The relative humidity can tell us where clouds are likely to form. But it can't tell us how humid the air feels. For that we turn to a measure called "dew point".

To understand the dew point, think of a clear, calm morning with dew on the grass. The dew formed overnight as the air near the ground cooled, and invisible water vapor in the air condensed out. The temperature at which the dew started to form is the dew point of the air. Since it's a temperature, dew point is measured in degrees.

The dew point is useful because the higher its value, the more water vapor is in the air, and thus the muggier you feel. A dew point of 60^oF is a good threshold to remember - above that, most folks agree that it's starting to feel humid. Once the dew point gets to 70^oF, it's downright tropical. In a typical summer around here, dew points might reach that uncomfortable level on 15 or 20 days.