MOVEMENTS OF THE ATMOSPHERE 41 



of high pressure, the distribution of these over the earth's surface 

 depends on a variety of factors, and is not coincident with regions 

 of corresponding temperature and humidity. Areas of high pres- 

 sure may also be produced by the crowding of warm rising air 

 against the cooler upper layers, as is the case in the higher atmos- 

 phere of the equatorial belt. 



Isobars. If the points of equal atmospheric pressure, due to 

 unequal heating, at a given time were united into a series of con- 

 tinuous surfaces, they would form a series of superposed planes 

 warped and dented into domes and ridges, and basins and troughs, 

 in a more or less conformable manner. So far as the layers are 

 affected by the heating, those representing the highest pressure 

 would lie below, and the successive surfaces upward would repre- 

 sent a regularly decreasing pressure. The elevations in these sur- 

 faces cover the areas of high pressure on the earth's surface, and 

 these elevations, when localized, w^ould be dome-like. The depres- 

 sions similarly cover the areas of low pressure. Since pressures 

 both higher and lower than 30 (30 inches of mercury) are found at 

 sea-level, this surface must be regarded as a horizontal plane cutting 

 the warped isobaric surfaces. The resulting lines of intersection are 

 the isobars, and it will be observed that, where the sea-level cuts a 

 local area of low pressvn"e, i. c, a depressed area in the isobaric sur- 

 faces, the resulting isobars will be irregular closed curves of decreas- 

 ing value toward the center, and crowded in proportion to the steep- 

 ness of the slope or gradient, whereas a local high pressure area 

 (a convex surface) will show closed curves of increasing value 

 toward the center. Over much of the earth's surface the isobars 

 are long, more or less parallel lines conforming in a general way 

 to the direction of the parallels of latitude. The winds are the 

 movements of the atmosphere from the regions of high to those of 

 low barometric pressure, at the same level. This may be regarded 

 as a movement down the isobaric slopes, and most readily and 

 strongly down the steepest gradient. 



Directions of Movement of the Air. Since the lower air of 

 the equatorial regions is heated more than that of the polar regions 

 by the direct action of the sun (insolation), it must expand most 

 at the equator and, rising, produce a belt of low pressure around 

 the globe. Into this will How the winds from the region of greater 

 pressure to north and south, along the bottom of the atmosphere, 

 while at the same time the rising air crowds against the higher 

 layers of the equatorial atmosphere which are not heated, and so 

 produces a surface of high pressure in the upper part of the equa- 

 torial belt, with a poleward gradient down which the air will move. 



