T

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 mech-
anisms are examined in theheating and coolingarticle. The rate at which heat is
transferred is expressed in watts (joules per second) and is maximized as tempera-
ture differences between objects become large. The net transfer of heat is always
from the hotter to the colder object.
All molecules in theEarthsystem vibrate and as they gain heat they vibrate
more rapidly. This energy of motion is kinetic energy and is responsible for radia-
tion 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 temper-
ature 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 tempera-
tures so it is a simple matter to calibrate the thermistor to known temperatures.
The third type of device is a radiometer, which is aremote sensingdevice that
detects invisible thermal infrared energy. By noting the most plentiful emitted
wavelengths from a target, its surface temperature can be determined. Satellite

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