Author Topic: Baro jump prior to downpour  (Read 2848 times)

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Offline aweatherguy

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Baro jump prior to downpour
« on: December 13, 2009, 02:17:56 AM »
I've been using a new barometer for a few months now. It has high resolution (better than 0.004 inHg) and I get readings every minute. It is installed inside the house -- no fancy static port or anything so it will have some error in high winds (like 50MPH or more).

I am beginning to see a pattern where the pressure rises very quickly just prior to the sudden onset of heavy rainfall. For example, one time there was an increase of 0.030 inHg in 3 minutes (0.6 inHg/hr) prior to a sudden downpour which had an initial rain rate of 4.9 in/hr. Another time there was an increase of 0.023 inHg in 5 minutes (0.276 inHg/hr) prior to a rain rate of 1.7 in/hr. In both cases the pressure increase ended just a minute or two before the rain began. None of these events were preceded by winds high enough to cause significant errors due to the barometer installation.

I'm guessing that others have been here before. My best guess at an explanation is the pressure rise is somehow related to the downdraft associated with the rainshaft. Does anyone know what's happening here?

blackjack52

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Re: Baro jump prior to downpour
« Reply #1 on: December 13, 2009, 03:45:42 PM »
Sounds like the accompanying front w/ the storms.

Offline DanS

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Re: Baro jump prior to downpour
« Reply #2 on: December 13, 2009, 04:19:09 PM »
Sounds like the accompanying front w/ the storms.

That's my thinking as well. During our "rainy season" here the barometer usually takes a dive just before a storm arrives. It gets to be a daily pattern where the baro graph will rise and reach it's peak and then in the afternoon drop pretty quick. That's usually when you can spot black sky approaching and the humidty start readings dropping fast as well. By the time the graph starts to level off we can expect the excitement to begin.

Offline floodcaster

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Re: Baro jump prior to downpour
« Reply #3 on: December 13, 2009, 06:07:08 PM »
I believe that phenomenon is called a pressure jump.
Bill


Offline Mark / Ohio

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Re: Baro jump prior to downpour
« Reply #4 on: December 14, 2009, 12:50:52 AM »
The bigger and faster the jump it seems the heavier the precip but also the shorter the duration of rain or snow.  I've found that when you see that if it is not raining or snowing it will be real soon.  Pretty neat to watch.
Mark 
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Offline aweatherguy

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Re: Baro jump prior to downpour
« Reply #5 on: December 15, 2009, 01:06:01 AM »
Well, that seems to square with my measly sample of two events -- the heavier rain came with the faster, bigger jump. As I understand it, heavy rainfall entrains a lot of air which creates a strong downdraft. I thought maybe that was increasing surface pressures just ahead of the storm. But...can heavy snowfall create such a downdraft? My theory/guess might not work for heavy snow...?

Seems like this could be used to give a (very short) warning for heavy precip events -- of course if you're watching Nexrad this is pretty useless...

Offline aweatherguy

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Re: Baro jump prior to downpour
« Reply #6 on: December 15, 2009, 02:03:46 AM »
Okay, mystery solved. Thanks to floodcaster for the pointer to "pressure jump". First, I found an article by Morris Tepper in the February 1950 issue of the Journal of Meterology. He proposed that jumps were associated with squall lines and explained it as shown in the attached diagram of a piston pushing air ahead of it.

Then I hit paydirt with this article in the Bulletin of American Meterological Society (Vol 58, No 9, September 1977).

http://ams.allenpress.com/archive/1520-0477/58/9/pdf/i1520-0477-58-9-920.pdf

They actually use (or at least did back then) pressure jumps to detect gust fronts/microbursts/wind shear around major airports! These jumps are caused by the outflow from these severe weather events. Cool. I gots me a wind shear detector!

Offline Mark / Ohio

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Re: Baro jump prior to downpour
« Reply #7 on: December 15, 2009, 11:43:59 PM »
Interesting!  Now Terminal Doppler Radar seems to be the new method of wind shear detection.

http://www.ll.mit.edu/mission/aviation/faawxsystems/tdwr.html

I would not mind having one of them in addition to a good barometer.  But I guess the one 30 miles to my North with have to suffice for now.   ;)
Mark 
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Offline Chris H.

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Re: Baro jump prior to downpour
« Reply #8 on: January 05, 2010, 01:10:29 AM »
This is pretty neat actually, though my house sees 10 degree temp jumps inside (if I'm correct, temperature has an effect on barometers) therefore my weather station's pressure graph is weird.

Just before a thunderstorm, I've actually witnessed this, the pressure spikes and then drops and levels out just as the rain starts to hit.
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