Discuss the basic daily and annual cycles of air temperature.

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.

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.

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.

• meteogram: Graphical representation of a time series of weather data, such as temperature or pressure.

• The main control of the daily cycle of air temperature is Earth’s rotation, which causes a place to move into and out of daylight. The time of highest temperature usually does not coincide with the time of maximum intensity of solar radiation.

• The seasonal temperature cycle is more pronounced the further you get from the equator.

• As a result of the mechanism by which Earth’s atmosphere is heated, the highest and lowest mean temperatures usually do not coincide with the periods of maximum and minimum incoming solar radiation.