Relative Humidity, Water Activity, and Cannabis: How Are They Connected?

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You can’t talk about cannabis without talking about humidity and water activity. The moisture content of your cannabis and the air around it are make-or-break factors that can wreak havoc on your flower if not kept in check. Too much moisture inside your flower breeds mold and mildew, while too little moisture can make your flower brittle and cause it to literally crumble in your hands. Whether you just planted clones or you’re storing finished and cured buds at home, controlling humidity and water activity is essential at every step.

The term “humidity,” though, doesn’t fully encompass the precise metrics that need to be monitored. Much of the focus is on relative humidity, which is a related but slightly different figure that can make a big difference, and the related term “water activity.” What are relative humidity and water activity, how does relative humidity differ from water activity and absolute humidity, and how do water activity and relative humidity relate to cannabis?

What is relative humidity?

Relative humidity measures water vapor that’s actually present in the air, as opposed to the maximum moisture content that could be present in the air. This is expressed as a percentage, calculated against the air’s current temperature. This more directly ties the humidity measurement to temperature.

Relative humidity matters because warmer air holds more moisture. The warmer your environment, the more humid the environment. This also means that you need to keep a closer eye on relative humidity as temperatures fluctuate to get an accurate assessment of the moisture levels in the air.


Absolute humidity tracks the amount of moisture that can be in the air regardless of the temperature, while relative humidity calculates moisture levels relative to the air’s temperature.

How do you calculate relative humidity?
You can calculate the relative humidity percentage by dividing the gram per cubic meter measurement of the maximum moisture possible in the air by the gram per cubic meter measurement of the water vapor present in the air. That number is then multiplied by 100 to come up with a percentage. You can also use a hygrometer placed in the area to get a relative humidity read without pen and paper, and don’t forget the thermometer to keep an eye on temperature!

What is water activity?
Water activity can be defined in two ways. It is the ratio of the vapor pressure of the water in a sample – say, your cannabis – to the vapor pressure of pure water at the same temperature. As such, water activity is expressed on a scale of zero to one.

What’s the difference between absolute humidity and relative humidity?

Water activity is also the relative humidity of the air in equilibrium with a sample in a sealed chamber. For cannabis, water activity is thus the relative humidity of the air surrounding your flowers in their container, assuming the water in your cannabis is in equilibrium with the water in the surrounding air.

In cannabis, water activity has gradually taken the place of another long-used metric: moisture content. That’s because moisture content describes the total amount of water in a substance, whereas water activity describes the extent to which water is bound to a substance’s molecules.

Think about it like this: A sopping wet paper towel has a moisture content value of 100% but a water activity of value of much lower than 1. That’s because the paper towel retains this moisture until you squeeze it out – or, put scientifically, until you exert energy upon the paper towel to release its water. The squeezed paper towel will now have a higher water activity value since most of the water present is now readily accessible – including to microbes that, in cannabis, can lead to spoiled flowers.

How do you measure water activity?

Sometimes, water activity is measured using equipment called capacitance hygrometers and resistive electrolytic hygrometers. However, the chilled mirror dew point hygrometer is the preferred water activity measurement method.

In chilled mirror dew point hygrometers, a temperature-controlled mirror, photodetector, and light beam stabilize the mirror’s temperature. This way, the rate of dew condensation matches the evaporation rate. With this condition achieved, equilibrium will exist between the water vapor in the air surrounding your cannabis and the water inside your cannabis.

With this setup, you can accurately determine the dew point temperature of the air around your cannabis. From there, you can easily use the dew point temperature equation to determine the relative humidity, and thus vapor pressure, of this air.

Additionally, since you know the temperature of your sample, you can use that in conjunction with the relative humidity and dew point temperature to determine the vapor pressure of your cannabis. Your final step is to determine your sample’s water activity, which is the ratio of the air vapor pressure to your cannabis’s vapor pressure.

How do relative humidity and water activity relate to cannabis?

Just like Goldilocks, your room temperature and your humidity cannot be too hot or too cold – it must be just right. Too humid (too high a water activity value), and you run into issues with bud rot, mildew, and mold development; too dry (too low a water activity value), and your cannabis may dry out too quickly. Since relative humidity and water activity reflect the moisture content of the air and cannabis quality is dependent on proper humidity levels, these more accurate moisture level assessments are a must for any cannabis cultivator.

Each stage of the cannabis growing cycle has its own temperature requirements. The temperature and humidity of your grow facility should be at these levels at each growing stage:

  • Seedling stage: The temperature should be set between 68°F and 77°F during the day, and slightly cooler at night. Humidity levels should hover between 40 percent and 60 percent. If you are growing your cannabis from clones, the humidity should be higher – as high as 80 percent – as the young plants thrive in warmer and more humid conditions.
  • Vegetative stage: According to Danny Danko, the temperature during the vegetative stage should be around 70°F to 78°F during the day, adjusted to no more than 15°F cooler at night. The relative humidity of this environment should hover between 45 percent and 55 percent.
  • Flowering stage: The flowering stage typically requires lower humidity levels as the buds grow and become more dense, and therefore create more opportunities for bud rot. Danny Danko recommends a relative humidity level no higher than 45 percent, gradually lowered to as low as 30 percent as the weeks go by. He also recommends a temperature between 68°F and 75°F.


At all stages, according to the ASTM International cannabis committee, the water activity of your cannabis should remain between 0.55 and 0.65. Notably, in California, state regulations allow for water activity levels in solid cannabis edibles to reach 0.85.


Relative humidity levels and temperature matter during the drying and curing process too. Growing experts recommend the following temperatures and humidity levels during the traditional drying and curing process:

  • Drying: The point of drying cannabis is to reduce its moisture content, and humidity reintroduces moisture to the plant material. To that end, cannabis cultivation guru Ed Rosenthal recommends drying cannabis at 68°F at no more than 55 percent relative humidity for the first few days of the lengthy drying process. The temperature should drop a few degrees after that, but the relative humidity should not dip below 50 percent, as this could cause the cannabis to dry too fast.
  • Curing: There are many methodologies for curing cannabis, each of which is crafted to intensify flavors and potency. Rosenthal recommends storing the cannabis in a dark room for 1 to 2 months at 64°F, keeping the relative humidity levels between 45 and 50 percent.

The ASTM International cannabis committee also recommends keeping water activity levels between 0.55 and 0.65 during drying and curing.


No matter the size of your grow or where your grow is located, relative humidity and water activity never go away. Moisture in the air is a fact in nearly every climate, which makes the traditional careful monitoring and control over relative humidity and water activity even more important, especially if an entire cannabis grow is on the line.

Thankfully, the Cryo Cure process is anything but traditional. Our patent-pending technology trims down the lengthy drying and curing process to just 13 hours, shortening the time cannabis and hemp flower spends exposed to the elements. By shrinking this window, there’s simply less time for something to go wrong.

How Cryo Cure helps minimize issues related to relative humidity

Through our machines’ customizable settings, the moisture content of the plant material is brought down to between 8 and 12 percent – dry enough to ward off dreaded molds and mildews, without drying it so much that the flower turns to dust. You can adjust these moisture content settings according to your needs, even as low as 1 percent for extractors looking to harvest phytocannabinoids and terpenes.

Best of all, the machines’ smaller footprint makes the internal environment easier to control. No longer will you need to heat or cool large warehouses – or pay the high energy bills that come with running that equipment. You’ll only have to worry about climate control within the machine’s walls, which takes up a fraction of the space of a traditional grow operation. The machine is only opened once during that 13-hour process, too, reducing exposure to the elements that can sour your cannabis or hemp flower.

Bottom line: the more control over your grow environment, the better handle you have over the relative humidity levels that can make or break your cannabis grow. Cryo Cure’s ability to shorten the drying and curing process – plus the machine’s smaller footprint – makes monitoring and adjusting relative humidity a much less daunting prospect. Contact Cryo Cure today to schedule your no-obligation consultation

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