Section 3.2:
Cycles of Air Temperature

Learning Objective

Discuss the basic daily and annual cycles of air temperature.

Section Content

There are two basic cycles of air temperature—the rather predictable daily temperature cycle and the annual temperature cycle associated with the seasons.

Daily Temperature Cycle

You know from experience that a rhythmic rise and fall of air temperature occurs almost every day. This fact is confirmed when observing a meteogram, a graph that shows how meteorological variables change over time (Figure 3.2). During much of the February week recorded, the temperature curve follows the daily temperature cycle. It reaches a minimum around sunrise and then climbs steadily to a maximum between 1 p.m. and 4 p.m. The temperature then declines until sunrise the following day.

Figure 3.2
Meteogram showing temperatures in Chicago, Illinois

The typical daily rhythm, with minimums around sunrise and maximums in the afternoon, occurred on most days. The obvious exception occurred on February 25, when the maximum was reached at midnight and temperatures dropped throughout most of the day.

The primary control of the daily cycle of air temperature is Earth’s daily rotation, which causes a location to move into daylight for part of each day and then into darkness. As the Sun’s angle increases throughout the morning, the intensity of sunlight also rises, reaching a peak at noon and gradually diminishing in the afternoon. During the night, the atmosphere and the surface of Earth cool as they radiate away heat. The minimum temperature, therefore, occurs about the time of sunrise when the Sun again begins to heat the ground.

The daily variation of incoming solar energy versus outgoing Earth radiation and the resulting temperature curve for a typical middle-latitude location at the time of an equinox is shown in Figure 3.3A. It is apparent from this graph that the time of highest temperature does not generally coincide with the time of maximum solar heating. By comparing Figure 3.3A and Figure 3.3B, you can see that the greatest amount of solar radiation occurs at noon but that Earth continues to receive more energy than it emits until a few hours after noon. In other words, as long as the amount of solar energy gained exceeds the amount of Earth radiation lost, the air temperature continues to rise. When the incoming solar energy no longer exceeds the rate of energy lost by Earth, the temperature begins to fall.

Figure 3.3
The daily cycle of incoming solar radiation, Earth’s radiation, and the resulting temperature cycle

This example is for a midlatitude site around the time of an equinox. A. As long as solar energy gained exceeds outgoing energy emitted by Earth, the temperature rises. When outgoing energy from Earth exceeds the input of solar energy, temperature falls. B. Note that the daily temperature cycle lags behind the solar radiation input by a couple of hours.

In dry regions, particularly on cloud-free days, the amount of radiation absorbed by the surface is generally high. Therefore, the maximum temperature at these locales often occurs quite late in the afternoon. Humid locations, in contrast, frequently experience a shorter time lag in the occurrence of their temperature maximum.

Although the rise and fall of daily temperatures usually reflects the general rise and fall of incoming solar radiation, this is not always the case. For example, a glance back at Figure 3.2 shows that on February 25, the maximum temperature occurred at midnight, after which temperatures fell throughout most of the day. If temperature records for a station are examined for a period of several weeks, apparently random variations are seen. Such irregularities are caused primarily by the passage of atmospheric disturbances (weather systems) that are often accompanied by variable cloudiness and winds that bring air having contrasting temperatures. Under these circumstances, the maximum and minimum temperatures may occur at any time of the day or night.

Annual Temperature Cycle

People who live in the tropics are accustomed to regularly warm temperatures. However, if you live in the midlatitudes you are familiar with warm, or even hot, summers and rather cool winters. As you move to even higher latitudes, this seasonal temperature cycle becomes more pronounced. For example, the average annual range of temperature is about 24°C (43°F) in Albuquerque, New Mexico, whereas the annual temperature range in Fairbanks, Alaska, located near the Arctic Circle, is about 40°C (72°F), nearly twice that of Albuquerque.

In most years, the months with the highest and lowest mean temperatures do not coincide with the periods of maximum and minimum incoming solar radiation. North of the tropics, the greatest intensity of solar radiation occurs at the time of the summer solstice in June, yet the months of July and August are generally the warmest of the year in the Northern Hemisphere. Conversely, the least amount of solar energy is received in December at the time of the winter solstice, but January and February are usually colder.

In the United States, the annual temperature cycle lags behind the period of the most intense solar heating by an average of 27 days. However, in areas located near a large body of water, the average lag time is 36 days. Midwestern St. Louis, for example, usually experiences its highest temperatures in July, whereas coastal San Francisco experiences its warmest temperatures typically in September, but occasionally as late as October. San Francisco’s extended lag in temperature is primarily due to the fact that large water bodies take much longer to heat up than land, a phenomenon discussed in detail in the next section.

Section Glossary

Section Summary

Section Study Questions

Try to answer the following questions on your own, then click the question to see the correct answer.

Although the intensity of incoming solar radiation is greatest at noon, the warmest part of the day is most often mid-afternoon. Why?

Although solar radiation is at maximum at noon and decreases after that time, it continues to provide more energy than is being lost for a few hours. Since the temperature is dependent on the total energy budget (gain versus loss), it continues to rise if there is a surplus. The maximum temperature occurs when solar energy gained no longer exceeds the energy loss.

Explain why the times of the year when a location experiences its highest and lowest temperatures do not coincide with occurrences of maximum and minimum annual solar radiation.

Expanding on the previous answer: seasonal temperatures continue to rise as long a location receives more heat energy than it emits into space. Although maximum Sun angle may occur on the summer solstice, temperatures will continue to rise if more solar energy is received than is emitted back into space, resulting in an energy surplus. Similarly, while the winter solstice may experience the lowest Sun angle, a location can expect temperatures to continue to fall if more energy is emitted into space than is received.