Bruce wrote:
Try - http://www.theweatheroutlook.com/com...cle.asp?id=182

Therw is a little unaccuracy in the text cited above, when it says:

"As the temperature drops the dew point remains constant, because the
total volume of water in the parcel is (basically) unchanged."

The meaning of the term "dew point" in this context is not clear, if
it means "temperature of the dew point", this is not true. The
temperature at which the water vapor will condense is related to its
partial pressure. The thing which remains constant while the parcels
ascends is the mass of water it contains, however, due to its expansion,
the partial pressure of water vapor drops, so the temperature at which
it will condense is lower than at ground level. This is why all these
diagrams have a network of lines usually called "mixing ratio lines",
each such line gives the relationship between total pressure (or
equivalently altitude) and temperature at which a parcel with this
mixing ratio (i.e. mass of water per mass of air, the thing which
remains constant) will condense. As expected from previous
considerations, along such a line the temperature drops as pressure
drops or altitude raises.

However the text cited above later correctly describes the use of these
mixing ratio lines.