If you live at more than 1000 feet above sea level, then the processing times and pressures given in almost all canning recipes don't apply to you. You need to adjust those numbers in order to can food safely at high altitudes. Don't worry - the adjustments are really simple.
But first let's look at why you can't just follow the same instructions your lower altitude-dwelling friends do (and by the way, the majority of people live at those lower altitudes, which is why most recipes are written with low altitude instructions).
Boiling water bath canning relies on a combination of high acidity in the food and the heat of boiling water to safely preserve the food. But water boils at different temperatures depending on the altitude. This is because the higher the altitude, the lower the atmospheric pressure.
Up to 1000 feet/305 meters above sea level, water boils at 212 F/100 C. But at 2500 feet/762 meters, water boils at just 207.1 F/97.3 C. Since the temperature of the water is part of the safety factor in boiling water bath canning, that temperature difference is significant.
Pressure canning relies on temperatures higher than that of boiling water to safely preserve low-acid foods (such as unpickled green beans). This too is affected by the lighter atmosphere at high altitudes.
To adjust recipes for high altitude canning, start with these two basic concepts:
For boiling water bath canning, add processing time at high altitudes.
For pressure canning, increase the pressure at high altitudes.
Here are the specifics:
Boiling Water Bath with Processing Time of More Than 20 Minutes
1001-3000 feet/305-914 meters - increase processing time by 5 minutes
3001-6000 feet/914-1829 meters - increase processing time by 10 minutes
6001+ feet/1829+ meters - increase processing time by 15 minutes.
For example, if the recipe calls for processing jars of tomatoes in a boiling water bath for 35 minutes and you live at 5000 feet above sea level, you'll need to process them for 45 minutes instead.
Boiling Water Bath with Processing Time of Less Than 20 Minutes
1001-6000 feet/305-1829 meters - increase processing time by 5 minutes
6001+ feet/1829+ meters - increase processing time by 10 minutes.
Okay, on to pressure canning.
Most pressure canning recipes call for processing at 10 psig (pounds of pressure per square inch guage). If you're using a pressure canner with a deadweight guage, the kind that shows 5-10-15 psig, increase the pressure to the 15 psig setting if you are more than 1000 feet above sea level.
For pressure canners with dial guages, adjust pressure in increments as follows:
1001-3000 feet/305-914 meters - increase pressure by 2 psig
3001-5000 feet/914-1524 meters - increase pressure by 3 psig
5001-7000 feet/1524-2134 meters - increase pressure by 4 psig
7001+ feet/2134+ meters - increase pressure by 5 psig
In other words, if the recipe calls for processing your jars of food in a pressure canner for 20 minutes at 10 psig, and you are at 3500 feet above sea level, you will still use the 20 minute processing time but you will increase the pressure to 13 psig.
There are a couple of other things to keep in mind about canning at high altitudes. These are less about safety than your valuable time.
Jellies will reach the gelling stage quicker at high altitudes, and a candy thermometer will not give you an accurate read on when they are ready. At sea level, a reading of 220 F/104.4 C is a semi-reliable way to test for the gel point. Above 1000 feet, that would give you more of a solid paste than a jelly.
Water takes longer to boil at high altitudes. This means it will take your boiling water bath or pressure canner longer to reach readiness. Keep that in mind when you're planning an afternoon of high altitude home canning!