what are good atmospheric conditions for weatherfax?

Very complex subject. Have a look at Frank Singleton's site (now sponsored) specifically:
http://weather.mailasail.com/Franks-Weather/Hf-Weather-Broadcast-Reception-Problems

Easiest approach is to listen to the different frequencies that the station you want transmits on before the fax you want and then use the frequency that gives best reception. Some days I have received Northwood clearly on 4610 and other days on 8040 at same time of day.
 
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