|RELATIVE HUMIDITY PITFALLS
METEOROLOGIST JEFF HABY
The relative humidity is one of the most misinterpreted and misunderstood pieces of weather information given with the current
conditions. Below is a discussion of relative humidity pitfalls:
a. The value of relative humidity often says little about if precipitation is falling or not. The relative humidity can be high with
or without precipitation and a low relative humidity can be experienced at the onset of precipitation. The misinterpretation that
often occurs is that a relative humidity of 100% means that it is raining. There are many situations in which a 100% relative humidity
can occur without precipitation. One way is through overnight cooling in which the temperature drops to the dewpoint. Another
way is when there is fog but precipitation is not falling. The value of relative humidity can be low at the onset of rain. It is
true the relative humidity can increase dramatically with rain but it may not rise all the way to 100% if the precipitation does
not last long enough or if the precipitation is light. In a sustained precipitation event the relative humidity can increase to 100%
but as mentioned precipitation does not require the 100% relative humidity value observed at the surface.
b. The temperature has a dramatic impact on the value of relative humidity. As the temperature increases, generally the relative humidity
decreases. Thus, the afternoons tend to have lower values of relative humidity when compared to the mornings. These increases
and decreases of relative humidity during the day do not indicate if precipitation is more or less likely but is only a reflection
of the temperature’s impact on the relative humidity value.
c. It is mistake to interpret a higher value of relative humidity as indicating there is more moisture in the air. This is only always
true when the temperature is constant. Air with a 40% relative humidity can have much more moisture than air with an 80% relative
humidity when the 40% relative humidity air is much warmer than the 80% relative humidity air. Because of this, relative humidity
information without knowing the temperature is fairly meaningless. It is important to combine both temperature with relative humidity
to get a sense of how much moisture is in the air. Better yet, it is more helpful to know the dewpoint.