### Author Topic: Rain vs Pressure  (Read 934 times)

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#### Stryder87

• Senior Member
• Posts: 64
##### Rain vs Pressure
« on: June 25, 2018, 12:47:13 PM »
Hi All

Because I've always been fascinated by the weather, I finally indulged myself and bought my first weather station a few weeks ago and have been doing a lot of reading on the different aspects of the weather, how things fit together, forecasting etc.

From what I've read and what I've been observing with my station, I'm seeing something that doesn't make sense and I was hoping you folks could help me understand it a bit more.  Everything I've read says that rain is usually preceded by a low-pressure system.  However, what I see in the attached screenshot from my station, there seem to be a couple oddities in the last three weeks that are making me scratch my head.

Box 1 is the first one that doesn't seem to match.  The pressure went up and then it rained, the rain stopped/paused, the pressure dropped and it continued to rain.  Box 2 is the opposite, as expected, the pressure dropped first then it rained, but the pressure rose and it rained again.  Box 3 matches everything I've read, the pressure dropped and it rained.

The pressure rising before it rained in box 1 & 2 seem to contradict everything I've read.  Since I've only really gotten through the basics, is there something that I'm missing that would explain this?  It doesn't make sense and I'm really puzzled by this.

Thanks.
Ray

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#### VaJim

• Virginia Beach Backyard Weather
• Forecaster
• Posts: 727
##### Re: Rain vs Pressure
« Reply #1 on: June 25, 2018, 12:55:16 PM »
Nice chart

I think what you'll find is that no one system is like the one that just left.  Each low pressure will vary.  The deeper the low the more obvious it will appear on a chart.  I've seen cases such as what you show, where the pressure either remained steady or rose slightly.  The other factor our chart doesn't really display is distance of the low center to your station.

You may want to take a look at any other stations in your area to see how their pressure tracked during the same time frame.
« Last Edit: June 25, 2018, 01:00:17 PM by VaJim »

#### Mattk

• Forecaster
• Posts: 1112
##### Re: Rain vs Pressure
« Reply #2 on: June 26, 2018, 04:24:33 AM »
Weather is not all controlled by surface systems which can be seen on surface charts etc Upper level systems control a lot of the weather which isn't going to show on ground based pressure instrumentation but these types of systems bring much of the rain in many places.

#### floodcaster

• Forecaster
• Posts: 578
##### Re: Rain vs Pressure
« Reply #3 on: June 26, 2018, 08:09:58 AM »
You can also experience pressure jumps just before/during passage of outflow boundaries or gust fronts.
Bill

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##### Re: Rain vs Pressure
« Reply #4 on: June 26, 2018, 09:39:34 AM »

• Member
• Posts: 13
##### Re: Rain vs Pressure
« Reply #5 on: June 26, 2018, 11:59:54 AM »
It's not just the pressure; if the humidity, temperature, and pressure are such that the dew point rises above 100% then it will rain or get foggy. Maybe...

#### Stryder87

• Senior Member
• Posts: 64
##### Re: Rain vs Pressure
« Reply #6 on: June 26, 2018, 01:22:55 PM »
Thanks for all the input.  You guys all make good points.  I think I'm trying to find more direct and obvious cause & effect observations on what's going on and keep forgetting that we're talking about weather here... it'll do what it wants!  haha

One quote from the book linked to on the Weather Doctor's page (The BC Weather Book:
From the Sunshine Coast to Storm Mountain) really applies to my area since I live on the West Coast of BC (Vancouver area): "Don't like the weather? Move a little further inland, closer to the coast, or to the other side of the mountain. That's sound advice for any BC resident because the climate of the province is one of the most complex in Canada, often changing dramatically in the space of just a few kilometres."

I suspect I'm going to find my head squirreling around a lot more now that I'm really trying to learn about how this all works and it's less an exact science than I'm used to.

Must... let... go... logic...

• Senior Contributor
• Posts: 200
##### Re: Rain vs Pressure
« Reply #7 on: June 26, 2018, 02:17:06 PM »
I recall reading somewhere that wind vector is also an important data point to consider (in addition to atmospheric pressure) when attempting to predict weather trends. I would assume such considerations would have to allow for one's specific geographic location.

#### Old Tele man

• Singing in the rain...
• Forecaster
• Posts: 1394
##### Re: Rain vs Pressure
« Reply #8 on: June 26, 2018, 02:52:57 PM »
I recall reading somewhere that wind vector is also an important data point to consider (in addition to atmospheric pressure) when attempting to predict weather trends. I would assume such considerations would have to allow for one's specific geographic location.
Correct, as:

HIGH pressure (higher temps & sunshine) is associated with Clockwise air rotations.
LOW pressure (lower temps & cloudiness) is associated with Counter-Clockwise air rotations.
« Last Edit: June 26, 2018, 02:55:16 PM by Old Tele man »
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#### CW2274

• Forecaster
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##### Re: Rain vs Pressure
« Reply #9 on: June 26, 2018, 04:12:47 PM »
Everything I've read says that rain is usually preceded by a low-pressure system.
Not at all. During monsoon season here, low pressure is rarely a factor in thunderstorm formation. Generally, all you need is ample surface based moisture, enough solar insolation to lift the moisture, and a favorable mid and upper level environment that won't hinder development (cap). However, when we do get a low come through during monsoon (easterly wave, inverted trough), things can really get cooking.

#### CW2274

• Forecaster
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##### Re: Rain vs Pressure
« Reply #10 on: June 26, 2018, 04:13:47 PM »
You can also experience pressure jumps just before/during passage of outflow boundaries or gust fronts.
Exactly. This is not only common, but expected.

#### so_whats_happening

• Member
• Posts: 2
##### Re: Rain vs Pressure
« Reply #11 on: July 24, 2018, 09:44:26 PM »
You can also experience pressure jumps just before/during passage of outflow boundaries or gust fronts.

This when you get small scale systems (usually MCS or thunderstorm scale) you get a rise in pressure right before the gust front then it will start to rain then pressures fall and then you will get the pressure to rise again as the rain continues to fall or towards the end of the storm. If I can find some outside my books Ill throw a picture of what im talking about.

For larger systems you tend to have not quite as large of a pressure pertubation but still there especially in the summer time with storm activity just depends on the activity going on. But I do agree with the other post there is random occurrences that do occur

anything