Difference Between Dew Point and Relative Humidity

Trying to understand why we reference dew point temperature rather than relative humidity when referring to how humid it is outside can be a little difficult to understand.  To try and get a better understanding, let's first discuss what each means.

Dew point temperature is a direct measurement of how much moisture is in the atmosphere.  We know how the weather makes us feel.  A humid afternoon makes it feel more 'sticky' outside, like Wednesday.  While afternoons like Thursday felt a little more comfortable.  That is a result of the dew point temperature.  Dew points Wednesday were in the low 70's, a very tropical air mass.  That made many of us feel uncomfortable.  Dew points by Thursday afternoon were in the upper 50's and low 60's.  This may have made you feel a little more comfortable when being outside.  Think of the dew point temperature and your comfort level as an almost direct correlation.

Relative humidity is a measurement of how saturated the air is.  When the relative humidity is less than 40%, the air feels dry.  When the relative humidity is above 80%, the air feels moist.  Relative humidity also changes throughout the day.  During the evening and overnight as the air temperature cools closer to the dew point temperature, the relative humidity gets closer to 100%.  And that's why often times in the morning we have a high relative humidity number.   Throughout the day as the air temperature warms, it gets farther away from the dew point temperature causing the relative humidity to decrease (most times).  If you're still a little uncertain, let's take this example.  On a hot and humid afternoon, like Wednesday, when the temperature was close to 90 degrees and the dew point was near 70 degrees, the relative humidity was only between 50% and 60%, but it felt very humid outside, even though the relative humidity was low.  This morning (Thursday) the relative humidity was in the low 80's but then dropped into the 60's during the afternoon.  This is why the dew point temperature is a better measurement of how humid it feels to you when outside.