We reference it in every weathercast we do, but barometric pressure is sometimes a hard thing to understand. The pressure reading really refers to the weight of the atmosphere above us, and we measure it with a barometer to see if it's going up or down. When a storm is approaching, the pressure goes down (hence, "low" pressure). Just because the pressure goes down, it doesn't necessarily mean rain or snow will soon follow, but it does give you an idea that something could be changing soon. Higher pressure is generally associated with calm weather because the air is sinking and exerting more force down toward the ground.
Watching the fluctuation in pressure with Sandy has been interesting, and once it comes on shore, it will have one of the lowest pressure readings and cause significant flooding and storm surge. Consider for a moment that most people sensitive to changes in the weather will notice the pressure when it drops to around 29.70". Sandy, even as a category 1 hurricane, has had pressure readings down around 28.00", but because it is merging with another system over the US, the pressure will likely drop to around 27.90 before it begins coming back up (or the storm starts weakening)
It's a big deal because so many people will be hit by the storm and power outages will be widespread. It could be several days before things get back to normal, even after the storm is long gone.
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