I moved to Logan, UT (∼4,500 ft.) a few years back and lived there for four years to obtain my bachelor’s degree in biology. Though I didn’t make candy a whole lot in college, I dabbled here and there, including a couple of attempts at caramel. My first few tries didn’t quite work out. Truthfully, they were a disaster. I wasn’t adjusting recipes for the high altitude, so each batch resulted in burned caramel and tears of frustration. It was then that I realized I wasn’t in Kansas anymore (Thought I would get the Wizard of Oz joke out of the way, as anybody who hears I’m from Kansas usually references it in some form.) and learned how to modify temperatures for candy making at high altitude. I’m now living just outside of Denver, CO, and these adjustments are finally becoming somewhat automatic for me.
How to adjust:
For every 500 feet above sea level, the boiling point of water (212° F at sea level) decreases by 1° F. Therefore, for every 500 feet in elevation in which you live, the temperature stated in your candy recipe needs to be reduced by 1° F. Here in Denver, CO where the altitude is around 5,300 feet, I lower the temperature by about 11º F (5300/500 ≈ 11).
The explanation behind this is pretty simple. As altitude increases, the density (and therefore pressure) of air decreases. Because of this decrease in air pressure, it takes less energy on average for water molecules to break free from their bonds with one another and evaporate, so water loss occurs at a faster rate. This causes sugar mixtures to become too hard or grainy if the temperature normally stated in recipes is not lowered. Adjusting the temperature allows for the correct amount of water evaporation so that the final candy product has the proper consistency and sugar concentration. Easy fix, right?