Author Topic: Barometric pressure increase during thunderstorms  (Read 947 times)

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Offline miraculon

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Barometric pressure increase during thunderstorms
« on: June 28, 2013, 08:57:12 PM »
I noticed two distinct barometer pressure jumps during recent thunderstorms. Here is a screen grab. The red plot on the wind direction graph is lightning. You can see the wind speed increase at the same time.

You can also see from the temperature plot that it cooled during this event, which would be consistent with a downdraft.

Are these downdrafts causing the pressure increase?

Greg H
« Last Edit: June 28, 2013, 09:00:04 PM by miraculon »




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Offline BigOkie

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Re: Barometric pressure increase during thunderstorms
« Reply #1 on: June 28, 2013, 11:00:13 PM »
Simply put?  No.

The temperature cooling?  That's usually rain-cooled air.   8-)

The pressure rise is typically due to evaporational cooling.  Low level thickness decreases, so the surface pressure tends to rise.  Happened to me last night, but it wasn't due to a thunderstorm...it was a gust front (outflow boundary) from a storm to my west.
« Last Edit: June 28, 2013, 11:03:39 PM by BigOkie »


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Offline SlowModem

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Re: Barometric pressure increase during thunderstorms
« Reply #2 on: June 29, 2013, 12:32:38 AM »
It's hard for me to tell what the lines are on your graphs.  However, I have seen barometer jumps right before a bad storm, too.  I know it happens, and it might be cooler air sinking.  I donno.   :-k

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Offline DaleReid

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Re: Barometric pressure increase during thunderstorms
« Reply #3 on: June 29, 2013, 09:23:11 AM »
Mr. Okie,

I read your reply with great interest since I, too, have had this question and would like to know the answer (s) to this effect.

My question is how does one separate out the effects of cold air downdraft (if there is such a thing, but I think it is the reason for the 'first gust' in front of a storm) from cold rain from higher up cooling the air surrounding it as it falls through, vs. the evaporative cooling you mention in your note?

Are each of these effects so distinct that there is a mechanism to detect which one is the major effect producer?  Which of these (and no doubt other causative factors) is the elephant and which is the flea for the total effect?

As one who can be misled by trying to apply one circumstance, wrongly, to explain another effect, I'd like to learn more about this effect. (example of a helium filled balloon moving away from the acceleration when one goes around a corner in a car with the windows closed, seems wrong until you know the physics behind it).

To me, opening a freezer door in the grocery and feeling the cold blast is attributable to in essence a downdraft of falling cold air, perhaps pulled down by the countless, near freezing, raindrops or whatever mechanism that downdrafts may occur.  Then there is the effect of the cold rain chilling the air at this level, and then, lastly in my way of evaluating this, would be evaporatative cooling, which isn't huge inside a storm area where the humidity is already nearly if not 100% (sorry to not talk dew points for this simplistic approach).  Also knowing how poorly I cool with evaporation when its hot and humid, and how poorly the evaporative coolers work for rooms when people try those instead of usual air conditioning, would make me think this would be a lessor effect.

Those graphs show a sizable shift in pressures and it would be interesting to see what a very sensitive barometer might show during the approach of a single cell, with the first gust hitting the instrument, then when rain started, noting the temperature of the rain and changes in the surrounding air temperature, but then somehow gathering similar data at each 1000' altitude (tough during a storm!) to see what these all are showing for an interplay.

As a pilot who would avoid storms of any sort, I know from ground school and practicality that there are multiple factors and what seems logical doesn't at all prove to be the major factor in many situations, so I'm intrigued by your bringing in this condition that I'd never even considered.

Rest assured that despite my devil's-advocate approach to this, that I'm all ears and very excited to learn something new so please don't take this as a one sided criticism, I'm hoping you can help us discover other facts about storms, with all their intrigue and excitement as to how they work. 
Dale

Offline BigOkie

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Re: Barometric pressure increase during thunderstorms
« Reply #4 on: June 29, 2013, 09:40:51 AM »
Mr. Okie,

I read your reply with great interest since I, too, have had this question and would like to know the answer (s) to this effect.

My question is how does one separate out the effects of cold air downdraft (if there is such a thing, but I think it is the reason for the 'first gust' in front of a storm) from cold rain from higher up cooling the air surrounding it as it falls through, vs. the evaporative cooling you mention in your note?

Are each of these effects so distinct that there is a mechanism to detect which one is the major effect producer?  Which of these (and no doubt other causative factors) is the elephant and which is the flea for the total effect?

As one who can be misled by trying to apply one circumstance, wrongly, to explain another effect, I'd like to learn more about this effect. (example of a helium filled balloon moving away from the acceleration when one goes around a corner in a car with the windows closed, seems wrong until you know the physics behind it).

To me, opening a freezer door in the grocery and feeling the cold blast is attributable to in essence a downdraft of falling cold air, perhaps pulled down by the countless, near freezing, raindrops or whatever mechanism that downdrafts may occur.  Then there is the effect of the cold rain chilling the air at this level, and then, lastly in my way of evaluating this, would be evaporatative cooling, which isn't huge inside a storm area where the humidity is already nearly if not 100% (sorry to not talk dew points for this simplistic approach).  Also knowing how poorly I cool with evaporation when its hot and humid, and how poorly the evaporative coolers work for rooms when people try those instead of usual air conditioning, would make me think this would be a lessor effect.

Those graphs show a sizable shift in pressures and it would be interesting to see what a very sensitive barometer might show during the approach of a single cell, with the first gust hitting the instrument, then when rain started, noting the temperature of the rain and changes in the surrounding air temperature, but then somehow gathering similar data at each 1000' altitude (tough during a storm!) to see what these all are showing for an interplay.

As a pilot who would avoid storms of any sort, I know from ground school and practicality that there are multiple factors and what seems logical doesn't at all prove to be the major factor in many situations, so I'm intrigued by your bringing in this condition that I'd never even considered.

Rest assured that despite my devil's-advocate approach to this, that I'm all ears and very excited to learn something new so please don't take this as a one sided criticism, I'm hoping you can help us discover other facts about storms, with all their intrigue and excitement as to how they work. 
Dale

I'm not disputing that there may be other factors.  I was simply answering the question that appeared to ask if a downdraft could be the only cause (as I read it).  And of course, it isn't.

I have a friend who is a meteo, and I too, am a pilot (without current paper, as it's a bit cost-prohibitive right now to fly).  I've asked him this before, and this is always his answer.  But note I also phrase it with 'typically'.  As everyone knows, weather is quite variable in nature.

But the question was as it relates to surface pressure, and not the dynamics within the storm.  Have you ever noticed how the dew point drops sharply before the advance of a storm?  Or as a gust front/outflow boundary moves through?  Look at that drop and correlate with the rise in pressure.  That's why I trust my meteorologist friend when he explained it as evaporational cooling.


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Offline Cutty Sark Sailor

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Re: Barometric pressure increase during thunderstorms
« Reply #5 on: June 29, 2013, 10:49:34 AM »
Evaporational cooling. --- As the rain falls, some of it evaporates, and that cools the immediate air volume, which also becomes a bit denser/heavier if you will. This cooler denser air will begin to sink, as a hot air baloon would without more heating. When it reaches the surface it might resemble a 'dome' as it spreads out. If you are outside this dome of cold air you might show a decrease of pressure until the cold air enters, or if you're inside the dome you may see an increase as the cold air flows downward. Which is exactly what your graph shows... a decrease, an increase, a decrease, then back to baseline...

Back in the late 70's/80's when I did a lot of night storm tracking with some others, we developed a sense of the cell features and movement from the air direction changes. I wouldn't be so brazen as to say we could sense the barometric changes, but anything is possible. A couple of us could plot the storm features pretty accurately at times...  :roll:
« Last Edit: June 29, 2013, 10:55:13 AM by Cutty Sark Sailor »






Offline Weather Display

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Re: Barometric pressure increase during thunderstorms
« Reply #6 on: June 29, 2013, 04:09:04 PM »
rising air in a thunderstorm is associated with a pressure drop
(and that pressure drop decreases the temperature, which causes more rain, which results in latent heat being released, which is then a feedback)
the rear flank down draft will be the "air pressure release" side of the CB, where the drop in pressure is countered by a rise in pressure, which can carry on rising as the storm passes if it was a line of CB's due to a change in air mass (dew point).i.e a cold front type passage)
sort of thing
as well

Offline DaleReid

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Re: Barometric pressure increase during thunderstorms
« Reply #7 on: June 29, 2013, 10:51:37 PM »
What an exciting discussion!

I marvel at nature and how things work, and you guys have opened a door to more understanding.

I love this group.  All my old meteorology texts are way too old (I took the course in the early 70s from Vernor Suomi) and had nothing listed for this topic.

I'll have to pay the University library a visit.  Thanks.