MNS Forums

MNS Forums (
-   14. Indoor (
-   -   Atmospheric Pressure (

cannatari 10-30-2011 05:09 PM

Atmospheric Pressure
I've seen and heard at least 2 conflicting pieces of advice regarding proper grow room air pressure and none have been accompanied by a conclusive reason as to why. I set out to answer the question myself. I learned of something called the Subtropical Ridge and it is responsible for all things barometric. It is basically a convergence of hot tropical air from the equator and cold air from the poles which takes place between 25-40 degrees latitude. The result is a zone of high barometric pressure, clear skies and dry, cold air. It is responsible for the world's deserts.

This chart represents 15 years of average barometric pressure. Jun, Jul, Aug on the top, Dec, Jan, Feb on the bottom:

If the chart looks like it came from an article on the distribution of landrace phenotypes, it's because they are the same thing. Barometric pressure is relevant to the intensity of sunlight in nature and it is that difference in intensity that provoked indica and sativa expressions. There is not enough air pressure at the equator to keep moisture in the ground so particles of moisture in the air diffuse sunlight is why it's less intense there.

I will leave all that this implies for discussion but the short answer for proper air pressure is that it depends on how much light your using, what type of plant your growing and whether you're vegging or flowering.:D


Atmospheric pressure - Wikipedia, the free encyclopedia

Subtropical ridge - Wikipedia, the free encyclopedia

High-pressure area - Wikipedia, the free encyclopedia

Barometer - Wikipedia, the free encyclopedia

sativaXindica 10-30-2011 05:43 PM

Greetings cannatari,

I also have been getting conflicting stories of what is better. A negeative pressure in my tent or a possitive pressure (inflating my tent). I was told that negative pressure helps control odor, which makes sense. But what about the plants actual health? What is better for the plant is more important to me? I was told that a negative pressure creates stagnant air that can encourage mold?

Would love to hear some response from members who have tried both ways.

Thanks for the chart cannatari,

Best Regards


cannatari 10-30-2011 06:12 PM

Hiya, sXi,

Transpiration increases in negative pressure is why I believe it encourages mold. Mold lives on a film of water and IMO, you can still have mold problems at 10% RH if the vegetation is supplying it with the moisture it requires.


cannatari 10-30-2011 06:23 PM

It also occurs to me that barometric pressure has an affect on the spectrum of sunlight as well as the intensity. Red light travels further than blue light. That is why the sky shifts toward the red spectrum at sunrise and sunset. Low pressure climates are subject to high amounts of moisture particles in the air which not only reduce the intensity of light but also shift it toward the red spectrum.


sativaXindica 10-30-2011 06:29 PM


Originally Posted by cannatari (Post 131373)
Hiya, sXi,

Transpiration increases in negative pressure is why I believe it encourages mold. Mold lives on a film of water and IMO, you can still have mold problems at 10% RH if the vegetation is supplying it with the moisture it requires.


Greetings cannatari,

I see what you mean how a limited amount of moisture can still produce mold becuase of the negative pressure. This means that the stagnant air that isn't being circulated leaves the plants in an almost vaccum like capsule. In return, no air being moved around can cause mold to grow even with the slightest amount moisture.

What if you had negative pressure but also had multiple fans blowing air around. Would that alieveate the stagnant air problem and allow the plants to "air out" so mold won't grow? I'm starting to worry a little about my set-up considering I do have a slight negative pressure. Fortuanutely I haven't had any mold problems to date. I have 3 passive intakes for air to come into my tent and 1 inline fan blowing air into the tent. But my exhaust inline fan sucking air out is far more powerful and creates a negative pressure. Though I don't have any mold problems yet, I would like to know what is best for my plants health.

One more thing cannatari, why do you believe that transpiation increases in negative pressure areas?

Best Regards


cannatari 10-30-2011 07:00 PM

It's not about stagnant air. There is no airspace in between the mold and the plant surface. Transpiration is like an NFT hydro for mold that runs 24 hrs a day in a negative pressure environment. It must be that the plant does not naturally transpire moisture 24 hrs a day and is therefore able to keep mold from proliferating with times of non-transpiration.

This article explains that plant transpiration doubles from sea level to 3,000 feet (decrease in atmospheric pressure) :

Plants and Altitude


tinleefloufa 10-30-2011 07:29 PM

howdy cannatari

how you do? thank you to share you research and such chart with respect to season spectrum. am much enjoy to think to landrace and wild variety with respect to illustration. am keen to similar illustration rather with respect to uv intensity. elevation and barometric pressure vary across earth and am keen with respect to such low pressure and elevation altitude to 3000 meter.

such dance mother nature dance with infinite variable, such barometric pressure and altitude not much study with respect to cannabis.

indeed proper depend with perspective, am appreciate you open mind and you topic. much respect cannatari.

much interest with respect to variable such barometric pressure and stomata variance analysis between variety within species. such with not to exclude altitude and uv intensity spectrum and other infinite variable. hehe :)

good on you and to share with mns am interest to you topic and eager with respect to hear other mns community opinion.

positive vibration

cannatari 10-30-2011 09:00 PM

Blow your mind on this!
None of this has really explained the effects of barometric pressure on phenotypical expression. I have shown how it affects sunlight and thus influences expression but is the pressure itself causing a change in the plant?

Here are what I consider to be the factors in expression:

Intensity of Light = Broadness of Leaf
Spectrum of Light = Incidences of Branches or Leaves
Latitude = Flowering Duration

What is missing in this map?

Atmospheric Pressure = Taste and Smell

Think about it. Desert plants have terpenes that deter plant eaters. Tropical plants have terpenes that attract pollinators. High altitude trees have terpenes that are anti-fungal. Those are the primary colors of the terpene palette and they are High, Low and Negative pressure environments.


cannatari 10-30-2011 11:10 PM

1 Attachment(s)
I took the liberty of overlaying the countries on the hPa map:


spice 10-31-2011 08:55 AM

great post cannatari

All times are GMT. The time now is 07:40 AM.

Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2020, vBulletin Solutions Inc.
SEO by vBSEO 3.6.0 PL2
All rights reserved, MR NICE SEEDBANK, NL