ix Climates of the World 137 



solid curve shows the range at Verkhoyansk, the finer line 

 that at the Lofoten Islands. In continental or land climates 

 the range of temperature is great and the rainfall very 

 small, in oceanic or sea climates the range of temperature 

 is small and the rainfall great. Prevailing winds carry an 

 oceanic climate for a considerable distance inland on the 

 west coasts of northern continents, and they carry a con- 

 tinental climate a considerable distance seaward on the 

 west shores of northern oceans. The student should verify 

 these statements by making a detailed comparison of the 

 two isothermal maps, and the map of rainfall (Plate VIII.) 

 192. Isobars. The invisible differences of atmospheric 

 pressure may be laid down on a map in the same way as 

 the invisible differences of temperature. Lines running 

 through places over which the atmospheric pressure, as 

 measured by the barometer and reduced to sea-level, is the 

 same, are called isobaric lines, or shortly isobars. Those 

 shown on the maps (Plates V. and VI.) express the pres- 

 sure in inches of mercury at intervals of every tenth of an 

 inch, and the spaces between them are coloured, so that 

 the regions of highest pressure are deep red, and those of 

 lowest pressure deep blue. When adjacent isobars are 

 drawn far apart on the map the barometric gradient ( 175) 

 between them is slight, and the wind set up is consequently 

 gentle ; but when the isobars are crowded closely together, 

 a steep gradient is indicated, giving rise to furious wind. 

 A gradient of 0-5 corresponds to a difference of pressure 

 of 0-20 inches in one degree of latitude ; a gradient of 

 15, giving rise to a hurricane, corresponds to a difference 

 of pressure equal to 0-60 inches in one degree of latitude. 

 In the maps the arrows are represented as flying with the 

 wind. The shortest path from a region of high to one of 

 low pressure is at right angles to the isobars, but in con- 

 sequence of the rotation of the Earth the actual path of the 

 wind is that stated in Ferrel's Law ( 89). The deviation 

 is proportional to the latitude, so that in the far north and 

 south, wind blows nearly parallel to the isobars. Dr. Buys 

 Ballot, the late eminent Dutch meteorologist, independently 

 discovered the Law of the Winds, which Dr. Buchan has 



