Permaculture Designers Manual
CHAPTER 5 – CLIMATIC FACTORS IN PERMACULTURE
Section 5.3 –
Patterning in Global Weather Systems in Permaculture
The Engines of the Atmosphere
Dense cold air flows continually off the polar ice caps. This high-pressure or down-draught air spirals out of the Polar Regions as persistent easterlies which affect high latitudes (60-80°) near the ice-cap themselves.
Long spokes of this air curve outward to Latitude 30°.
As the spiral itself is caught up in (and generated by) earth spin, these cold cells of air drive a series of contra-rotating low-pressure cells (turning clockwise in the southern hemisphere and anti-clockwise in the northern).
These in turn mesh with rotating spirals of high-pressure air which have risen at the equator and are falling at Latitudes 15-40°.
The high pressure mid-latitude cells turn anticlockwise in the southern hemisphere and clockwise in the northern.
Thus from Latitude 50-20°, and In the “roaring forties“, about 15-18 alternating high-low pairs of great cells circulate the earth, all of them as smaller spiral systems around the great polar spiral itself (Figure 5.3)
On westerly coasts, the alternation of cold polar and warm high pressure air arrives at about H)–day intervals, although some great high-pressure cells persist in place, thus blocking westward movement of winds and creating static oceanic conditions that can affect oceanic over-turn and thus fisheries (e.g. the el Nino effect).
These great processions are disturbed and deflected by continents, stubborn high-pressure cells over cool land masses, and the relative intensity of the air cells, so that irregular cold-warm fronts arrive at any one site.
Just as polar air is sometimes drawn strongly towards the equator in the lows, so warm tropical air masses are entrained in the outer circulation of the highs and bring heavy warm rains towards the poles.
High level jet streams may speed up or block this procession and the jet stream itself may also break up under stresses caused by shear.
The disturbances and impedances in the system cause cold fronts to pile up against each other and deflect pole-wards at high-pressure cells, and a sequence of warm and cold front rains (the cyclonic or spiral rains) of earth results.
All these wind belts shift north or south with the sun annually and to some slower extent as a result of the 18 year moon cycle, so that periods of drought and excessive rain can result.
The system appears chaotic and subject only to short-term prediction, but of late we are learning to assess some of the effects of the long-term cycles.
The great spiral circulation of the south Polar Regions is shown in Figure 5.3.
About 12-18 cold fronts (cloud bands) circle from west to east around the poles, arriving as “cyclonic fronts” every 10 or so days on coasts in that region.
They affect areas up to 30° south, with four or so large fronts continuous with (and probably driving) cloud up to 10° south or north latitude, mostly along the western margins of South America, Africa, and the South Atlantic.
It is now clear that it is the oceanic circulation that drives the air masses, rather than the opposite.
The fronts are dragged in a curve to the west as the earth spins to the east. Each cloud front is a result of the meeting of cold polar and warm sub-polar air masses or high-pressure cells.
The low-pressure areas rotate clockwise, the highs anti-clockwise in a series of cog-like spirals or tori that travel every month around the poles.
Rotation is in the opposite sense in the northern hemisphere. It is the cold, dense, dry polar air sweeping off the ice-caps, and the hot rising air of the equatorial calms which drives these great wheels; clear-air (descending) intrusions are of hot-dry and cold-dry continental air (Australia, Africa) or air descending from the equatorial (rising) congruence (Figure 5.4).