Supercooling and Superheating Written by: HON Kai­kwong December 2010 It is well known that boiling water using microwave oven could be dangerous. The threat actually
involves a physical phenomenon called "superheating". Generally speaking, a substance
undergoes a "phase change" from liquid state to gaseous state when it is heated to its boiling
point. Taking water as an example, pure water boils at 100oC under standard atmospheric
pressure. However, containers with smooth surface, such as a glass, and the relatively static
heating environment inside a microwave oven are unfavourable for the formation of steam
bubbles. Water is prevented from converting into steam even it is heated to or above its boiling
point, thus delaying the boiling process. The water is then said to be in a "superheated"
state. Sudden boiling could be triggered when the superheated water is perturbed, such as by
adding sugar or stirring with a teaspoon. The abrupt expansion of steam bubbles could splash
hot water causing scalding (Fig. 1). Fig. 1 The danger of superheating water. Conversely, in an environment lacking "condensation nuclei" which facilitate the solidification
process, a liquid would be "supercooled" when its temperature drops below the freezing point
without solidifying. Supercooled water actually occurs naturally in the atmosphere. Temperature
decreases with height in the troposphere. Depending on latitude, temperature generally falls
below 0 oC at altitudes above 3 km to 5 km ­ the height of middle and high clouds. In lack of
"condensation nuclei" such as suspended particulates or existing ice crystals to trigger freezing
at such altitude, water droplets in "cold clouds" with temperatures below freezing point can exist
in the form of supercooled water (Fig. 2). Experiments even showed that supercooled water
droplets suspended in air can remain in the liquid state at temperatures as low as ­40 oC! Fig. 2 Regions with possible supercooled water (shaded blue) can be identified using processed infra­red satellite imagery. Back to content References: 1. C.D. Ahrens, Essentials of Meteorology 2. J.M. Wallace, P.V. Hobbs, Atmospheric Science: An Introductory Survey 3. Wikipedia