Altitude, but it's not such a silly question. I've been canning here for a few years and just realized that we're enough about sea level that I should have been adding another 5 minutes to the hot water bath canning. I make sure the gauge on the pressure canner reads a little bit over 240, just to be on the safe side.
Hmmm, I hadn’t thought of it before, but you could easily rack up 1,000’ of altitude change just from a low passing, or move to sea level from a high.
OK, now I am going to have to start watching that barometer next time I can, just out of curiosity. If today it is 29.92” tomorrow night as the low comes in, If it makes it down to 28.82 my effective altitude is then 1,000 feet.
Maybe I should then alter my time or pressure... The Ball Blue Book would have to grow several pages to accommodate that calculation and the related charts.
CottonBall - SEE - there I go again with the engineering calculation stuff...