Relative Humidity and Cigar Storage
Relative humidity is a measure of the amount of water in the air compared with the amount of water the air can hold at the temperature it happens to be when you measure it.
Air temperature (Celsius) / Water vapor air can hold at that temp (Saturation vapor pressure)
30°C (86°F) / 30 grams per cubic meter of air
20°C (68°F) / 17 grams per cubic meter of air
10°C (50°F) / 9 grams per cubic meter of air
Based on the above "table," here's how relative humidity works: Imagine that you measure the air's temperature at 30° Celsius (86°F) and you measure its humidity at 9 grams per cubic meter of air. You would divide 9 by 30 and multiply by 100 to get a relative humidity of 30%. In other words, the air actually has 30% of the water vapor it can hold at its current temperature. Cool the air to 20°C (68°F). Now we divide 9, the vapor actually in the air, by 17, the vapor it could hold at its new temperature, and multiply by 100 to get a relative humidity of 52.9% (53% rounded off). Finally, when the air cools to 10°C (50°F), we divide 9 by 9 and multiply by 100 to get a relative humidity of 100%. The air now has all the vapor it can hold at its new temperature.
Once again, the way you measure "relative humidity" is to divide the actual vapor pressure by the saturation vapor pressure and then multiply by 100 to convert to a percent. So, to get the "ideal" 70°/70% conditions in your humidor (give or take a degree or two) you would need a temperature of 68°F and saturation vapor pressure of 11.55 grams per cubic meter of air.
What this amounts to is, the less heat you have the more humidity required, and vice versa to keep the climate balanced.
Now that you've taken "Humidity 101," let me direct your attention to a illuminating article in the July/August 2008 issue of Cigar Snob magazine by Colin Ganley. Titled "Keeping Your Cigars," Ganley reports that the ideal conditions for your storing and aging your cigars are actually very cool temperatures with an average relative humidity of 67%. Indeed, from my own experience I've found that an RH closer to 65% is quite suitable.
I suggest you read "Keeping Your Cigars" because you'll really learn a lot about humidity levels, but there are some other great tips about tobacco beetle issues and proper storage of your cigars. When the window opens, on the left-hand side of the page where it says "Go to page," type in 18 and click "GO."