What you missing is RELATIVE humidity. 50% relative humidity at 20 degrees f and 50% relative humidity at 70 degrees are two very different things. If I remember correctly the air can hold 100% more moisture by weight for every 20 degrees increase in temperature.
When you look at the humidity chart in the link you posted you'll see almost 80% for January. But then go look at the temperature chart for January.
When you look at the humidity chart in the link you posted you'll see almost 80% for January. But then go look at the temperature chart for January.