On a barometer-what is the threshold between high & low pressure?
Standard barometric pressure is 29.92" of 1013.2 millibars. Readings above those values are considered 'high' and below them, 'low'.
But what meteorologists really watch isn't just the exact pressure, but it's rate of change. The faster the pressure is rising or falling, the faster the change. Both increasing and decreasing pressure can indicate nasty winds; but decreasing pressure is most likely to indicate increasing clouds and rain.