2.1: Temperature

Air Temperature

It is commonly thought that air temperature is a measure of how cold or hot an object is. We instinctively know that the higher an object’s temperature, the hotter it will be, and the lower its temperature, the colder it will be. But what does temperature really measure? The answer lies in the motion of the molecules.

Air molecules are constantly in motion, colliding with one another and with objects. As we increase the temperature of a volume of air, the air molecules speed up, and the frequency of collisions increases. As air temperature decreases, molecules slow down and collide less frequently. What is going on here? Temperature is a relative measure of the average kinetic energy (kinetic meaning motion) of the molecules in a system. Simply put, air temperature is a measure of the average speed of air molecules. Warmer air temperatures indicate that air molecules move faster, whereas cooler temperatures indicate that they move more slowly.

Figure \(\PageIndex{1}\): A comparison between the three major temperature scales: Celsius, Kelvin, and Fahrenheit. (CC BY 4.0; Neel Desai)

Table 2.1.1: Comparison between the three major temperature scales.

Reference point	K (Kelvin)	°C (Celsius)	°F (Fahrenheit)
Boiling point of water	373.1	100	212
Highest recorded air temperature (Death Valley, CA)	329.7	56.7	134.1
Average room temperature	293.1	20	68
Freezing point of water	273.1	0	32
Lowest recorded air temperature (Vostok Station)	183.8	-89.2	-128.6
Absolute zero	0	-273.1	-459.7

Temperature Scales

Surface air temperature is commonly given in degrees Fahrenheit (°F) in the United States. If you were born and raised in the US, this is the temperature scale you probably use in your daily life and what you’ll see when you look at the daily weather. You’ll know that 32°F is the temperature at which water freezes, and 212°F is the temperature at which water boils. The highest air temperature ever recorded on Earth is 134.1°F in Death Valley, CA, in July 1913, while the lowest is -128.6°F at the Soviet Vostok Station in Antarctica in July 1983. The average room temperature is around 68°F.

Degrees Celsius (°C) is also commonly used, especially internationally. Celsius is a convenient scale to use because water freezes at 0°C and boils at 100°C, as illustrated in Figure \(\PageIndex{1}\). The information from Figure 2.1.1 is also provided in Table 2.1.1. The record for the highest and lowest air temperatures mentioned earlier, °F, correspond to 56.7°C and -89.2°C, respectively. A difference of 1°C is larger than a difference of 1°F by about 1.8 times. To convert between the two, use the following equations.

\[{ }^{\circ} C=\frac{5}{9} \times \left({ }^{\circ} F-32\right) \nonumber \]

\[{ }^{\circ} F=\left(\frac{9}{5} \times { }^{\circ} C\right)+32 \nonumber \]

The Kelvin scale (K) is almost always used as a temperature unit in scientific equations and is convenient in that it contains no negative numbers. The Kelvin scale begins at 0 K, or absolute zero, where atoms and molecules would theoretically be at rest due to the absence of thermal motion. The Kelvin scale is also sometimes known as the absolute temperature scale. The lowest possible temperature is 0 K, but it does not occur naturally. The coolest naturally occurring place in the universe is the Boomerang Nebula, located in the constellation Centaurus and approximately 5,000 light-years from Earth. The temperature is measured at 1 K, only 1°C above absolute zero. A difference of 1 K is equivalent to 1°C, making the conversion linear and straightforward.

\[K={ }^{\circ} C+273.15 \nonumber \]

Based on this, absolute zero is -273.15 °C or -459.7 °F. Keep in mind that degrees Celsius (°C) and degrees Fahrenheit (°F) always have the degree ° symbol in front, but Kelvin (K) never uses this symbol. Typical values of temperature on Earth’s surface on the Kelvin scale are around 260-310 K. Within atmospheric sciences, mostly the Kelvin and Celsius scales are used. The Fahrenheit scale is used only when discussing local weather.