Storm Boris was the result of what meteorologists call a "cut-off low". This is where part of the jet stream becomes detached from the main area of strong winds and circles over the same region for several days. An area of low pressure developed in it, with heavy and persistent rain (and mountain snow) pummelling the same areas again and again. Record heat ahead of Storm Boris, and moisture-laden air being drawn in from above a "warmer than normal" Mediterranean, clashed with much cooler air trying to work in from the north which helped to make the storm even worse. This resulted in record rainfall across a large area. In the Czech Republic, up to 500mm of rain fell in just five days. For a few places this was nearly 10 times the normal September rainfall.