5.1: Temperature

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Michael E. Ritter
University of Wisconsin-Stevens Point via The Physical Environment

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Temperature is a measure of the average speed of the random motion of molecules that comprise a substance. Basically, it is a measure of how hot or cold something is. In Chapter 4 we made an important distinction between temperature and heat. Heat is the total molecular motion associated with a substance while temperature is the average motion of molecular that make up a substance.

There are two ways that the temperature of the air can change. There can be a physical exchange of heat between the air and the earth surface in the presence of a temperature gradient. For example, if the surface is warmer than the air above, heat will be transferred into the air to warm it up. This is a case of diabatic temperature change. The temperature of the air can also change without a physical exchange of heat. This is called an adiabatic temperature change that occurs with rising parcels of air. As a parcel of air rises it expands. In the process of expansion requires the expenditure of its internal energy and as a result the parcel cools. We'll look at this process in more detail in Chapter 7.

Measuring Temperature

Surface temperature is the temperature of the earth surface, not the temperature of the air above it. Air temperature is measured at a height of 4 to 6 feet above the surface. For accurate results, instruments should be shielded from the Sun and away from buildings and other structures that could affect temperature measurements. A Stevenson screen, like that shown in Figure \(\PageIndex{1}\) is ventilated to allow air through it.

Stevens Screen — Figure \(\PageIndex{1}\): Stevenson screen for measuring weather conditions like air temperature.

One of two scales of measurement are common to most of us, the Fahrenheit scale in the United States and Celsius in most other countries. The Celsius scale is an accepted international system of measurement because it is based on a decimal scale having 100 units between the freezing and boiling points of water. The Fahrenheit scale sets the freezing point of water at 32^oF and 212^oF for boiling. In much scientific research the Kelvin scale is used. It is based on the "kelvin", a unit increment of temperature. Unlike the Fahrenheit and Celsius scales, the kelvin is not referred to as a "degree". It is an absolute scale given that at absolute zero all molecular movement ceases. It too has 100 units between the freezing and boiling points of water. Absolute zero is -273^o on the Celsius scale.

Figure \(\PageIndex{2}\): Temperature Scales

Temperature Statisics

A variety of statistics are available to easily communicate the volumes of temperature data available. The daily temperature range for a place is the difference between the highest and lowest temperatures over a 24 hour period. The average daily temperature is the sum of highest and lowest temperature of the day divided 2. This the sum of the average daily temperature divided by the number of days in the month. The annual temperature range is the difference between the highest and lowest monthly temperature for a place. The average annual temperature is the sum of the mean monthly temperature divide by 12.

Video: Taking Earth's Temperature (Courtesy NASA)
This short video describes how researchers use models to understand the Earth's changing temperature.