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The temperature at which water boils declines as elevation rises, so a longer time is required to prepare hard-cooked eggs at higher altitudes. Colorado cooks will need to do their own testing with hard-cooked eggs to obtain the desired results, but a good place to start is to use the following method adapted from the American Egg Board.
You can also learn more about high altitude cooking adjustments by visiting http://farmtotable.colostate.edu/prepare-high-altitude.php
The following method for preparing hard-boiled eggs has been modified for cooks living above 5,000 feet:
High Altitude Hard-boiled Eggs
For easier peeling, use eggs that are 7-10 days old.
Place eggs in saucepan large enough to hold them in single layer. Add cold water to cover eggs by 1 inch. Cover pan and heat on HIGH just to boiling. Reduce heat and simmer for 5 minutes.
Remove pan from burner. Let eggs stand, covered, 15 minutes large eggs/20 minutes extra-large eggs.
Drain immediately and serve warm, or cool completely under cold running water or in bowl of iced water, then refrigerate.
Get lots of great egg information and recipes at http://www.incredibleegg.org/