6 THE AIR 



an isobaric map the direction and force of the wind at 

 any point, it is sufficient to remember that the wind 

 blows from the higher pressure to the lower pressure, 

 with a force proportional to the gradient (that is, to 

 the closeness with which the isobars are drawn), and 

 in a direction tending to the right of a line drawn per- 

 pendicular to the isobars in the northern hemisphere, 

 and to the left in the southern hemisphere. It must 

 be remembered that an isobaric or wind chart for the 

 year or for a month represents the average condition 

 for the year or for that month, and may not accurately 

 depict the condition at any particular moment of time. 



If the surface of the Earth were entirely covered by 

 the sea, so that there was no land to disturb the normal 

 circulation of the atmosphere, the conditions of weather 

 which actually prevail only in mid-ocean would prob- 

 ably hold good for the whole world, which would be 

 girdled by concentric belts of climate as follows : 



i. An Equatorial belt of permanently low pressure 

 right round the Earth, lying approximately along the 

 Equator, forming a region of calms with ascending 

 air, in which occasional sharp squalls, accompanied by 

 lightning, thunder, and heavy rain, often occur. This 

 belt is the trough of minimum pressure, to which the 

 winds of the whole tropical region blow, and on which 

 all the other pressure belts depend. It would move 

 with the season, following the sun northward in the 

 northern summer, southward in the southern summer, 

 and displacing the parallel belts on each side in the 

 same directions. The belt, as it actually exists on the 

 oceans, occupies a mean position between 5 and 10 N., 

 on account of the indirect influence of land, and as it 

 swings northward and southward across any given 

 parallel, the rainy season of that parallel occurs. The 

 belt is known to sailors as the " Doldrums." 



