Hello , meteorology wizards.

The problem as it stated:
Non-saturated air parcel is adiabatically lifted from 10 C 900 mbar up
to 660 mbar, where fog appears.
I ploted it on the tephigram:
http://img503.imageshack.us/my.php?i...ewpoint7ma.jpg

The question is:
What the dew point temp of the parcel @ 660 mbar, @800 mbar?

For 660 mbar the answer is rather easy - it's around Td= -17 C.
For 800 mbar, i got confused:
a) assuming that water mixing ratio is const, we follow blue line and
get Td =-15 C for 800 mbar
it looks like it's too high for this pressure level, maybe I should
b) follow the red line of wet adiabat to get Td=-5 C , which seems more
reasonable for Tparcel=0 C

Thanks in advance for your answers.