Hi all, the obvious reason is because the crop season finishes around late august & early september, however, because of the location of the circles
ie- something like 98% of all cropcircles appear on ground that has a chalk aquifer beneath it.
The Aquifer Connection: During the mid-1990s two British researchers, Glenn Broughton (now living in Vermont) and Steve Page, wondered why crop
circles occur in some locations (in many cases year after year) and not in others? By plotting crop circle occurrence on a geological map of southern
England they were able to demonstrate that an overwhelming majority of British crop circles during the years 1993-98 occurred over aquifers lying
beneath either deep deposits of chalk or greensand. For more details see "Aquifer Research" (page 2) on the BLT Research Links page.
Glenn Broughton & Steve Page
Chalk is a highly porous rock. BLT's John Burke suspected that the Spring rainwater percolating slowly down through the chalk over the summer months
might increase the ground electrical charge in these areas as the summer progressed. He further speculated that this increase in ground electrical
charge might be enough to function as an attractor to the complex upper atmospheric plasma system hypothesized by W.C. Levengood as the causative
agency behind crop circles--and, further, that this would be particularly evident along the edges of the chalk layers, where the surface water has
farther to travel to reach the underground aquifer.
Repeated ground-electrical-charge measurements were carried out over several summers in southern England in the early 1990s, in areas where crop
circles regularly occur. It was found that the ground-electrical-charge did increase as the summer progressed (and as the aquifer below the chalk
lowered), thus increasing the likelihood that ground-electrical-charge might be an "attractor" to the hypothesized atmospheric energy system--the
increase in ground electrical charge, although slight, might explain why at least some of the circles occur where they do.
When Burke examined a graph of water-table levels in the Wiltshire area of the U.K. for the recent years during which crop circles have been reported,
he discovered another interesting fact: the British Water Authority had, in two specific years, begun to pump the aquifer during the summer months and
it was during those two years (when the aquifer was at it's lowest point) that the greatest number of crop circles had been reported.
A number of researchers have raised the question as to whether or not a magnetic anomaly in the earth's crust might account for the question of
"why" crop circles occur so often in certain locations and not in others. A German team, using a geomagetometer, mapped out several British fields
in which crop circles existed, and found that there were magnetic anomalies in the earth's crust in most of these fields--but not necessarily
directly beneath the circles themselves. This German study was not published and all of the details are not available, but more work along these lines
might clarify a possible connection.