Question:

Greetings! We are going on vacation across America and will be staying at many different elevations. What is considered High Altitude for baking?

Answer:

The principle adjustments recommended for cookies baked at higher altitudes (generally considered to be above 3,000 feet) are to increase the water slightly, to help the dough come together, and to decrease the amount of chemical leavens (baking powder, baking soda) used.

For additional help please read more at the King Arthur Flour website.

Souce: kingarthurflour.com/learn/high-altitude-baking.html