One thing I rarely mention in my recipes is the necessity to adjust cooking and processing times if you live more than 1,000 feet above sea level. I don’t bring it up often because even in my 20th floor apartment, I don’t come close to being that high up (the bulk of Philadelphia is at sea level and the highest portion of the city doesn’t go more than 500 feet above sea level).
Thing is, not all of you live in my lovely city and so elevation is something you do need to keep in mind. The reason it has an impact in canning is that once you get more than 1,000 feet above sea level, the temperature at which water boils gets lower (there’s a calculator here that allows you to plug in your altitude and get your specific boiling point).
If you use a thermometer to monitor the progress of your preserves, you don’t have to do too much to adjust during cooking. Just know that when your jam comes to a boil, it could still be a few degrees shy of 212° and may still have quite a way to go before reaching its set point.
However, elevation has more of an impact on the processing of preserves because once water boils, it can’t get any hotter. This means that even if your canning pot is happily boiling away, it might not be as hot as you think. The way that the USDA and the National Center for Home Food Preservation has you compensate for this temperature differential is by increasing processing time. Here’s the guide for making these adjustments.
1,001 to 3,000 feet, add 5 minutes
3,001 to 6,000 feet, add 10 minutes
6,001 to 8,000 feet, add 15 minutes
8,001 to 10,000 feet, add 20 minutes
If you live above 1,000 feet, you also have to adjust the amount of pressure you apply during pressure canning. The rule of thumb is that you need an additional 1/2 pound of pressure for every 1,000 feet you are above sea level. If you have a weighted gauge canner, you’ll just use the 15 pounds of pressure setting for any recipe that calls for 10.