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Hi guys.
I have some questions concerning humidity (absolute and relative), dew
point, and air density.

Understanding humidity and dew point:

Dewpoint and relative humidity are related.
Short answer
Water is evaporating and condensing all the time in the atmosphere there is a point where
they are equal..

Dewpoint:
The temperature to which a given air parcel** must be cooled at constant pressure in order
for equalization* to occur.

Humidity:
The ratio of the vapor pressure to the equalization* vapor pressure at a given
temperature with respect to water, usually expressed as a percentage.

* "equalization" is usually referred to as "saturation" but "saturation" is a hold over
from an18th century understanding of science when it was thought that air was like a
liquid solution.


** Technically - the temperature and pressure are that of the water vapor.


Does it make sense for the weather channel to report a dew point below
freezing?
Is this a result of a calculation from measured relative humidity, air
temperature, and barometric pressure? Or can you actually condense
water out of the air below freezing?

Even at subfreezing temperatures most hygrometers are sensitive to relative humidity with
respect to water rather than that of ice because of "supersaturation".


Is there a source of standard functions relating relative humidity,
absolute humidity, pressure and temperature?


Yes, try NOAA or the American Meteorological Society websites, they probably have
glossaries or other resources


If I want a more accurate method of measuring absolute humidity, can I
cool a plate and record the temperature when moisture condenses on the
plate and use that as dew point to calculate relative and absolute
humidity? (This is how they determine humidity in the chamber used to
house the Declaration of Independence without opening the chamber.)

Thanks in advance for any help.