What VPD means for your cigars (and why your hygrometer is lying about it)
Vapor Pressure Deficit explains why a humidor at 70% RH can still dry your cigars. The science, the math, and the fix.
TL;DR. Relative humidity is just a ratio, not an amount. If you see 70% on your hygrometer, it’s not telling you how much water is actually in the air — it’s telling you what percent full the air is for that temperature. Your cigars definitely notice the difference. Vapor Pressure Deficit, or VPD, wraps temperature and humidity into one clear number: how thirsty the air really is.
Picture this: it’s last July, and an enthusiast keeps their small Daniel Marshall humidor running a locked-in 68% RH all month. The hygrometer is spot-on, the Boveda packs are fresh, and they’d just spent an hour sealing up the lid. Still, something goes wrong. A prized Padrón 1964 tries to canoe, and a Connecticut Macanudo — meant as a gift — splits at the cap right when they cut it. Out of frustration, not strategy, they knock the box down to 62% over a weekend. The next Padrón? Burns perfectly even.
What gives? The number on the dial never changed. The cigars sat at 68% all along, but the air in July was a whole different animal than in May. And a dial can’t know the difference.
Turns out, there’s a name for what was really going on: Vapor Pressure Deficit, or VPD. Greenhouse growers and tobacco folks have been using this idea for nearly 100 years. In the cigar world, there’s a homespun version of this wisdom (“don’t trust 70/70 in summer”
The hygrometer isn’t lying
“Relative humidity” means exactly what it says — it’s relative. It tells you how much water vapor is in the air, as a percentage of what the air could hold at that temperature
But here’s the bit most people ignore. We talk about “70% RH” like that’s a set amount, as if “70%” means you’re walking around with 70 grams of water in the air. Nope. The air at 25°C (77°F) can hold way more water than air at 16°C (61°F). So 70% at the higher temp really means you’ve got a lot more water floating around.
Your hygrometer’s not lying — it’s just reporting a ratio. The actual mass of water floating in the air depends totally on the temperature, and your dial can’t show you that.
The number your hygrometer won’t show
There’s a number called saturation vapor pressure (usually es or SVP) — basically, how much water air could possibly hold at a certain temperature. People measure it in kilopascals (kPa), and the relationship to temperature is steep. The formula looks messy
es(T) = 0.6108 · exp ( 17.27 · T / (T + 237.3) )
You don’t have to memorize it. Just know: as temperature goes up, air can hold a ton more water. For example, bumping the temperature up just 5°C near room temp means the air can hold about a third more water
| Temperature | SVP (kPa) | What 70% RH means here |
|---|---|---|
| 10°C | 1.23 | 70% means pretty dry air |
| 16°C | 1.82 | 50% more water than at 10°C |
| 21°C | 2.49 | 37% more than at 16°C |
| 25°C | 3.17 | another 27% more |
| 30°C | 4.24 | now you’re holding 3.5× what 10°C could |
So if your humidor is sitting at 70% RH, how much moisture is really in there jumps all over the place depending on the time of year, location, what your room and lid are made of, HVAC, you name it.
VPD: the honest number
Vapor Pressure Deficit is simple. It’s the difference between what the air could hold at your current temperature, and how much it’s actually got right now.
VPD = es(T) × (1 − RH/100)
This number tells you how “thirsty” the air is. Greenhouse pros have known for decades — VPD predicts how hard air will pull moisture from a leaf (or a cigar), way better than RH alone
Play with the numbers and here’s what you see. You can reach the same VPD in lots of different ways. More importantly, two identical RH readings can mean very different VPDs if the temps are different.
Stable storage range for cigars.
For example, a cool cabinet at 16°C and 70% RH gives you 0.55 kPa. Move your humidor to a summer room at 25°C and keep it at 70% RH? Suddenly you’re at 0.95 kPa — almost twice the air “thirst” for the same number on the dial. That was me last July. My Boveda packs kept the ratio perfect. The air itself just wanted more water out of the cigars.
The 70/70 myth, finally explained
“70 degrees Fahrenheit and 70% RH” became the standard cigar answer sometime in the late 1900s. It’s fine for Cuban fields or a wine cellar. For someone in the desert with the AC cycling on and off? Pretty lousy.
It comes down to VPD. At 21°C and 70% RH, VPD sits at 0.75 kPa — a nice balance that keeps wrappers smooth and doesn’t let mold get going. Push that same 70% up to 27°C (80°F) and VPD shoots to 1.07 kPa. Now the air is a lot drier and a paradise for tobacco beetles, which hatch right above 75°F
All that advice people pass around — “lower RH in summer,” “aim for 62–65% if it’s hot out” — really says: dial back RH as temps rise, to keep VPD in the safe zone. It works because you’re pulling VPD back to where it needs to be, even when the temperature’s fighting you. Cigar smokers figured this out long ago by trial and error, before anyone named it.
What the single hygrometer misses
Ever notice the air feels more humid near your humidifier? One guy on a forum moved his hygrometer from the side next to the humidifier to the other side overnight, and it dropped from 68–70% down to 64%
What to actually do
If you only remember three things, let it be these, in order:
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Track temperature as carefully as humidity. A 5°C swing changes the impact of RH by about a third in either direction. If your humidor is somewhere that gets hot or cold, the “fixed” RH you always aim for actually means something different as the seasons roll by.
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Lower your RH target as you raise the temp. The cigar community’s advice stands tall here, backed by math. Try to keep your VPD in the 0.5–0.9 kPa sweet spot. Use a calculator (like the one above) to check what RH and temp combos put you there.
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Use more sensors for bigger humidors. Walk-ins, cabinet-style, or big towers mix air unevenly, so one sensor usually just flatters you. Put sensors top and bottom, near and far, so you actually see what’s happening across the space.
Every time The Cabinet reads a temperature and humidity from your humidor, it calculates VPD. That’s how I caught the Daniel Marshall last summer. If you only have a basic hygrometer, it’ll show the symptoms — split caps, cigars that burn weird, the classic curse of “the box looks fine but smokes poorly.” VPD explains why.
And I’ll admit something: I keep the dashboard open in a browser tab, not just for emergencies. Most mornings, the VPD numbers don’t budge, even as the AC kicks in or the radiator wakes up. That’s how I know the cigars are okay, and that quiet “is the box alright?” anxiety? It kind of fades out. In its place, nerd calm. If you’re still reading this far into a post about vapor pressure, you know exactly what I’m talking about. Welcome to the club.
— Roger
Sources
- Über einige meteorologische Begriffe
The empirical equation for saturation vapor pressure used throughout meteorology and agriculture for nearly a century. The Cabinet's VPD computation uses Tetens' formulation directly.
- Crop Evapotranspiration — FAO Irrigation and Drainage Paper 56
The standard reference for vapor-pressure-deficit calculation in agricultural science — the framework that took VPD from atmospheric physics into practical crop and storage management.
- Vapour-pressure deficit
Plain-language overview of VPD with the same Tetens-derived formula horticulturalists, greenhouse operators, and tobacco specialists have used for decades.
- The 70/70 Myth: Why a Lower RH Is Often Better for Your Cigars
The canonical writeup of the 70/70 problem in cigar circles — including the desktop-humidor case that mirrors the anecdote in this post.
- "Dried Out Cigars in 68 RH"
The forum case where repositioning a hygrometer to the opposite side of the humidor dropped its reading from 68–70% to 64% overnight. Single-point RH measurement hides spatial gradients — and VPD asymmetries.