UPDATE: Fondriest Environmental has a new water quality online knowledge base. This resource provides an updated and comprehensive look at water temperature and why it is important to water quality. To learn more, check out: Water Temperature.
Water temperature, of course, expresses how hot or cold the water is. Technically, heat is an indicator of the kinetic energy of water, or energy of motion. Increasing temperature indicates increasing energy, or molecular motion, of water.
Water temperature affects the growth and reproduction of living organisms. Many animals use temperature as a signal for when to reproduce and when to migrate. Generally, animals and plants grow faster at warmer temperatures, although all organisms have an upper temperature limit.
Water temperature has a tremendous impact on water density. Differences in water temperature and density cause stratification.
A property that is unique to water versus other substances is that it is most dense at 4 degrees Celsius, or 39 degrees Fahrenheit, and is less dense at either higher or lower temperatures. Most other substances continue to become denser as their temperature drops. Water at higher or lower temperatures will float on top of water that is 4 degrees C. Because ice is less dense than the underlying water, it floats, insulating the warmer water below. Without this unique property, life on Earth would probably not exist.
Temperature is usually expressed in degrees Fahrenheit or Celsius. Zero degrees Celsius is equal to 32 degrees Fahrenheit, and 25 degrees Celsius is equal to 77 degrees Fahrenheit.
Water temperature can be measured with a thermistor, which is a metallic device that undergoes a predictable change in resistance in response to temperature changes. This resistance is measured and converted to a temperature reading in Celsius, Fahrenheit, or Kelvin.