It takes longer for water to boil at a higher altitude because the air pressure is lower. Further, water evaporates faster but gases expand more. For those of you living in higher altitudes, changes are you have already learned to adjust your cooking and baking recipes. It is no different when pressure canning.
When pressure canning, the total processing time per recipe remains the same but the pounds of pressure must be adjusted based on the number of feet you are above sea level. Use this simple chart to make adjustments for either a dial-gauge or a weighted-gauge pressure canner. If you are not sure of your altitude, be sure to click this link.