|BACKYARD METEOROLOGY: MOISTURE
METEOROLOGIST JEFF HABY
The two main types of moisture measurements are dewpoint and relative humidity. Dewpoint is the temperature the air needs to be cooled
in order for saturation to occur. Relative humidity is the percent of moisture in the air relative to the maximum amount of moisture
that could be in the air for a certain temperature. Just like with temperature, an intuitive feel can be gained for the dewpoint and
relative humidity of the air. It is more difficult to guess than with temperature since small changes in the amount of moisture can
result in significant changes in the dewpoint and relative humidity. It can be easier to guess the relative humidity fairly accurately
when the air feels very dry or very moist. The dewpoint is easier to guess fairly accurately when the air is very moist. On any
given day, the relative humidity is going to vary significantly during the day. The dewpoint will typically be more stable when
the air mass stays the same.
When precipitation occurs, the dewpoint and relative humidity will increase. If the precipitation is persistent then eventually the
dewpoint and relative humidity will reach their maximum values indicating saturated air and 100% relative humidity. Another clue
to a high relative humidity is dew or frost on the ground in the morning. When dew or frost occurs, the temperature will be near
the dewpoint and the relative humidity will be near 100%. With an intuitive feel for the temperature, the dewpoint and relative
humidity can be guessed fairly accurately when there is dew or frost on the ground.
The relative humidity tends to decrease rapidly during the morning hours on a typical day. This is because this time of day generally
experiences the greatest warming. As temperature increases the relative humidity decreases.
When the air has abundant moisture it is easier to feel this moisture in the air, especially at warm temperatures. This is a major
factor in the development of the heat index. Warm humid air will feel warmer than dry humid air even when they
have the same measured temperature.