Bars are a standard unit used in measuring air pressure. Millibars are 1/1,000 of a bar. The measure of 1 Bar is average pressure at sea level.
Weather patterns, and particularly storms are tracked and predicted by changes in air pressure. An approaching storm is always preceded by a drop in air pressure. The eye of a hurricane is very low pressure and it is the primary structure that holds the hurricane together. The center of a tornado is extremely low pressure.
Low pressure isn’t what causes things so you should not look at it like that, it is just an indicator. As a storm gathers up energy it evacuates much of the surrounding area, pulling in air, which causes the low pressure. The extreme case of low pressure is the tornado where the low pressure center is caused by the tornado attempting to create a vacuum. In turn the vacuum pulls it back in and the result is a vortex structure.
Community, please correct my mistakes, I am not an expert on this stuff.
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