HOW AND WHY DOES ALTITUDE AFFECT BAKING?
The higher in elevation, or distance above sea-level, the lower the air pressure. Low air pressure can lead to batters that rise quickly then deflate once they leave the oven, or cause baked goods to dry out as liquids evaporate at a higher rate. At higher elevations, the thinner and drier air weighs less, putting less pressure on baked goods, which contributes to batter rising more quickly. This environment can cause baked goods to fall flat when you take them out of the oven, which is why a common modification is to reduce leavening agents when baking at high altitudes.
Another effect of lower air pressure is faster evaporation. When liquids evaporate at a higher rate, it can dry out baked goods and also create higher concentrations of sugar, since water and sugar bind together.
To accommodate for these conditions, high altitude baking will call for adjustments in baking times, oven temperature, amount of liquid, fats, leavening agents and even the type of flour you’re using.