People probably notice changes in air temperature more often than they notice changes in any other element of weather. At a weather station, the temperature is monitored on a regular basis from instruments mounted in an instrument shelter ▼. The shelter protects the instruments from direct sunlight and allows a free flow of air.

The daily maximum and minimum temperatures underlie much of the basic temperature data compiled by meteorologists:

• By adding the maximum and minimum temperatures and then dividing by two, the daily mean temperature is calculated.

• The daily range of temperature is computed by finding the difference between the maximum and minimum temperatures for a given day.

• The monthly mean is calculated by adding together the daily means for each day of the month and dividing by the number of days in the month.

• The annual mean is an average of the 12 monthly means.

• The annual temperature range is computed by finding the difference between the highest and lowest monthly means.

Mean temperatures are particularly useful for making comparisons, whether on a daily, monthly, or annual basis. It is quite common to hear weather reporters make statements such as “Last month was the hottest July on record” or “Today, Chicago was 10 degrees warmer than Miami.” Temperature ranges are also useful statistics because they give an indication of extremes.

To examine 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. Therefore, all points through which an isotherm passes have identical temperatures for the time period indicated. Generally, isotherms representing 5°  or 10°  temperature differences are used, but any interval may be chosen. The figure below ▼ 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 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 stations.

Maps with isotherms are valuable tools because they make the temperature distribution visible at a glance. Areas of low and high temperatures are easy to pick out. 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. You can see this in the figure above ▲. The isotherms are closer in Colorado and Utah (steeper temperature gradient), whereas the isotherms are spread farther in Texas (gentler temperature gradient). Without isotherms, a map would be covered with numbers representing temperatures at dozens or hundreds of places, which would make patterns difficult to see.

• Daily mean temperature is an average of the daily maximum and daily minimum temperatures, whereas the daily range is the difference between the daily maximum and daily minimum temperatures. The monthly mean is determined by averaging the daily means for a particular month. The annual mean is an average of the 12 monthly means, whereas the annual temperature range is the difference between the highest and lowest monthly means.

• Temperature distribution is shown on a map by using isotherms, which are lines of equal temperature. Temperature gradient is the amount of temperature change per unit of distance. Closely spaced isotherms indicate a rapid rate of change.

• annual mean: With respect to air temperature, the average of the 12 monthly temperature means.

• annual temperature range: The difference between the highest and lowest monthly temperature means over 12 months.

• daily mean temperature: Determined by averaging the hourly readings or, more commonly, by averaging the maximum and minimum temperatures for a day.

• daily range: The difference between the maximum and minimum temperatures for a day.

• isotherm: A line connecting points of equal temperature on a map; used to examine distribution of temperature data over large areas.

• monthly mean temperature: Calculated by averaging the daily temperature means for an entire month.

• temperature gradient: The amount of temperature change per unit of distance.