How It Works Magazine » freezing

What is absolute zero?

Jonny O'Callaghan — Sat, 17 Dec 2011 08:00:36 +0000

Asked by Dan

Absolute zero is a temperature 273 degrees Celsius (459 degrees Fahrenheit) below freezing. Nothing can ever be colder than absolute zero, just as it is impossible to have less than no thickness. The colder something is the harder it is to cool it any further. This is the bottom of the scientiﬁc temperature scale, or zero degrees Kelvin. This way of measuring temperature is named after Lord William Kelvin who devised the theory behind the unit in the middle of the 19th century.

Tacye Phillipson, Science curator, NMS

How cold would the sea have to be to freeze?

Robert Jones — Thu, 03 Jun 2010 14:56:29 +0000

Pure water contains no impurities and freezes at 0°C. However, seawater – on average – contains 3.5 per cent of dissolved salts, which lowers the freezing point; this phenomenon is called freezing point depression. Freezing point depression is a colligative property of matter, which means it depends on the number of molecules present, not on the specific type of molecule or their mass (i.e. the dissolved substance doesn’t have to be salt, but any soluble substance, it’s the amount of dissolved molecules that is important). In terms of seawater, the average freezing point is about -2°C. However, it does vary around the world due to differing amounts of chemicals (e.g. salts) dissolved.

Britain’s big freeze – 7 January 2010

Helen — Fri, 08 Jan 2010 10:52:05 +0000

Image courtesy of NASA

The Moderate-Resolution Imaging Spectroradiometer (or MODIS) on board NASA’s Terra satellite captured this incredibly chilling image of Great Britain on 7 January as most areas experienced freezing temperatures of -10C on the coldest night of the winter so far. Isolated areas were even subjected to lows of -22C in the Scottish Highlands.

Snowfall followed by freezing temperatures left areas dangerously icy, resulting in widespread chaos on the roads. Schools across the nation were closed and many businesses sent staff home. The emergency services, including roadside assistance, were inundated with incidents.

One possible reason for the severely cold conditions across the Northern Hemisphere this last month is an extreme negative phase of Arctic Oscillation (AO). AO refers to a state of seesawing atmospheric pressure over the Arctic. The negative phase of AO causes high pressure over the polar region and low pressure at mid-latitudes.

MODIS images the entire Earth every one to two days. The grey areas against the white land surface on the map are actually the cities of Manchester, Birmingham and London.