How does atmospheric pressure affect boiling point?

When the is equal to the vapor pressure of water, the water will boil.

Let's go back a second. "Vapor pressure" is just a term that's used when dealing with a liquid that's evaporating (i.e. becoming a vapor). Because this evaporated liquid is now a gas, it has a pressure like any other gas.

If the atmospheric pressure is exactly 1 atm, the boiling point of water is 100.0 degrees Celsius. This is because the vapor pressure of water is 1 atm at this temperature. If you increase the pressure, the boiling point will increase because more energy will be needed to raise the vapor pressure to the increased atmospheric pressure. Likewise, if you decrease the pressure, the boiling point will decrease.

This is handy in a couple of ways: 1) If you want water to stay a liquid, just pressurize it a lot. This is done in pressure cookers to allow food to cook more quickly in water at a temperature of several hundred degrees. 2) If you want water to evaporate quickly, just decrease the pressure. A piece of equipment called a "Rotovap" removes solvents from chemical in this way.