METEOROLOGIST JEFF HABY
The term relative in relative humidity refers to being relative to temperature. It can also be interpreted as the actual mixing ratio
(or actual vapor pressure) relative to the saturation mixing ratio (or saturation vapor pressure). The relative humidity by itself
can not be used to tell how much moisture is in the air. A relative humidity of 75% in warm air has much more moisture than a relative
humidity of 75% in cold air.
A percent is found by taking a decimal value and multiplying by 100. A percent can also be thought of as being out of 100. For example,
if the mixing ratio is 8 g/kg and the saturation mixing ratio is 16 g/kg, then the relative humidity is 8/16 = 0.5 = 0.5 * 100 = 50%.
This can be thought of as the air having half the moisture it could have for the given temperature.
Relative humidity can change dramatically as temperature changes. If the moisture in the air stays about the same, increasing the
temperature will lower the relative humidity. This is why the relative humidity tends to decrease during the day light hours
(since temperature typically increases). A morning with frost and 100% relative humidity can turn into an afternoon with mild
temperatures and a relative humidity of 40%. Relative humidity tends to increase at night since temperature
is typically decreasing overnight.
How does the general public many times interpret relative humidity. Below are several misinterpretation and the reason for the mistake.
Misinterpretation: Relative humidity indicates how high the chance for precipitation is. 100% indicates precipitation is occurring
and low values indicate precipitation is not occurring.
Comments: Although higher relative humidity often occurs with precipitation, it can occur for other reasons such as overnight cooling. Also,
in elevated precipitation events, precipitation can occur with an initially low relative humidity at the surface.
Misinterpretation: Relative humidity is an indicator of the amount of moisture in the air. High relative humidity indicates lots of
moisture in the air while low relative humidity indicates very little moisture in the air.
Comments: Temperature information must also be included since cold and saturated air will have low amounts of moisture and warm air
with low relative humidity can actually have quite a bit of moisture in the air.
Misinterpretation: Relative humidity tells if there are clouds present or not. A high value indicate lots of clouds while a low
value indicates no clouds.
Comments: This can occur in many situations but not always. It is possible to have a clear morning with a relative humidity of
100% and it can be cloudy with a low relative humidity at the surface.
Misinterpretation: Relative humidity is the dewpoint divided by the temperature.
Comments: This will typically not give an accurate value on its own. The relative humidity is found by the mixing ratio divided by the
saturation mixing ratio or the vapor pressure divided by the saturation vapor pressure. This value is then converted to a percent.