Dewpoint is an absolute measure of the water content of the air.
By comparison, relative humidity is a percent indication that
changes depending on the temperature of the "parcel" of air being
observed.
For any parcel of air, the temperature defines the saturation
vapor pressure. Lets call it Es instead of "small e sub s", since
I can't subscript in text mode. Es increases in an exponential-like
fashion as temperature increases. (Es is a weak function of other
variables, such as pressure) So, Dewpoint is defined as the
temperature to which a parcel of air must be lowered to make water
condense from the parcel. This means that the temperature would be
lowered to that temperature whose Es corresponds to the actual vapor
pressure in the parcel, E. Lower the temp, the parcel becomes
saturated. Since conditions at the surface are very rarely such that
the parcel can become super-saturated, water vapor condenses to the
liquid phase and E remains at Es, whatever that is, as the temperature
continues to lower.
In short, dewpoint is an absolute indication of the amount of water
vapor in the parcel. Relative humidity, Rh, is a comparison of the
actual water content in the parcel, E, to the saturation vapor
pressure of the parcel at the current temperature. This is why Rh
changes quickly. Change the temperature and Es for the parcel is
different. E didn't change though, so (E/Es)*100 is a different
Rh.
One more thing: The drier the air is, low Rh, the faster liquid
water will evaporate. The more water that evaporates in cooler pads,
the greater the rate that latent heat of vaporization needs to be
supplied to the vaporizing water. The only place it can come from
is the air. The swamp cooler works better at lower relative humidities.
That's why Rh is such an interesting number for the daily news reports.