Temperature

All matter contains heat (whether or not it is hot to the touch). How much heat is present relates to a combination of the mass of an object, its temperature, and how that mass is arranged. “Heat” and “temperature” are commonly used as synonyms. Instead, they are related but separate concepts. Heat is the aggregate internal energy of an object and is transferred by conduction, convection, and radiation; these mechanisms are examined in the heating and cooling article. The rate at which heat is transferred is expressed in watts (joules per second) and is maximized as temperature differences between objects become large. The net transfer of heat is always from the hotter to the colder object.

All molecules in the Earth system vibrate and as they gain heat they vibrate more rapidly. This energy of motion is kinetic energy and is responsible for radiation of electromagnetic energy. Temperature is an expression of the average kinetic energy of a substance and this measurement provides a ready determination of how much and which wavelengths of energy are being emitted. There are three common ways by which temperature is measured. The first dates back to Renaissance Europe and is the liquid-in-glass thermometer. In this device, a liquid is placed in a sealed glass tube with a bulbous reservoir on the bottom. The liquid volume visibly expands on heating and contracts on cooling. The first thermometers contained water, but now have liquids with freezing temperatures below freezing. Mercury was used for many years but its toxicity and cost has caused it to be largely replaced by red-dyed alcohol. The other two devices became quite common in the late 20th century because of their abilities to measure temperature without a human being present. The second device is a thermistor, which is based on precise calibration of electrical resistance in a metal probe. Small—

typically a centimeter or less long—thermistors are frequently made of parallel platinum alloy wire heads together with semiconductor paste. A small current is passed from a source to the thermistor and the resistance measured. The amount of resistance decreases considerably over small increases in atmospheric temperatures so it is a simple matter to calibrate the thermistor to known temperatures. The third type of device is a radiometer, which is a remote sensing device that detects invisible thermal infrared energy. By noting the most plentiful emitted wavelengths from a target, its surface temperature can be determined. Satellite radiometers are able to “take the temperature” of clouds, Earth’s water and land surfaces, and vertical atmospheric temperature profiles.

There are three scales of temperature in common use. They are Celsius, Kelvin, and Fahrenheit. The Celsius scale has 100 degrees between the freezing and boiling of pure water at sea level pressure. This is the scale in the most common use in the world and is the International System of Units (metric system) official scale. A Kelvin is the same “size” as a degree Celsius but the Kelvin scale has different zero point. Kelvins are based on absolute temperature. Every molecule vibrates until it is as cold as absolute zero (−273°C), which is defined as 0 Kelvins. Average room temperature is 293K (22°C). Kelvins are used heavily in the sciences, especially physics, chemistry, and the atmospheric sciences. The Fahrenheit degree is about 5/9 the “size” of a Celsius degree and there are 212 degrees between freezing and boiling with 0°F being considerably colder than 0°C. Very few countries besides the United States use Fahrenheit as their primary temperature scale.