The boiling point of a liquid is the temperature at which its vapor pressure is equal to the pressure of the gas above it.The normal boiling point of a liquid is the temperature at which its vapor pressure is equal to one atmosphere (760 torr). Microscopic view inside a bubble in boiling water. read more
When atmospheric pressure > vapor pressure, there is a net force for the atmosphere to push down on the water, leading to relatively small liberation of water vapor. When vapor pressure >= atmospheric pressure, the water vapor can do as it pleases because there is no net force pushing down on it. read more
If the pressure is maintained externally, say by a piston, there is no possibility for the water bubbles to form at any other pressure than the vapor pressure. However when you boil water at the bottom of a pot, two phenomena alter the situation slightly. read more
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