The Jet Streams

The paths of the Northern and Southern Hemisphere jet streams normally vary according to the season. In each hemisphere, the jet streams gravitate towards their respective poles in the summer and towards the equator in the winter, bringing precipitation as they move. This means that the cool temperate regions are moist in the summer and the warm temperate regions are moist in the winter. This is why both the cool temperate and the warm temperate regions in the Northern Hemisphere, where most of the rain falls on land, are good for agriculture and why most of the first civilizations in the Old World flourished in these climate zones.

The seasonally undulating paths of the northern and southern jet streams are normally consistent for many years. Occasionally, however, they change their course. When this happens, areas once receiving plentiful rainfall may suddenly and inexplicably suffer massive drought, with devastating consequences. In the summer of 1972, for example, the Northern Hemisphere jet stream did not move as far north as usual. The Sahel suffered a severe drought and parts of India had a 60% shortfall in harvest. While some regions receive less rain than usual other receive more and usually at a time they least need it.

Water in the form of rain or irrigation is essential for agriculture of course, but if it rains for too long at one time during the growing season it can be very deleterious especially when plant pathogens are involved. The spores of most plant pathogenic fungi require free-standing moisture to germinate and gain ingress into the host. If plants are not able to dry out between rain storms they are vulnerable to plant diseases. Serious crop losses and famine in ancient Egypt and Rome occurred in growing seasons experiencing unusually heavy rainfall during the growing season.

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