Section 3.1:
Air Temperature Data

Learning Objective

Calculate five commonly used types of temperature data and interpret a map that depicts temperature data using isotherms.

Section Content

Temperatures recorded daily at thousands of weather stations worldwide provide much of the temperature data compiled by meteorologists and climatologists. Hourly temperatures may be recorded by an observer or obtained from automated observation systems that continually monitor the atmosphere. At many locations, only the maximum and minimum temperatures are recorded.

Mini-Lecture Video - Air Temperature Data (Click to watch the video)

Basic Calculations

A location’s daily mean temperature is determined by averaging the 24 hourly readings or by adding the maximum and minimum temperatures for a 24-hour period and dividing by 2. From the maximum and minimum, the daily temperature range is computed by finding the difference between these figures. Other data involving longer periods are also compiled:

Mean temperatures are especially useful for making daily, monthly, and annual comparisons. It is common to hear a weather reporter state, “Last month was the warmest February on record,” or “Today Denver was 10° warmer than Chicago.” Temperature ranges are also useful statistics because they give an indication of extremes, a necessary part of understanding the weather and climate of a place or an area (Box 3.1).

Box 3.1

Hottest and Coldest Places in the United States

Most people living in the United States have experienced temperatures of 100°F or more. When statistics for the 50 states are examined, we find that every state has a maximum temperature record of 100°F or higher. Even Alaska has recorded a temperature this high—set June 27, 1915, at Fort Yukon, a town along the Arctic Circle in the interior of the state.

Maximum Temperature Records

The highest accepted temperature record for the United States—as well as the entire world—is 134°F. This long-standing record was set at Death Valley, California, on July 10, 1913. Summer temperatures at Death Valley are consistently among the highest in the Western Hemisphere. During June, July, and August, temperatures exceeding 120°F are to be expected. Fortunately, Death Valley has few human summertime residents (Figure 3.A).

Figure 3.A
Almost a record!

On June 30, 2013, 100 years after Death Valley set the all-time high recorded temperature, it came close to equaling it. On that date, Death Valley’s air temperature peaked at 129.2°F.

Why are summer temperatures at Death Valley so high? In addition to having the lowest elevation in the Western Hemisphere (53 meters [174 feet] below sea level), Death Valley is a desert. Although it is only about 300 kilometers (less than 200 miles) from the Pacific Ocean, mountains cut off the valley from the ocean’s moderating influence and moisture. Clear skies allow maximum solar radiation to strike the dry, barren surface. Because no energy is used to evaporate moisture, as occurs in humid regions, all the energy is available to heat the ground.

Minimum Temperature Records

We expect extremely cold temperatures during winter in high-latitude places that lack the moderating influence of the ocean. Further, we would expect stations located at high elevations to be especially cold. All these criteria apply to Prospect Creek, located north of the Arctic Circle in the Endicott Mountains of Alaska, which holds the record of −80°F. In the lower 48 states, the record of −70°F was set in the mountains at Rogers Pass, Montana, on January 20, 1954. Remember that many other places have experienced equally low or even lower temperatures, but these were not recorded at an official recording station.


To display the distribution of air temperatures over large areas, isotherms are commonly used. An isotherm is a line that connects points on a map that have the same temperature (iso = equal, therm = temperature). Therefore, all points through which an isotherm passes have identical temperatures for the time period indicated. Generally, isotherms representing temperature differences of 5° or 10° are used, but any interval may be chosen. Figure 3.1 illustrates how isotherms are drawn on a map. Notice that most isotherms do not pass directly through the observing stations because the station readings may not coincide with the values chosen for the isotherms. Only an occasional weather station temperature will be exactly the same as the value of the isotherm, so it is usually necessary to draw the lines by estimating the proper position between observing stations.

Figure 3.1 Isotherms

The large map shows high temperatures for a spring day. Isotherms are lines that connect points of equal temperature. Showing temperature distribution in this way makes patterns easier to see. On television and in many newspapers, temperature maps are in color, as shown in the inset map. Rather than label isotherms, these maps label the area between isotherms. For example, the zone between the 60°F and 70°F isotherms is labeled “60s.”

Tutorial Video - Isotherm Maps (Click to watch the video)

Isothermal maps are valuable tools because they make temperature distribution clearly visible and make areas of low and high temperatures easy to identify. In addition, the amount of temperature change per unit of distance, called the temperature gradient, is easy to visualize. Closely spaced isotherms indicate a rapid rate of temperature change, whereas more widely spaced lines indicate a more gradual rate of change. For example, notice in Figure 3.1 that the isotherms are more closely spaced in Colorado and Utah (steeper temperature gradient), whereas the isotherms are spread farther apart in Texas (gentler temperature gradient). Without isotherms, a map would be covered with numbers representing temperatures at tens or hundreds of places, which would make patterns difficult to see.

You might have wondered . . . 

What’s the hottest city in the United States?

It depends on how you define “hottest.” If average annual temperature is used, then Key West, Florida, is the hottest, with an annual mean of 78°F for the 30-year span 1981–2010. However, if we look at cities with the highest July maximums during the 1981–2010 span, then the desert community of Bullhead City, Arizona, has the distinction of being hottest. Its average daily high in July is a blistering 112°F!

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.

How are the following temperature data calculated: daily mean, daily range, monthly mean, annual mean, and annual range?

Daily mean: An average of the 24-hourly readings or, more commonly, an average of the daily maximum and the daily minimum. Daily range: The difference between the daily maximum and the daily minimum. Monthly mean: A figure computed by adding the daily means for each day of the month and dividing by the number of days in the month. Annual mean: An average of the 12 monthly means. Annual range: The difference between the warmest and the coldest monthly means.

What are isotherms, and what is their purpose?

An isotherm is a line that connects points of equal temperature. Isotherms are drawn on 

maps to show the distribution of temperature. They make the important aspects of temperature distribution more easily visible.