Temperature | Definition, Scales, Units, & Facts (2024)

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What is temperature?

Temperature is the measure of hotness or coldness expressed in terms of any of several scales, including Fahrenheit and Celsius. Temperature indicates the direction in which heat energy will spontaneously flow—i.e., from a hotter body (one at a higher temperature) to a colder body (one at a lower temperature).

When is air temperature the highest?

On average, the time of day when air temperature is the highest is usually about 3 PM, as heat continues to build up after noon. However, after 3 PM the Sun is low enough in the sky for more heat to be lost than absorbed, causing a gradual decline in air temperature.

Why is air temperature lower at night?

During the night there is no direct sunlight, so there is more solar energy being lost than absorbed, causing air temperature during the night to be relatively cooler than during the day.

Why does temperature decrease with higher altitude?

Temperature decreases at higher altitudes because there is less air exerting pressure than at sea level. According to Gay-Lussac’s law, pressure and temperature at a constant volume are directly proportional, so if there is a decrease in pressure, temperature will consequently decrease.

When do temperature inversions occur?

Temperature inversions occur when the normal heat gradient of the atmosphere—air is warmer near the ground and cooler in the sky—is reversed. A temperature inversion typically occurs in the late evening and disappears just after sunrise. A visual cue that a temperature inversion has happened is fog (or smog in an urban environment).

temperature, measure of hotness or coldness expressed in terms of any of several arbitrary scales and indicating the direction in which heat energy will spontaneously flow—i.e., from a hotter body (one at a higher temperature) to a colder body (one at a lower temperature). Temperature is not the equivalent of the energy of a thermodynamic system; e.g., a burning match is at a much higher temperature than an iceberg, but the total heat energy contained in an iceberg is much greater than the energy contained in a match. Temperature, similar to pressure or density, is called an intensive property—one that is independent of the quantity of matter being considered—as distinguished from extensive properties, such as mass or volume.

Three temperature scales are in general use today. The Fahrenheit (°F) temperature scale is used in the United States and a few other English-speaking countries. The Celsius (°C) temperature scale is standard in virtually all countries that have adopted the metric system of measurement, and it is widely used in the sciences. The Kelvin (K) scale, an absolute temperature scale (obtained by shifting the Celsius scale by −273.15° so that absolute zero coincides with 0 K), is recognized as the international standard for scientific temperature measurement.

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In certain fields of engineering, another absolute temperature scale, the Rankine scale (see William Rankine), is preferred over the Kelvin scale. Its unit of measure—the degree Rankine (°R)—equals the Fahrenheit degree, as the kelvin equals one Celsius degree.

The Réaumur (°Re) temperature scale (or octogesimal division) was widely used in parts of Europe in the 18th and 19th centuries; it later was used primarily to measure the temperature of mixtures during brewing, of syrups in the production of certain food products, and of milk during cheese making.

This article was most recently revised and updated by Adam Augustyn.