High altitude cooking can be tricky. This is primarily due to the decrease in barometric pressure. At sea level, the boiling point of water is 212° F. But as the pressure drops, so does the temperature of the boiling point.
The rate of drop is about 2° F. for every 1,000 feet (305 m.) elevation gain — so, at 10,000 feet (3,048 m.) the boiling point is only 192° F. As a result of this, cooking times nearly double every 5,000 feet (1,524 m.), or 10 degree drop in boiling point temperature.
For example, if the cooking time for any given dish is five minutes at sea level, at 10,000 feet it would be nearly 20 minutes! So basically what this boils down to (pardon the pun!) is an increase in cooking times of about 20% per 1,000 foot altitude gain.