sphere. At many locations, only the maximum and minimum temperatures are obtained ( Box 3.1 ). Basic Calculations The daily mean temperature is determined by averag- ing the 24 hourly readings or by adding the maximum and minimum temperatures for a 24-hour period and di- viding by 2. From the maximum and minimum, the daily temperature range is computed by fi nding the difference between these fi gures. Other data involving longer periods are also compiled: · The monthly mean temperature is calculated by adding together the daily means for each day of the month and divid- ing by the number of days in the month. · The annual mean temperature is an average of the 12 monthly means. · The annual temperature range is computed by fi nding the difference between the warmest and coldest monthly mean temperatures. 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 warm- est February on record” or “Today Omaha was 10° warmer than Chicago.” Temperature ranges are also useful statis- tics because they give an indication of extremes, a necessary part of understanding the weather and climate of a place or an area. Isotherms 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 tempera- ture ( iso = equal, therm = temperature). Therefore, all points through which an isotherm passes have identical tempera- tures for the time period indicated. Generally, isotherms
CHAPTER 3 Temperature 61 representing 5° or 10° differences in temperature are used, but any interval may be chosen. Figure. 3.2 illustrates how iso- therms 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. Isothermal maps are valuable tools because they make temperature distribution clearly visible at a glance. Areas of low and high temperatures are easy to pick out. In addi- tion, 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 3.2 that the iso- therms are more closely spaced in Colorado and Utah (steeper Box 3.1 North America’s Hottest and Coldest Places Most people living in the United States have experienced temperatures of 38°C (100°F) or more. When statistics for the 50 states are examined for the past century or longer, we find that every state has a maximum temper- ature record of 38°C 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.
- Winter '17