Having thus disposed of the extremes and ranges, we may compute what is called the “variability” of temperature, i. e., the average difference between the means of two successive days in a given month, and the corresponding average for the entire year. These data are of considerable importance in medical climatology. We may also compute the frequency of occurrence of various values among the temperature data above enumerated. The most frequent value is often quite different from the average value. Many climatologists compute the number of days, in an average year, on which the temperature rises to 77 degrees (Fahr.) or above (“summer-days”), and the number of days on which the temperature does not rise above the freezing point (“winter-days”). Especially valuable in agricultural regions are data of the average and extreme dates of the last frost in spring and of the first frost in autumn. These define the length of the “growing season.” Statistics of the temperature of the ground at the surface and at various depths below the surface are also of agricultural interest. From the foregoing outline it will be seen that a bewildering variety of climatic statistics may be computed merely from observations of temperature, and the same is true of the other elements. Moreover, the list set forth above is by no means exhaustive even for temperature. In fact, there is almost no limit to the number of ways in which the raw material of climatic data--i. e., the original records of observation--may be grouped, averaged, or otherwise treated in order to bring out certain features of the climate that may conceivably serve some useful purpose. The reader will now be able to understand why a treatise on the climate of a single locality often fills a substantial volume. The numerical data contained in such a work are generally supplemented by text descriptions and by various graphic devices, such as curves showing the normal fluctuation or “march” of a weather element during a day, year, or other interval. Works which deal with the climates of larger areas, such as whole countries, are usually accompanied by climatic charts. These charts furnish a quick and easy way of getting a general idea of the climate of a region. Among the more important climatic charts are the following: 1. _Temperature (isothermal) charts._ These include charts showing the distribution of normal temperatures for months, seasons, and the year; normal range of temperature for similar periods; highest and lowest temperatures ever recorded at the different stations; etc. Lines known as _isotherms_ are so drawn as to pass through places having identical values of the element in question (mean temperature, highest temperature, etc.).