The humidity term we always see reported is relative humidity. Not to get too technical, but it's the ratio of the actual vapor pressure and the saturated vapor pressure. In other words, the amount of moisture in the air and the amount of moisture the air would have if it was saturated at the specific air temperature. Now we never see vapor pressures reported on the news (although we use them often in meteorology), but you do ocassionally see dewpoint. That's the temp the air would have if it was completely saturated. When the temp equals the dewpoint you have a relative humidity of 100%. You need saturation to form clouds and precipitation aloft. At the surface you tend to get fog or diamond dust at very cold temperatures. The air can have 100% relative humidity at any temperature. Humditiy is not a good indicator of the actual amount of moisture in the air. Dewpoint is a more direct measure. A temp of 90F and dewpoint of 70F feels very oppressive to us. A temp of 30F and dewpoint of 30F just feels cold and raw. The former has a relative humidity well below 100% and the latter has a relative humidity of 100%.
On a relatively calm, clear night with an arctic airmass in place, the ground loses heat to space via radiation loss and the surface attempts to cool down toward the dewpoint. Often the amount of moisture in the air is similar outside and inside of the coop if you have good ventilation. So, if your inside coop temp tends to match the outside air temp at night, your relative humidity will run similar to the outside values. If your coop runs 10F warmer inside than out, you will tend to have a lower relative humidity in the coop assuming good ventilation. Cold wind is bad when directly hitting the birds, but it does tend to mean lower relative humidities. Keep the drafts off of the birds and they should have no problem with those types of airmasses so long as you have cold hardy breeds.
I like to put a ceramic heat emitter directed on my leghorns on the radiational cooling nights where it can be -20F with 90% RH by morning. Again, if you're near saturation outside and your coop temp is similar to the outside air, you're going to have trouble bringing your RH down much unless you add a little heat or a dehumidifier. I'm struggling with my leghorn dilemma right now too. It's currently -2F with a dewpoint of -15F and a relative himidity of 54%, but my leggies got a tough of frost bite with the outside air at 3F and 90% humidity.
Took this pic a last year? maybe to show roost height temps compared to outside temps, which were around 18* at the time. 
