How do glider pilots gain height over larged ploughed fields?

Answer 1

By circling in rising air called Thermals. This air rises because it has been warmed by contact with the sunlit ground and warm air is lighter than cold air so it is bouyant. Thermals tend to be shaped somewhere between a column and a bubble, rather like the stuff in those 1970's lava lamps. Check this for more info: http://www.gliding.co.uk/whatisgliding/soaring.htm