The expansion of alcohol in a thermometer is one of many commonly encountered examples of thermal expansion, which is the change in size or volume of a given system as its temperature changes. The most visible example is the expansion of hot air. When air is heated, it expands and becomes less dense than the surrounding air, which then exerts an upward force on the hot air to, for example, make steam and smoke rise, and hot air balloons float. The same behavior happens in all liquids and gases, driving natural heat transfer upward in homes, oceans, and weather systems. Solids also undergo thermal expansion. Railroad tracks and bridges, for example, have expansion joints to allow them to freely expand and contract with temperature changes.
Water is the most important exception to this phenomenon. Whilst water does expand with increasing temperature (its density decreases) at temperatures greater than 4 °C (40 °F), it is densest at +4 °C, and also expands with decreasing temperature between +4 °C and 0 °C (40 °F and 32 °F). A striking effect of this phenomenon is the freezing of water in a pond. When water near the surface cools down to 4 °C, it is denser than the remaining water and thus sinks to the bottom. This “turnover” leaves a layer of warmer water near the surface, which is then cooled. However, if the temperature in the surface layer drops below 4 °C, that water is less dense than the water below and thus stays near the top. As a result, the pond surface can freeze over. The layer of ice insulates the liquid water below it from low air temperatures. Fish and other aquatic life can survive in 4 °C water beneath the ice, due to this unusual characteristic of water.
