|EXPLAINING DEWPOINT AND RELATIVE
TO THE PUBLIC
METEOROLOGIST JEFF HABY
The public can have a tough time understanding the difference between the "meaning" of RH and dewpoint. Meteorologists
have the challenge of explaining these concepts to the general public.
The public has a good grasp on how the weather makes them feel. One approach to explaining dewpoint would be to
say, dewpoints above 65 F make it feel sticky and humid outside while dewpoints less than 65 F are comfortable
with respect to the stickiness of the air. The higher the dewpoint is, the more moisture that is in the air.
The higher the dewpoint is above 65 F, the stickier it will feel outside (feels like you have to breathe in
a bunch of moisture with each breath). 75 F or above dewpoint, the air really feels sticky and humid.
RH can be more difficult to explain. The public pretty much understands that a RH of 100% means it is either
foggy, very wet, or saturated outside. One misconception people have is that the RH is 100% only when it
is raining. Example 1: The RH is often 100% in the early morning hours when temperature has dropped to dewpoint.
Example 2: When rain first begins, it takes time for the air to saturate. RH is often much less than 100% when
it is raining (it takes time and lots of evaporation to saturate air that previously has a RH of 50% for example).
If the rain is not heavy enough or does not last long enough, the rain will not saturate
a previously drier
RH can be explained to the public as the "closeness the air is the saturation". When the RH is less than
feels dry outside, and when the RH is greater than 80% it feels moist outside (dewpoint will determine
if it is uncomfortably moist or just regularly moist). Between 40 and 80% RH is comfortable if the temperature
is also comfortable.
The worst combination for human comfort is a high dewpoint (65 F or above) combined with a high RH. If the
dewpoint is above 65, it will generally always feel uncomfortably humid outside. Obviously, the temperature
could climb to over 100 and result in a low RH, but the quantity of moisture in the air is still high and
will be noticed.
The optimum combination for human comfort is a dewpoint of about 60 F and a RH of between 50 and 70% (this
would put the temperature at about 75 F). The air feels dry outside when BOTH the dewpoint is below 60 F
AND the RH is less than 40%.
Now the dilemma, how does the public differentiate the "meaning" between a high dewpoint and a high RH when
they both indicate the air is humid??? Dewpoint is related to the quantity of moisture in the air while
relative humidity is related to how close the air is to saturation. How the public is to understand
this difference in meaning can be a challenge. The challenge can be overcome by describing how
the weather feels and relate that information to the current dewpoint and relative humidity.