Eskimo?? 

Jeff

Bob:

So much for us cold weather Minnesotans going to Texas for winter warmth.  I still prefer Hawaii over Texas/Arizona.  Never in the 50s in Hawaii.

If your uke sounded good before, and now is off it most likely is the strings.  One year is more than enough.  Get a new set.

Don