The actual movements of convection are much more complex. Calms alternate with eddying movements of great intensity. All general movements are deflected by the rotation of the earth on its axis—easterly in tropical latitudes, and westerly beyond the tropics. There are therefore three wind belts, one of easterly and two of westerly motion. Each of these has also a northerly and a southerly component; moreover, all three belts shift alternately north and south with the apparent movement of the sun. The belt of tropical easterly, or Trade Winds, extends a little further north than New Orleans in summer and its northern edge recedes as far south as Havana in winter. The position of the wind belts, month by month, is
The migration of the heat belt.
shown on the Coast Pilot charts of the United States Hydrographic Office.
The apparent motion of the sun, due to the inclination of the earth’s axis, carries the zone of greatest warmth far north in June and far south in December. Thereby the warmth of tropical regions is carried well into the temperate zones, and thereby the production of foodstuffs is extended to about the sixtieth parallel of latitude, north and south.
All this complexity of movement adds to the diffusion of warmth. The warm air of tropical regions is mixed with the cold air of circumpolar regions. Complex as they are, the general movements of diffusion may be classified as the horizontal movements which include the winds, and the vertical
