At what temperature does water have to freeze?  The answer: Not necessarily at zero degrees Celsius (32 degrees Fahrenheit). Water can exist in liquid form well below this threshold, notably in a so-called \"supercooled\" state that is well into the minus zone. The reason: if you want to form ice from liquid water, you need a \"seed\" of ice from the liquid -- a crystal that becomes the nucleus around which other crystals form. But in very pure water, which has no contaminants or particles around which the critical nucleus can form, this can be difficult to achieve because of the unusual thermodynamics of H20. Until now, supercooled water has been measured right down to around -41 C (-42 F), although scientists have long suspected that the temperature at which it unconditionally has to freeze is somewhat lower. They have been unable to find out for sure because ice crystallises so fast at this temperature that it is impossible to measure accurately the properties of the remaining liquid. Chemists Valeria Molinero and Emily Moore at the University of Utah used computer modelling to simulate the behaviour of supercooled water at the microscopic level. Their programme mimicked what would happen when 32,768 water molecules were cooled, factoring in the heat capacity of water, its density and compressibility. After thousands of computer hours, the answer came back. The temperature at which water absolutely must freeze, no questions asked, is -48 C (-55F). When water approaches this temperature, it becomes less dense and becomes easier to compress and its structure changes. As a result, each molecule links up loosely with four others to form tetrahedron, or pyramid-like, shapes. The investigators describe this as an \"intermediate ice\" that is halfway between the structure of the liquid and halfway between the full structure of ice. The research, published on Wednesday in the British journal Nature, is more than an exercise in scientific curiosity. Atmospheric scientists delving into global warming need to know temperatures and rates at which water freezes and crystallises into ice. Water as chill as -40 C (-40F) has been found in clouds. \"You need that to predict how much water in the atmosphere is in the liquid state or crystal state,\" Molinero said. \"This is important for predictions of global climate.\"