The distribution of rainfall on the land is dependent on the direction of the rain-bringing wind and the configuration of the surface. Thus when the rain-bringing wind meets a mountain range, it deposits a great rainfall on the exposed slopes, but passes over as a dry wind which yields little rain to the region beyond. In places where the wind changes with the season, as in southern Asia, the distribution of rainfall is entirely different during the continuance of the different monsoons. All these questions of normal climate can be more easily illustrated on maps than explained by words. But the reader must be cautioned against taking the condensed and generalised representations of small-scale maps as showing all that is known on the subject. Even the magnificent plates in the ‘Atlas of Meteorology,’ which forms part of Bartholomew’s Physical Atlas, cannot show everything that is known; and in many parts of the world so little has yet been ascertained as to the climatic conditions that generations of observers will be required to make it possible for meteorologists to draw a uniform trustworthy map of the whole world showing the distribution of any one element of climate. _Isothermal maps._—The principle of an isothermal map is that of representing the distribution of temperature by drawing lines through all the places where the temperature is the same at a given time. It is usual to take this time as an average month in an average year. Thus in a map of isotherms for January (see p. 50), what is shown is not the temperature of any particular day in any particular January, but that of an average day in a long series of Januaries. Hence it is not likely that the _exact_ distribution of temperature shown in the map will ever be found on any January day; but it is to be expected that most days in every January will have a distribution of temperature which is very similar to that shown. The same is of course true of maps showing pressure, or rainfall, or any other average condition.