Reading Pressure Trends: What the Numbers Mean
Atmospheric pressure at sea level averages 1013 hPa. Most people know that low pressure means bad weather and high pressure means good weather — but the rate of change matters more than the absolute value.
A drop of 3 hPa or more in three hours is a strong signal of approaching bad weather. In maritime climates (northwest Europe, Atlantic seaboard), this often precedes frontal passages by 12-18 hours. My log shows that every storm with sustained winds above 50 km/h in the past year was preceded by at least a 4 hPa three-hour drop.
Rapid rises after a low can be equally dramatic. A fast pressure recovery (3+ hPa in three hours) often produces the gusty, showery weather behind a cold front — the sky looks cleaner but the wind picks up. For cyclists and sailors this matters: post-frontal conditions can be rougher than the front itself.
The most interesting patterns are the ones that don't follow the textbook. Last November I recorded a 9 hPa rise in six hours — officially meteorological bomb (explosive cyclogenesis in reverse). The sky was cloudless throughout. Checking synoptic charts afterwards, a blocking high was accelerating into position from the Azores. No rain, no clouds, but the pressure moved like a storm.
StormLeaf's three-hour rate-of-change chart is what I look at first every morning. The absolute value comes second.