When 100°C isn’t 100°C

The Boiling Point of Water

water boiling

Every school pupil knows that water boils at 100°C (212°F) – this is a bit of conundrum. Obviously 100°C can not be anything other than 100°C but when it comes to boiling water that’s not always the case.

The boiling point of water depends on air pressure and at sea level water does indeed boil at 100°C. 

Temperature Settings on Kettles

Luckily for those of us living in the UK there aren’t many places above 200ft above see level, so we’re okay. The temperature scale on variable temperature kettles are spot on.

However if you live in southern Africa, then things are slightly different, some of these cities are 4000ft above sea level. This means that water here boils at around 96°C. Only a few degrees different but enough to affect our brewing of our tea.  

The reverse happens in those places in the world which are below sea level. The temperature of boiling water in these places is actually above 100°C. However no places  on earth are low enough to have any real affect

How does Air Pressure affect Boiling

A simple explanation is that water boils more easily when the air pressure is lower. For each one thousand feet we go up, the boiling point of water drops by one degree.

The attached video explains, in simple terms, how water boils and the affects of atmospheric pressure.

How Water Boils Around the World

  • Mount Everest

    8848m (29,028 feet) above sea level. Water boils at 69.94°C

  • London

    14m 46 feet) above sea level. Water boils at 99.96°C

  • The Dead Sea

    427m (1,400 feet) below sea level. Water boils at 101.4°C