Here's an explanation from a Ph.D. at UCLA from 2009:
5.0 Humidification
Why create a humid environment with a constant relative humidity of 70%?
Cigars are naturally hygroscopic products. In common with many organic substances, they dry out in the absence of humidity in the air, or absorb moisture from the ambient air. They establish an equilibrium with the atmospheric humidity which surrounds them.
At 68% a cigar will slowly dry out and loose essential oils. At 74% and higher, organic molecules will break down out-of-order, producing unwanted tastes. More importantly, at 80% or higher, you're leaving your stogies wide open to grow mold. Neither cigars or humidors are a particularly sterile environment. Note that these are relative humidities - cigars should be stored at 70 - 73% RH regardless of temperature.
The "Variable" Humidity Myth...
Many self-proclaimed "authorities" have been making a serious mistake of confusing moisture content with relative humidity lately. At least one well-known Cigar distributor has added strength to this legend by distributing a table recommending different relative humidities at different temperatures. Don't fall for this "urban legend"! Cigars should be kept at 70% RH REGARDLESS of temperature or you WILL ruin them...
The modern myth of varying humidity with temperature is based on an entirely false premise. Cigars don't care about absolute moisture content. They need just enough moisture to keep the tobacco pliable, but not so much to swell it. This occurs at 70-73% REGARDLESS of temperature.
Tobacco should be kept at a relative humidity which just keeps the leaves pliable without swelling them. Since that pliability is based on the interaction of the cellular membranes with the air surrounding them, relative humidity--not absolute moisture content--is what's important. This isn't chemistry, it's mechanics - on a cellular level. Ideally, the correct point is 70-73% regardless of temperature. At 70% relative humidity (a measure which by definition is independent of temperature) cigar leaves become pliable without swelling. At 75% or higher, the cells begin to swell; at 68% or lower, they can become brittle. These points are regardless of the ambient temperature or the absolute moisture content.
Does it make any sense to a rational person to keep their cigars at close to 100% humidity at 60 degrees? Anyone living in cold climates can attest to the fact that at 60 deg, their cigars are perfect at 70%, and (as they would at any temperature) their cigars are prone to mold and swelling at humidities above 80%. Those living in the tropics will tell you the same thing - their cigars MUST be kept at 70% humidity at storage temperatures of 80 degrees. According to "the table", those cigars should be at 50% humidity? I'll tell you what: If the humidity drops below 68%, your cigars will become dry and crack - I couldn't even imagine how badly dried out they'd be at 50%!
Try it yourself. The "logic" of varying RH with temp falls apart in the real world. Moisture content is NOT why we keep cigars at 70% humidity...
Here's a more scientific explanation debunking the myth of varying humidity from David E. Patton, Ph.D. at the Department of Physiology, UCLA School of Medicine...
"There has been extensive discussion on A.S.C. concerning the effects of temperature on humidity and its application to proper cigar storage. Much of the confusion concerning these concepts comes from not understanding what is happening at the molecular level. My goal is to explain some of the relevant concepts and then to put the concepts together in such a way as to give an intuitive understanding of how they relate to cigar storage.
Cigars are made from tobacco leaves. Tobacco leaves (like most plant material) consist primarily of carbohydrates and proteins. Protein and carbohydrate molecules contain many binding sites for water molecules. A certain proportion of the water binding sites need to be occupied by water molecules in order for the tobacco leaves to be adequately pliable, to burn properly and to age properly. So when we think about humidity as it relates to cigar storage, the most relevant factor to consider is: Are the correct proportion of water binding sites occupied? Water molecules bind to other molecules via hydrogen bonds. Van der Waals interactions also participate. This applies to water bound to protein and carbohydrate molecules (e.g. tobacco leaves) or to other water molecules (e.g. liquid water). For the purposes of this discussion, water bound to tobacco leaves will be treated like liquid water.
Temperature is the main factor determining whether a water molecule will be more likely to be in either the gas or liquid (or bound) phases. This is because at higher temperatures, water molecules (like any other molecule) will have more kinetic energy. The more kinetic energy a molecule has, the higher its probability of being in the gas phase. This is because it will have sufficient kinetic energy to break out of the hydrogen bonds and Van der Waals interactions that would otherwise keep it bound. At lower temperatures molecules have less kinetic energy so when they collide with a carbohydrate molecule, for example, its kinetic energy is insufficient to break away from it. The important point here is that at higher temperatures, a water molecule is more likely to be in the gas phase and less likely to be bound. At lower temperatures a water molecule is more likely to be bound and less likely to be in the gas phase.
Another point that needs to be explained here is the concept of relative humidity. Simply stated, relative humidity is the ratio of the concentration of water in the gas phase divided by the maximal concentration of water the air can hold (the saturating concentration) at a given temperature. Air holds more water at higher temperatures. Therefore, if you hold the relative humidity constant and increase the temperature, the concentration of water molecules in the gas phase will increase. If you think about this superficially you may think that because the concentration of water molecules increases in the gas phase as you increase the temperature (holding relative humidity constant)that your cigars will become over-humidified. This is WRONG. Remember, as you increase the temperature, the water molecules are less likely to be bound to the tobacco and more likely to be in the gas phase. Thus, to keep the same proportion of water binding sites in the tobacco occupied by water molecules, the concentration of water molecules in the gas phase must be increased when temperature is increased."
Again, cigars should be stored at 70 - 73% relative humidity, regardless of temperature. Period.
here's why!!
