Topic: Storm warning BC coast

[larryjb]

We had a storm warning with wind and rain warnings on the BC coast.  I don't have my weather station yet, but I was monitoring the BP readings from the airport using an app and saw the BP rising, and rising, and rising.  Everything I've read and understood suggests that I should have been expecting a drop, but a big drop never came.  The big storm hasn't come yet either.  Yes, the BP is falling again, about the same rate that it rose.  I guess it could still drop rapidly, but I was beginning to doubt my knowledge, especially given that it's very basic. 

So, what conditions can cause the BP to rise (101.5 to 102.5 in about 12 hours) while a storm is barrelling down beside us?

[galfert]

Pressure changes daily in what can be described as rising and falling smoothly and gradually on an almost predictable cycle. This happens every day. There are two big reasons for this; the moon and the sun. The moon causes gravitational tides of atmospheric pressure. The sun causes thermal atmospheric tides. The sun also causes gravitational tides but to an almost negligible effect. There are some other factors also with negligible effects, but I don't want to write a book. Just research atmospheric tides to learn more.

Therefore what you were experiencing was just normal daily atmospheric tide changes. The storm is not yet close enough for you to see its effects of really causing the pressure to suddenly drop. If it is a large enough storm it probably is dropping the pressure as it approaches, but the atmospheric tides may be the bigger immediate influence on an observed pressure increase (within a few hour time frame). But the daily average is most likely lower due to the storm if it is large enough and close enough.

This is the reason why when watching a barometer day to day and keeping manual records to always do it at the same time every day, with noon being the recommended hour (it tends to stabilize around this time and makes calibration easier). You want the atmospheric tides to not be a factor.

[larryjb]

I had superimposed the atmospheric tides onto the graph already, and the rise I saw was definitely independent of the tide.

I looked at the graph again. The low was Friday morning at about 6 AM, at about 101.4 kPa. The high was at 6AM Saturday morning, about 102.6 kPa.

[galfert]

There is one other part I left out. At the edge of a storm there is actually a higher pressure phenomenon caused by the shifting winds as the storm moves in. This can result In a quick rise and then sudden drop as the front has moved in.

[larryjb]

I'll see if I can attach a screenshot.  It sounds like a rise caused by shifting winds as you say.  But, we didn't get the big storm. I saw it coming on the RADAR, but it seemed to pass us by.

[galfert]

Yes, if the storm misses you but you are close enough as it passes, you can experience a pressure increase due to the increased winds. Then there is no sudden drop because it missed you. You only got the edge.