Isobars.— In order to present graphical! the distribution of atmospheric pressure ore the earth's surface it is customary to draw Gee through points having the same sea-level pre sure. These lines are called isobars.
Barometric Gradient.— The distance to tween isobars shows the steepness of it barometric gradient. The unit of horizora distance usually employed is the distance of 1' on a meridian. Thus, if the difference in Fs sure between two points is one inch and th distance between them is equal to the length 10° on a meridian the barometric gradient! 0.1 of an inch.
Distribution of Pressure.— The average all the pressures of the year show belts of Ezt pressure at about latitude 35° north and and a belt of low pressure at the equator, al: which move north and south appreciably the annual march of the sun.
W. J. Humphreys computes the vertical !r:: perature gradient to he as follows: There is a close relation between tempera ture and pressure. If one takes the pressures for all the warm months and of all the cold months it will be found continents have low pressures in summer s high pressures in winter, and that the of the lows and of the highs will tear definite relation to the size of the confiner area and its distance from the equator. conditions arc reversed on the oceans, the occurring in winter and the highs in surrr-r The heat of summer raises the land to a temperature than it does the water. The expands, rises and overflows to the occr-s leaving a deficient pressure on the confine" and an excess on the seas. In winter !" land loses heat much more rapidly by racEat'lt than does the water. It contracts, and in the higher levels flows back from the water.
The average hourly pressure for a consider able period will show the barometer rises from 4 A.m. to 10 A.m.; then falls to 4 P.M., and rises to a second daily maximum at 10 P.M.; when it slowly falls to a secondary minimum at 4 A.M. These variations are greatest at the equator, diminishing with latitude until they are hardly distinguishable at the poles.
General In the tropics the winds blow almost continuously from the northeast in the northern hemisphere and from the southeast in the southern hemisphere, with irregular winds, mostly from the west in extra tropical regions. The explanation of the trade winds is that the heat of the sun raises the temperature and expands the air most where its rays fall nearly perpendicularly. This causes
the air to ascend in the region of the equator and to flow in along the surface from either side. Since, however, air moving from higher to lower latitudes passes successively over parallels having greater easterly motion than its own, or rather greater than the parallel whence it started, it would, on account of its momentum, fall behind the diurnal motion of the earth and be deflected more and more to the west. Over the warmest belt it would rise. In rising it would cool by expansion, so that in flowing away from the equator, as it must do to make room for the air continually flow ing in at the surface, its tendency would be to gradually fall, reaching the surface in tem perate latitudes and contributing to create the belt of high pressure existing at latitude each side of the equator.
The air aloft in going from low to high lati tudes passes successively over parallels of lati tude having less and less easterly motion, so that it runs ahead of the diurnal rotation of the earth or is deflected more and more to the east, giving us the prevailing westerly winds of the temperate zones when this air comes to earth.
From the isothermal level downward to the surface, between latitudes 30° and the poles, there are cyclonic circulations, central at the poles. The centrifugal force of these winds, because they run ahead of the earth, is greater than it would be if they ran with it, and con sequently the air of high latitudes, as it en circles the globe, tends to pile up over latitudes farther south.
Local previously indicated, the permanent circulations are due to equatorial heating and polar cooling and to the great sub permanent high pressure and low pressure sys tems created by the different heating effects of continents and oceans. Recently these systems have been called Great Centres of Action. They arc built up and disintegrated and change their geographical positions slowly. While they do not have a velocity of translation, like the ordi nary cyclones and anti-cyclones that move across continents and oceans and cause the changes of weather that occur from day to day, they do profoundly influence the intensity of storms and the path along which they may move. They, therefore, determine, in a general way, whether the character of the weather for weeks at a time shall be wetter or drier, colder or warmer, than the seasonal average.