92 PHYSICAL

 
 

N. WIND.

  
  

  

S. WIND.

 

N. WIND.

  

 

Fig. 82, Direction of Wind as Affected by Rotation.

sphere, and constant south-easterly winds in the
southern. Several causes, however, exist to pre-
vent this simple circulation of the air between the
equatorial and polar regions.

The equatorial currents do not continue as upper
eurrents all the way to the poles, but fall and become
surface currents, replacing the polar winds, which
rise and continue for a while toward the equator as
upper currents.

244, Causes of Interchange of Surface. and

Upper Currents.—The causes which produce this”

shifting of the equatorial and polar currents are:

(1.) The equatorial currents become cold—

(a.) By the cold of elevation ;

(6.) By expansion ;

(c.) By change of latitude.

The equatorial currents therefore fall and are
replaced by the polar currents, which have been
gradually ‘growing warmer by continuing near
the surface of the earth.

(2.) As the equatorial currents approach the
poles they have a smaller area over which to
spread, and, being thereby compressed, are caused
to descend and become surface currents.

This interchange between the equatorial and polar cur-
rents takes place at about lat. 30°. It varies, however,

with the position of the sun, moving toward the poles ©

when the sun is nearly overhead, and toward the equator
when the sun is in the other hemisphere.

The interchange in the position of the equatorial and
polar currents is represented in Fig. 83.

As the equatorial currrents fall, they divide,

GEOGRAPHY.

 

  

Zone of Variable Winds.

  
    
  

Calms of Cancer.

Zone of the North-east Trades.

Zone of the Calms.

A | Zone of the South-east Trades.

* Calms of Capricorn.

Zone of Variable Winds. y
Zone of Polar Winds,

s

 
 

 

   

 

  
    

Fig. 88. Interchange of the Equatorial and Polar Currents, Wind Zones.

part going to the poles, and part returning to the
equator.

The general system of the aérial circulation
thus indicated is more regular over the oceans
than over the land. Over the continents the
greater heat of the land during summer causes
a general tendency of the wind to blow toward
the land; similarly, the greater cold of the land
during winter causes a tendency of the wind to
blow toward the sea.

245. Classification of Winds.—Winds are di-
vided into three classes:

(1.) Constant, or those whose direction remains
the same throughout the year.

(2.) Periodical, or those which, for regular pe-
riods, blow alternately in opposite directions.

(8.) Variable, or those which blow in any di-
rection.

~ 246, Wind Zones.—The principal wind zones

are the zone of calms, the zones of the trades, the
zones of the calms of Cancer and Capricorn, the
zones of the variable winds, and the zones of the

‘polar winds.

Zone of Calms.—In parts of the ocean near the
equator the ascending currents are sufficiently
powerful to neutralize entirely the inblowing
polar currents, and thus produce a calm, which,
however, is liable at any moment to be disturbed
by powerful winds. The boundaries,of the zone
vary with the season; they extend from about 2°
to 11° north latitude.