Why does the boiling point of water change at high elevation (maybe in Denver, Colorado, the Mile-High City) compared to sea level (maybe in Ocean County, New Jersey)?

Explanation:

This is due to a reduction in atmospheric pressure – pressure due to the weight of the overlying air column). Remember that the higher the altitude the lower the air density and subsequently the lower the air pressure.

Therefore when heating water at higher altitude the vapor pressure of the water at the surface overpowers the atmospheric pressure – which is slightly lower than at sea level – even before the water temperatures reach 100 degrees centigrade.