Freezing Water

Last Updated on Tuesday, 13 January 2009 16:25

The unusually cold weather is over, for the moment at least, with temperatures locally having dropped as low as -10°C. These temperatures have resulted in ice along the seashore, something not often seen on the South Wales coast. This has prompted a number of people to ask:

At what temperature does sea water freeze?
Simple answer: sea water freezes at -2°C.

As with all simple answers there are many complex ones as well. Staying on the simple tack, the two major factors are temperature and salinity. Temperature is readily appreciated. Go for a swim off Rhossili in January and you may well find it a touch chilly, but the salinity level of the water would probably not cross your mind.

Salinity is the saltiness or dissolved salt content of a body of water. This depends on many different elements and in turn is influenced by temperature. A given quantity of sea water will be 96.5% water with the remaining 3.5% being composed of salts.

Traditionally salinity was expressed as parts per thousand (ppt) which is approximately grams of salt per litre of solution. Since 1978 a new standard was generally adopted. The Practical Salinity Scale (PSS) is based on the conductivity ratio of a sea water sample to a standard potassium chloride (KCl) solution. The general figure used for sea water is 35 PSS.