METEOROLOGY AND CLIMATOLOGY. 39 
the temperature is above 80° and it may be assumed to be below 85°. 
The method of representation is unsatisfactory in such a case. 
Bearing these facts in mind, the study of isotherm maps will be found 
t o give an excellent general idea of the distribution of climate at sea- 
level, and if the contour lines of 600 and 6000 feet are traced on the 
maps the areas within which corrections of over -2° and -20° have to 
be made to the isothermal values to get the temperature at the place will 
be easily recognised. 
Isobaric Maps .—Isobars are drawn from the data of the height of the 
barometer corrected to sea-level values and to the temperature of 32° F., 
exactly in the same way as isotherms are drawn from the data of thermo¬ 
meter readings or contour lines from data of elevation measurements. 
The practical value of the study of isobars is very great, because of 
the importance of assuming a probable value of sea-level pressure in 
reducing the barometric or boiling-point thermometer readings for 
determining elevation, and also because of the intimate relation between 
the form and proximity of isobars and the direction and force of the 
winds. 
Barometric gradient is measured by the difference between the isobars 
per unit of length. For instance, gradient is frequently expressed in 
the number of hundredths of an inch difference between barometers 
fifteen nautical miles apart. The greater the gradient of pressure is the 
more closely together must the isobars be drawn in order to represent 
it. For example, in the isobaric map for January (p. 50) a very 
steep gradient is shown [on the east coast of Asia, north of Japan, 
and a remarkably gentle gradient in the interior of Asia from the 
Black Sea eastward. The steeper the gradient the stronger is the 
wind. 
The arrows in the isobaric maps (which are represented flying with the 
wind) show the average directions of the wind over the world for the 
months in question. The relation they bear to the isobars becomes clear 
on inspection, although, on account of the greater number of observa¬ 
tions available for some parts of the world than for others, all the arrows 
are not drawn with the same amount of certainty, and the direction of a 
few contradicts that of most. As a general rule, the following facts may 
be taken as absolutely established: (1) Wherever there is a region of 
high pressure the wind blows out from it in all directions. (2) Wherever 
