METEOROLOGY AND CLIMATOLOGY. 
37 
towards the equator, and the westerly anti-trades blow towards the poles. 
These are also subject to the annual change; but the influence of the 
different action of land and sea on the distribution of pressure exercises 
a greater influence than does the difference of latitude. As the greater 
heating and cooling of the land each day causes the phenomena of daily 
land and sea breezes, so the greater heating and cooling of the land 
between summer and winter causes seasonal land and sea winds, blowing 
from land to sea in winter, from sea to land in summer. Generally 
speaking, the pressure is greater—in the same latitude—where the air is 
cooler, so that outside the frigid zones cold areas are usually areas of 
high pressure, from which wind blows out in every direction, while warm 
areas are areas of low pressure towards which wind blows in on every 
side. 
Rainfall on the land is dependent in its distribution 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 f 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 
