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relative humidity
RELATIVE HUMIDITY
The coastal sections of
the Pacific Northwest has a considerably moist climate, characterized by steady
winter rains and summer. As you travel inland from the coast and across the
mountains, a drier climate will be found. This difference in "wetness"
and "dryness" across the area has a lot to do with how comfortable
you feel outdoors, and is referred to as humidity.
There are many ways of
discussing humidity. The quantity the National Weather Service uses is known
as relative humidity; it is the percent of saturation of air.
Warm air can hold more
moisture than cold air. Therefore, air containing a certain amount of moisture
will have a lower relative humidity on a warm afternoon than that same air on
a cold night. The percent of saturation is lower for warm air than for cold
air, given a certain amount of water vapor, since warm air can hold more water
than cold. That is why it feels cool and damp on a summer day on the coast,
and hot and dry in the inland valleys
Related to relative humidity
is dew point temperature. This is the temperature at which dew begins to form
on various objects. Warm air contains more moisture than cool air. As air cools,
the moisture it holds is given up and deposited on objects cooler than the air.
For example, on a warm sunny day an iced drink will form moisture on the outside
of the glass. The moisture forming on the outside of the glass is dew. The dewpoint
temperature is important to agriculture. When dew begins forming, as a result
of the temperature reaching the dewpoint temperature, heat is given off to the
air and usually results in a less rapid drop in the air temperature.
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