The case against Relative Humidity
many consider RH to be a good indicator for how the humidity will affect comfort level. This is incorrect however because as the name suggests, the metric is relative to various conditions. It only describes how much water is in the air relative to what the current temperature can hold. (Give non-weather example) What that means is 80% RH can describe quite different amounts of moisture in the air. One possible solution is to memorize this chart and the various conditions that both variables, air temperature and RH could produce. The better solution is to get acquainted with the dew point.
Carrying Capacity of Air
If you've ever put sugar in hot liquid like tea or coffee, you may have noticed that the sugar dissolves somewhat easily when compared to colder water. (If you don't believe me - try it!)